The Dairy Science and Technology eBook

This is an educational area focused on milk, dairy products, and dairy technology, and is one book in our Dairy Education Series. This site was developed and is continually maintained by:   

Professor H. Douglas Goff
University of Guelph
Canada

I hope you find this site useful as a reference, or for teaching or training purposes. If you do use information from this site, please cite your source as Professor H. Douglas Goff, Dairy Science and Technology Education Series, University of Guelph, Canada.

Reproduction of this site, or any portions thereof, in either print or electronic form without permission of the author is strictly prohibited.

The Dairy Science and Technology Book Table of Contents

Introduction to Dairy Science and Technology: Milk History, Consumption, Production, and Composition

Introduction

 This course is about the study of milk and milk-derived food products from a food science perspective. It focuses on the biological, chemical, physical, and microbiological aspects of milk itself, and on the technological (processing) aspects of the transformation of milk into its various consumer products, including beverages, fermented products, concentrated and dried products, butter and ice cream.

Milk is as ancient as humankind itself, as it is the substance created to feed the mammalian infant. All species of mammals, from humans to whales, produce milk for this purpose. Many centuries ago, perhaps as early as 6000-8000 BC, ancient peoples learned to domesticate species of animals for the provision of milk to be consumed by them. These included cows (genus Bos), buffaloes, sheep, goats, and camels, all of which are still used in various parts of the world for the production of milk for human consumption.

Fermented products such as cheeses were discovered by accident, but their history has also been documented for many centuries, as has the production of concentrated milks, butter, and even ice cream.

Technological advances have only come about very recently in the history of milk consumption, and our generations will be the ones credited for having turned milk processing from an art to a science. The availability and distribution of milk and milk products today in the modern world is a blend of the centuries old knowledge of traditional milk products with the application of modern science and technology.

The role of milk in the traditional diet has varied greatly in different regions of the world. The tropical countries have not been traditional milk consumers, whereas the more northern regions of the world, Europe (especially Scandinavia) and North America, have traditionally consumed far more milk and milk products in their diet. In tropical countries where high temperatures and lack of refrigeration has led to the inability to produce and store fresh milk, milk has traditionally been preserved through means other than refrigeration, including immediate consumption of warm milk after milking, by boiling milk, or by conversion into more stable products such as fermented milks.

World-wide Milk Consumption and Production

The world dairy situation - facts and figures regarding production and consumption - is presented by the International Dairy Federation here.

Total milk consumption (as fluid milk and processed products) per person varies widely from highs in Europe and North America (60-90 kg) to lows in Asia (<30 kg). However, as the various regions of the world become more integrated through travel and migration, these trends are changing, a factor which needs to be considered by product developers and marketers of milk and milk products in various countries of the world.

Even within regions such as Europe, the custom of milk consumption has varied greatly. Consider for example the high consumption of fluid milk in countries like Finland, Norway and Sweden compared to France and Italy where cheeses have tended to dominate milk consumption. When you also consider the climates of these regions, it would appear that the culture of producing more stable products (cheese) in hotter climates as a means of preservation is evident. Table 1 illustrates milk per capita consumption information from various countries of the world (older data from 2011) while Table 2 shows the quantity of raw milk produced around the world.

Table 1. Per Capita Consumption of Milk (L) and Milk Products (kg) in Various Countries, 2011 data (www.dairyinfo.gc.ca).

Table 2. Cow Milk Production (‘000 tonnes) in Selected Countries of the World, 2011 data (www.dairyinfo.gc.ca)

Milk Composition

The role of milk in nature is to nourish and provide immunological protection for the mammalian young. Milk and honey are the only articles of diet whose sole function in nature is food. It is not surprising, therefore, that the nutritional value of milk is high.

Table 3. Composition of Milk from Different Mammalian Species (per 100 g fresh milk).

Table 4. The changing yield and composition of milk over the last two decades in Canada, as a function of breed (Canadian Dairy Information Centre, www.dairyinfo.gc.ca, 2010).

Now you can return to the home page Table of Contents and work through the various topics within this Education Series systematically, or you can select any topic of interest for further, in-depth information. I hope you enjoy!

Milk Production and Biosynthesis

Milk is the source of nutrients and immunological protection for the young cow. The gestation period for the female cow is 9 months. Shortly before calving, milk is secreted into the udder in preparation for the new born. At parturition, fluid from the mammary gland known as colostrum is secreted. This yellowish coloured, salty liquid has a very high serum protein content and provides antibodies to help protect the newborn until its own immune system is established. Within 72 hours, the composition of colostrum returns to that of fresh milk, allowing to be used in the food supply.

The period of lactation, or milk production, then continues for an average of 305 days, producing as much as 9000-10,000 or more kg of milk. This is quite a large amount considering the calf only needs about 1000 kg for growth.

Within the lactation, the highest yield is 2-3 months post- parturition, yielding 40-50 L/day. Within the milking lifetime, a cow reaches a peak in production about her third lactation, but can be kept in production for 5-6 lactations if her health and milk yield are still good.

Graph representing Milk Yield (kg/day) and Weeks of Lactation. Milk Yield declines at -9% per month after peak lactation

About 1-2 months after calving, the cow begins to come into heat again. She is usually inseminated about 3 months after calving so as to come into a yearly calving cycle. Heifers are normally first inseminated at 15 months so she's 2 when the first calf is born. About 60 days before the next calving, the cow is dried off. There is no milking during this stage for two reasons:

- milk has tapered off because of maternal needs of the fetus
- udder needs time to prepare for the next milking cycle

The life of a female cow can be summerized as follows: 

 Age 

0                          Calf born
15 mos                Heifer inseminated for first calf
24 mos                First calf born - starts milking
27 mos                Inseminated for second calf
34 mos                Dried off
36 mos                Second calf born - starts milking
Cycle repeats for 5-6 lactations. 

Automatic Milking

Milking of cows on-farm is usually done twice per day in milking parlors. Modern milking units cn be applied automatically in robotic milkers, or manually in herring-bone style or rotary milking parlor configurations. The milking unit simulates the suckling action by oulsating a rubber inflation to massage the teat and remove the milk. Vacuum is applied to create the pulsating opening and closing of the rubber inflation, but the vacuum is not applied directly to the teat. Milk is withdrawn at 37°C and must be immediately cooled to < 4°C. 

 

Diagram of an Automatic Milking Machine showing both the Massage Phase and the Expansion Phase

Effects of Milk Handling on Quality and Hygiene

Cleanliness

The environment of production has a great effect on the quality of milk produced. For many reaosns including cow and udder health, food safety and dairy food quality, the production of the highest quality milk (lowest microbial content) should be the goal. Hygienic quality assessment tests include sensory tests, dye reduction tests for microbial activity, total bacterial count (standard plate count), sediment, titratable acidity, somatic cell count, antibiotic residues, and added water.

The two common dye reduction tests are methylene blue and resazurin. These are both synthetic compounds which accept electrons and change colour as a result of this reduction. As part of natural metabolism, active microorganisms transfer electrons, and thus rate at which dyes added to milk are reduced is an indication of the level of microbial activity. Methylene blue turns from blue to colorless, while resazurin turns from blue to violet to pink to colourless. The reduction time is inversely correlated to bacterial numbers. However, different species react differently. Mesophilics are favoured over psychrotrophs, but psychrotrophic organisms tend to be more numerous and active in cooled milk. 

Temperature

Milk production and distribution in the tropical regions of the world is more challenging due to the requirements for low-temperature for milk stability. Consider the following chart illustraing the numbers of bacteria per millilitre of milk after 24 hours: 

5°C 2,600
10°C 11,600
12.7°C 18,800
15.5°C 180,000
20°C 450,000

Traditionally, this has been overcome in tropical countries by stabilizing milk through means other than refrigeration, including immediate consumption of warm milk after milking, by boiling milk, or by conversion into more stable products such as fermented milks. 

Mastitis and Antibiotics

Mastitis is a bacterial and yeast infection of the udder. Milk from mastitic cows is termed abnormal. Its SNF, especially lactose, content is decreased, while Na and Cl levels are increased, often giving mastitic milk a salty flavour. The presence of mastitis is also accompanied by increases in bacterial numbers, including the possibility of human pathogens, and by a dramatic increase in somatic cells. These are comprised of leukocytes (white blood cells) and epithelial cells from the udder lining. Increased somatic cell counts are therefore indicative of the presence of mastitis. Once the infection reaches the level known as "clinical' mastitis, pus can be observed in the teat canal just prior to milking, but at sub-clinical levels, the presence of mastitis is not obvious. 

Somatic Cell Count (000's/ml) Daily Milk Yield (kg): 1st Lactation
0-17 23.1 29.3
18-34 23.0 28.7
35-70 22.6 28.0
71-140 22.4 27.4
141-282 22.1 27.0
282-565 21.9 26.3
566-1130 21.4 25.4
1131-2262 20.7 24.6
2263-4525 20.0 23.6
>4526 19.0 22.5

Antibiotics are frequently used to control mastitis in dairy cattle. However, the presence of antibiotic residues in milk is very problematic, for at least three reasons. In the production of fermented milks, antibiotic residues can slow or destroy the growth of the fermentation bacteria. From a human health point of view, some people are allergic to specific antibiotics, and their presence in food consumed can have severe consequences. Also, frequent exposure to low level antibiotics can cause microorganisms to become resistant to them, through mutation, so that they are ineffective when needed to fight a human infection. For these reasons, it is extremely important that milk from cows being treated with antibiotics is withheld from the milk supply.

The withdrawal time after final treatment for various antibiotics is shown below (Note: for illustration only, practitioners shoudl consult individual drug therapy recommendations):

Amoxcillin 60 hrs.
Cloxacillin 48 hrs.
Erythromicin 36 hrs.
Novobiocin 72 hrs.
Penicillin 84 hrs.
Sulfadimethozine 60 hrs.
Sulfabromomethozine 96 hrs.
Sulfaethoxypyridozine 72 hrs.

Anti-Microbial Systems in Raw Milk

There exists in milk a number of natural anti-microbial defense mechanisms. These include:

  • lysozyme - an enzyme that hydrolyses glycosidic bonds in gram positive cell walls. However, its effect as a bacteriostatic mechanism in milk is probably negligible.
  • lactoferrin - an iron binding protein that sequesters iron from microorganisms, thus taking away one of their growth factors. Its effect as a bacteriostatic mechanism in milk is also probably negligible.
  • However, lactoperoxidase is significant - an enzyme naturally present in raw milk that catalyzes the conversion of hydrogen peroxide to water. When hydrogen peroxide and thiocyanate are added to raw milk, the thiocyanate is oxidized by the enzyme/ hydrogen peroxide complex producing bacteriostatic compounds that inhibit Gram negative bacteria, E. coli , Salmonella spp., and streptococci. This technique is being used in many parts of the world, especially where refrigeration for raw milk is not readily available, as a means of increasing the shelf life of raw milk. Good sanitation and hygiene practices remain critical to produce good quality milk! See, for example, the commercial product Stabilak in Nicaragua, http://stabilak.com.ni/en/meet-stabilak/. "This method should only be used in situations when technical, economical and/or practical reasons do not allow the use of cooling facilities for maintaining the quality of raw milk. Use of the LP-system in areas which currently lack an adequate infrastructure for collection of liquid milk, would ensure the production of milk as a safe and wholesome food, which otherwise would be virtually impossible. The method of activating the LP-s in milk is to add about 10 ppm (parts per million) of thiocyanate (preferably in powder form) to the raw milk to increase the overall level to 15 ppm (around 5 ppm is naturally present). The solution is thoroughly mixed for 30 seconds and then an equimolar amount (8.5 ppm) of hydrogen peroxide is added (generally in the form of a granulated sodium carbonate peroxyhydrate). The activation of the lactoperoxidase has a bacteriostatic effect on the raw milk and effectively extends the shelf life of raw milk for 7–8 hours under ambient temperatures of around 30oC or longer at lower temperatures. This allows adequate time for the milk to be transported from the collection point to a processing centre without refrigeration." (Benefits and potential risks of the lactoperoxidase system of raw milk preservation: report of an FAO/WHO technical meeting, FAO Headquarters, Rome, Italy, 28 November - 2 December 2005.)

Milk Biosynthesis

Milk is synthesized in the mammary gland. Within the mammary gland is the milk producing unit, the alveolus. It contains a single layer of epithelial secretory cells surrounding a central storage area called thelumen, which is connected to a duct system. The secretory cells are, in turn, surrounded by a layer of myoepithelial cells and blood capillaries.

Diagrams of the Udder Quarter, Alveolus, and Secretory Cell

The raw materials for milk production are transported via the bloodstream to the secretory cells. It takes 400-800 L of blood to deliver components for 1 L of milk.

  • Proteins: building blocks are amino acids in the blood. Casein micelles, or small aggregates thereof, may begin aggregation in Golgi vesicles within the secretory cell.
  • Lipids:
    • C4-C14 fatty acids are synthesized in the cells
    • C16 and greater fatty acids are preformed as a result of rumen hydrogenation and are transported directly in the blood
  • Lactose: milk is in osmotic equilibrium with the blood and is controlled by lactose, K, Na, Cl; lactose synthesis regulates the volume of milk secreted

The milk components are synthesized within the cells, mainly by the endoplasmic reticulum (ER) and its attached ribosomes. The energy for the ER is supplied by the mitochondria. The components are then passed along to the Golgi apparatus, which is responsible for their eventual movement out of the cell in the form of vesicles. Both vesicles containing aqueous non-fat components, as well as liquid droplets (synthesized by the ER) must pass through the cytoplasm and the apical plasma membrane to be deposited in the lumen. It is thought that the milk fat globule membrane is comprised of the apical plasma membrane of the secretory cell.

Milking stimuli, such as a sucking calf, a warm wash cloth, the regime of parlour etc., causes the release of a hormone called oxytocin. Oxytocin is relased from the pituitary gland, below the brain, to begin the process of milk let-down. As a result of this hormone stimulation, the muscles begin to compress the alveoli, causing a pressure in the udder known as letdown reflex, and the milk components stored in the lumen are released into the duct system. The milk is forced down into the teat cistern from which it is milked. The let-down reflex fades as the oxytocin is degraded, within 4-7 minutes. It is very difficult to milk after this time.

Milk Grading and Defects

The importance of milk grading lies in the fact that dairy products are only as good as the raw materials from which they were made. It is important that dairy personnel have a knowledge of sensory perception and evaluation techniques. The identification of off-flavours and desirable flavours, as well as knowledge of their likely cause, should enable the production of high quality milk, and subsequently, high quality dairy products.

An understanding of the principles of sensory evaluation are necessary for grading. All five primary senses are used in the sensory evaluation of dairy products: sight, taste, smell, touch and sound. The greatest emphasis, however, is placed on taste and smell.

The Sense of Taste

Taste buds, or receptors, are chiefly on the upper surface of the tongue, but may also be present in the cheek and soft palates of young people. These buds, about 900 in number, must make contact with the flavouring agent before a taste sensation occurs. Saliva, of course, is essential in aiding this contact. There are four different types of nerve endings on the tongue which detect the four basic "mouth" flavours -sweet, salt, sour, and bitter. Samples must, therefore, be spread around in the mouth in order to make positive flavour identification. In addition to these basic tastes, the mouth also allows us to get such reactions as coolness, warmth, sweetness, astringency, etc. 

The Sense of Smell

We are much more perceptive to the sense of smell than we are to taste. For instance, it is possible for an odouriferous material such as mercaptain to be detected in 20 billion parts of air. The centres of olfaction are located chiefly in the uppermost part of the nasal cavity. To be detectable by smell, a substance must dissolve at body temperature and be soluble in fat solvents.

Note: The sense of both taste and smell may become fatigued during steady use. A good judge does not try to examine more than one sample per minute. Rinsing the mouth with water between samples may help to restore sensitivity.

Milk Grading Techniques

Temperature should be between 60-70° F (15.5-21° C) so that any odour present may be detected readily by sniffing the container. Also, we want a temperature rise when taking the sample into the mouth; this serves to volatize any notable constituents.

Noting the odour by placing the nose directly over the container immediately after shaking and taking a full "whiff" of air. Any off odour present may be noted.

Need to make sure we have a representative sample; mixing and agitation are important.

Agitation leaves a thin film of milk on the inner surface which tends to evaporate giving off odour if present.

During sampling, take a generous sip, roll about the mouth, note flavour sensation, and expectorate. Swallowing milk is a poor practice.

Can enhance the after-taste by drawing a breath of fresh air slowly through the mouth and then exhale slowly through the nose. With this practice, even faint odours can be noted.

Milk has a flavour defect if it has an odour, a foretaste or an aftertaste, or does not leave the mouth in a clean, sweet, pleasant condition after tasting.

Characterization of Flavour Defects - ADSA

Lipolytic or Hydrolytic rancidity

Rancidity arises from the hydrolysis of milkfat by an enzyme called the lipoprotein lipase (LPL). The flavour is due to the short chain fatty acids produced, particularly butyric acid. LPL can be indigenous or bacterial. It is active at the fat/water interface but is ineffective unless the fat globule membrane is damaged or weakened. This may occur through agitation, and/or foaming, and pumping. For this reason, homogenized milk is subject to rapid lipolysis unless lipase is destroyed by heating first; the enzyme (protein) is denatured at 55-60° C. Therefore, always homogenize milk immediately before or after pasteurization and avoid mixing new and homogenized milk because it leads to rapid rancidity.

Some cows can produce spontaneous lipolysis from reacting to something indigenous to the milk. Late lactation, mastitis, hay and grain ratio diets (more so than fresh forage or silage), and low yielding cows are more susceptible.

Lipolysis can be detected by measuring the acid degree value which determines the presence of free fatty acids. Lipolytic or hydrolytic rancidity is distinct from oxidative rancidity, but frequently in other fat industries, rancid is used to mean oxidative rancidity; in dairy, rancidity means lipolysis.

Characterized: soapy, blue-cheese like aroma, slightly bitter, foul, pronounced aftertaste, does not clear up readily

Oxidation

Milk fat oxidation is catalysed by copper and certain other metals with oxygen and air. This leads to an autooxidation reaction consisting of initiation, propagation, termination.

RH --- R + H initiation - free radical

R + O2 ---- RO2 propagation

RO2 + RH --- ROOH + R

R + R --- R2 termination

R + RO2 --- RO2R

It is usually initiated in the phospholipid of the fat globule membrane. Propagation then occurs in triglycerides, primarily double bonds of unsaturated fatty acids. During propagation, peroxide derivatives of fatty acids accumulate. These undergo further reactions to form carbonyls, of which some, like aldehydes and ketones, have strong flavours. Dry feed, late lactation, added copper or other metals, lack of vit E (tocopherol) or selenium (natural antioxidates) in the diet all lead to spontaneous oxidation. It can be a real problem especially in winter. Exposure to metals during processing can also contribute.

Characterized: metallic, wet cardboard, oily, tallowy, chalky; mouth usually perceives a puckery or astringent feel

Sunlight

Often confused with oxidized, this defect is caused by UV-rays from sunlight or fluorescent lighting catalyzing oxidation in unprotected milk. Photo-oxidation activates riboflavin which is responsible for catalyzing the conversion of methionine to methanal. It is, therefore, a protein reaction rather than a lipid reaction. However, the end product flavour notes are similar but tends to diminish after storage of several days.

Characterized: burnt-protein or burnt-feathers-like, "medicinal"-like flavour

Cooked

This defect is a function of the time-temperature of heating and especially the presence of any "burn-on" action of heat on certain proteins, particularly whey proteins. Whey proteins are a source of sulfide bonds which form sulfhydryl groups that contribute to the flavour. The defect is most obvious immediately after heating but dissipates within 1 or 2 days.

Characterized: slightly cooked or nutty-like to scorched or caramelized

Transmitted flavours

Cows are particularly bad for transmitting flavours through milk and milk is equally as susceptible to pick-up of off flavours in storage. Feed flavours and green grass can be problems so it is necessary to remove cows from feed 2-4 hrs before milking. Weeds, garlic/onion, and dandelions can tranfer flavours to the milk and even subsequent products such as butter. Barny flavours can be picked up in the milk if there is poor ventilation and the barn is not properly cleared and cows breathe the air. These flavours are volatile so can be driven off through vacuum de-aeration.

Characterization: hay/silage, cowy/barny

Microbial

There are many flavour defects of dairy products that may be caused by bacteria, yeasts, or moulds. In raw milk the high acid/sour flavour is caused by the growth of lactic acid bacteria which ferment lactose. It is less common today due to change in raw milk microflora. In both raw or processed milk,  fruity  flavours may arise due to psychrotrophs such as Pseudomonas fragi. Bitter or putrid flavours are caused by psychrotrophic bacteria which produce protease. It is the proteolytic action of protease that usually causes spoilage in milk. Malty flavours are caused by S.lactis var. maltigenes and is characterized by a corn flakes type flavour. Although more of a tactile defect, ropy milk is also caused by bacteria, specifically those which produce exopolysaccharides. 

Miscellaneous Defects

  • astringent
  • chalky
  • chemical/medicinal - disease - associated or adulteration
  • flat - adulteration (water)
  • foreign
  • salty - disease associated
  • bitter - adulteration

More information on off-flavours in milk can be found in Clarke et al. 

Milk flavour is graded on a score of one to 10. Some flavour defects, even if only slightly present, can decrease the score drastically. The following are suggested flavour scores for milk with designated intensities of flavour defects. 

 

Flavour Criticisms

 

Slight

Intensity of Defect

Definite

 

Pronounced

Astringent 8 7 5
Barny 7 5 3
Bitter 7 5 3
Cooked 9 8 6
Cowy 6 4 1
Feed 9 7 5
Flat 9 8 7
Foreign 5 3 0
Garlic/onion 5 3 1
High acid 3 1 0
Bacterial 5 3 0
Lacks Freshness 7 5 3
Malty 7 5 3
Oxidized 7 5 3
Rancid 7 5 3
Salty 8 6 4
Unclean 7 5 3

Dairy Chemistry and Physics

Composition and Structure: Overview

The role of milk in nature is to nourish and provide immunological protection for the mammalian young. Milk has been a food source for humans since prehistoric times; from human, goat, buffalo, sheep, yak, to the focus of this section - domesticated cow milk (genus Bos). Milk and honey are the only articles of diet whose sole function in nature is food. It is not surprising, therefore, that the nutritional value of milk is high. Milk is also a very complex food with over 100,000 different molecular species found. There are many factors that can affect milk composition such as breed variations (see introduction), cow to cow variations, herd to herd variations - including management and feed considerations, seasonal variations, and geographic variations. With all this in mind, only an approximate composition of milk can be given:

  • 87.3% water (range of 85.5% - 88.7%)
  • 3.9 % milkfat (range of 2.4% - 5.5%)
  • 8.8% solids-not-fat (range of 7.9 - 10.0%):
    • protein 3.25% (3/4 casein)
    • lactose 4.6%
    • minerals 0.65% - Ca, P, citrate, Mg, K, Na, Zn, Cl, Fe, Cu, sulfate, bicarbonate, many others
    • acids 0.18% - citrate, formate, acetate, lactate, oxalate
    • enzymes - peroxidase, catalase, phosphatase, lipase
    • gases - oxygen, nitrogen
    • vitamins - A, C, D, thiamine, riboflavin, others

The following terms are used to describe milk fractions:

  • Plasma = milk - fat (skim milk)
  • Serum = plasma - casein micelles (whey)
  • solids-not-fat (SNF) = proteins, lactose, minerals, acids, enzymes, vitamins
  • Total Milk Solids = fat + SNF

Not only is the composition important in determining the properties of milk, but the physical structure must also be examined. Due to its role in nature, milk is in a liquid form. This may seem curious if one takes into consideration the fact that milk has less water than most fruits and vegetables. Milk can be described as:

  • an oil-in-water emulsion with the fat globules dispersed in the continuous serum phase
  • colloid suspension of casein micelles, globular proteins and lipoprotein partilcles
  • solution of lactose, soluble proteins, minerals, vitamins other components.

Looking at milk under a microscope, at low magnification (5X) a uniform but turbid liquid is observed. At 500X magnification, spherical droplets of fat, known as fat globules, can be seen. At even higher magnification (50,000X), the casein micelles can be observed. The main structural components of milk, fat globules and casein micelles, will be examined in more detail later.

Milk Structure diagram

Milk Lipids - Chemical Properties

The fat content of milk is of economic importance because milk is sold on the basis of fat. Milk fatty acids originate either from microbial activity in the rumen, and transported to the secretory cells via the blood and lymph, or from synthesis in the secretory cells. The main milk lipids are a class called triglycerides which are comprised of a glycerol backbone binding up to three different fatty acids. The fatty acids are composed of a hydrocarbon chain and a carboxyl group. The major fatty acids found in milk are:

Long chain

  • C14 - myristic 11%
  • C16 - palmitic 26%
  • C18 - stearic 10%
  • C18:1 - oleic 20%

Short chain (11%)

  • C4 - butyric*
  • C6 - caproic
  • C8 - caprylic
  • C10 - capric

* butyric fatty acid is specific for milk fat of ruminant animals and is responsible for the rancid flavour when it is cleaved from glycerol by lipase action.

Saturated fatty acids (no double bonds), such as myristic, palmitic, and stearic make up two thirds of milk fatty acids. Oleic acid is the most abundant unsaturated fatty acid in milk with one double bond. While the cis form of geometric isomer is the most common found in nature, approximately 5% of all unsaturated bonds are in the trans position as a result of rumen hydrogenation.

Triglycerides account for 98.3% of milkfat. The distribution of fatty acids on the triglyceride chain, while there are hundreds of different combinations, is not random. The fatty acid pattern is important when determining the physical properties of the lipids. In general, the SN1 position binds mostly longer carbon length fatty acids, and the SN3 position binds mostly shorter carbon length and unsaturated fatty acids. For example:

  • C4 - 97% in SN3
  • C6 - 84% in SN3
  • C18 - 58% in SN1

The small amounts of mono- , diglycerides, and free fatty acids in fresh milk may be a product of early lipolysis or simply incomplete synthesis. Other classes of lipids include phospholipids (0.8%) which are mainly associated with the fat globule membrane, and cholesterol (0.3%) which is mostly located in the fat globule core.

Milk Lipids - Physical Properties

 The physical properties of milkfat can be summerized as follows:

  • density at 20° C is 915 kg m(-3)*
  • refractive index (589 nm) is 1.462 which decreases with increasing temperature
  • solubility of water in fat is 0.14% (w/w) at 20° C and increases with increasing temperature
  • thermal conductivity is about 0.17 J m(-1) s(-1) K(-1) at 20° C
  • specific heat at 40° C is about 2.1kJ kg(-1) K(-1)
  • electrical conductivity is <10(-12) ohm(-1) cm(-1)
  • dielectric constant is about 3.1

*the brackets around numbers denote superscript

At room temperature, the lipids are solid, therefore, are correctly referred to as "fat" as opposed to "oil" which is liquid at room temperature. The melting points of individual triglycerides ranges from -75° C for tributyric glycerol to 72° C for tristearin. However, the final melting point of milkfat is at 37° C because higher melting triglycerides dissolve in the liquid fat. This temperature is significant because 37° C is the body temperature of the cow and the milk would need to be liquid at this temperature. The melting curves of milkfat are complicated by the diverse lipid composition:

trans unsaturation increases melting points
odd-numbered and branched chains decrease melting points

Crystallization of milkfat largely determines the physical stability of the fat globule and the consistency of high-fat dairy products, but crystal behaviour is also complicated by the wide range of different triglycerides. There are four forms that milkfat crystals can occur in; alpha, ß , ß ' 1, and ß ' 2, however, the alpha form is the least stable and is rarely observed in slowly cooled fat.

Milkfat Structure - Fat Globules

More than 95% of the total milk lipid is in the form of a globule ranging in size from 0.1 to 15 um in diameter. These liquid fat droplets are covered by a thin membrane, 8 to 10 nm in thickness, whose properties are completely different from both milkfat and plasma. The native fat globule membrane (FGM) is comprised of apical plasma membrane of the secretory cell which continually envelopes the lipid droplets as they pass into the lumen. The major components of the native FGM, therefore, is protein and phospholipids. The phospholipids are involved in the oxidation of milk. There may be some rearrangement of the membrane after release into the lumen as amphiphilic substances from the plasma adsorb onto the fat globule and parts of the membrane dissolve into either the globule core or the serum. The FGM decreases the lipid-serum interface to very low values, 1 to 2.5 mN/m, preventing the globules from immediate flocculation and coalescence, as well as protecting them from enzymatic action.

It is well known that if raw milk or cream is left to stand, it will separate. Stokes' Law predicts that fat globules will cream due to the differences in densities between the fat and plasma phases of milk. However, in cold raw milk, creaming takes place faster than is predicted from this fact alone. IgM, an immunoglobulin in milk, forms a complex with lipoproteins. This complex, known as cryoglobulinprecipitates onto the fat globules and causes flocculation. This is known as cold agglutination. As fat globules cluster, the speed of rising increases and sweeps up the smaller globules with them. The cream layer forms very rapidly, within 20 to 30 min., in cold milk.

Homogenization of milk prevents this creaming by decreasing the diameter and size distribution of the fat globules, causing the speed of rise to be similar for the majority of globules. As well, homogenization causes the formation of a recombined membrane which is much similar in density to the continuous phase.

Recombined membranes are very different than native FGM. Processing steps such as homogenization, decreases the average diameter of fat globule and significantly increases the surface area. Some of the native FGM will remain adsorbed but there is no longer enough of it to cover all of the newly created surface area. Immediately after disruption of the fat globule, the surface tension raises to a high level of 15 mN/m and amphiphilic molecules in the plasma quickly adsorb to the lipid droplet to lower this value. The adsorbed layers consist mainly of serum proteins and casein micelles.

Homogenized MilkFat Globules

Fat Destabilization

While homogenization is the principal method for achieving stabilization of the fat emulsion in milk, fat destabilization is necessary for structure formation in butterwhipping cream and ice cream. Fat destabilization refers to the process of clustering and clumping (partial coalescence) of the fat globules which leads to the development of a continuous internal fat network or matrix structure in the product. Fat destabilization (sometimes "fat agglomeration") is a general term that describes the summation of several different phenomena. These include:

Coalescence:

an irreversible increase in the size of fat globules and a loss of identity of the coalescing globules;

Flocculation:

a reversible (with minor energy input) agglomeration/clustering of fat globules with no loss of identity of the globules in the floc; the fat globules that flocculate ; they can be easily redispersed if they are held together by weak forces, or they might be harder to redisperse to they share part of their interfacial layers;

Partial coalescence:

an irreversible agglomeration/clustering of fat globules, held together by a combination of fat crystals and liquid fat, and a retention of identity of individual globules as long as the crystal structure is maintained (i.e., temperature dependent, once the crystals melt, the cluster coalesces). They usually come together in a shear field, as in whipping, and it is envisioned that the crystals at the surface of the droplets are responsible for causing colliding globules to stick together, while the liquid fat partially flows between they and acts as the "cement". Partial coalescence dominates structure formation in whipped, aerated dairy emulsions, and it should be emphasized that crystals within the emulsion droplets are responsible for its occurrence.

Partial Coalescence of Fat Globules

A good reference for more information on fat globules can be found in Walstra or Fox and McSweeney, Lipids.

Milk Lipids - Functional Properties

Like all fats, milkfat provides lubrication. They impart a creamy mouth feel as opposed to a dry texture. Butter flavour is unique and is derived from low levels of short chain fatty acids. If too many short chain fatty acids are hydrolyzed (separated) from the triglycerides, however, the product will taste rancid. Butter fat also acts as a reservoir for other flavours, especially in aged cheese. Fat globules produce a 'shortening' effect in cheese by keeping the protein matrix extended to give a soft texture. Fat substitutes are designed to mimic the globular property of milk fat. The spreadable range of butter fat is 16-24° C. Unfortunately butter is not spreadable at refrigeration temperatures. Milk fat provides energy (1g = 9 cal.), and nutrients (essential fatty acids, fat soluble vitamins).

Milk Proteins

The primary structure of proteins consists of a polypeptide chain of amino acids residues joined together by peptide linkages, which may also be cross-linked by disulphide bridges. Amino acids contain both a weakly basic amino group, and a weakly acid carboxyl group both connected to a hydrocarbon chain, which is unique to different amino acids. The three-dimensional organization of proteins, orconformation, also involves secondary, tertiary, and quaternary structures. The secondary structure refers to the spatial arrangement of amino acid residues that are near one another in the linear sequence. The alpha-helix and ß -pleated sheat are examples of secondary structures arising from regular and periodic steric relationships. The tertiary structure refers to the spatial arrangement of amino acid residues that are far apart in the linear sequence, giving rise to further coiling and folding. If the protein is tightly coiled and folded into a somewhat spherical shape, it is called a globular protein. If the protein consists of long polypeptide chains that are intermolecularly linked, they are called fibrous proteins. Quaternary structure occurs when proteins with two or more polypeptide chain subunits are associated.

Milk Protein Fractionation

The nitrogen content of milk is distributed among caseins (75%), whey proteins (18%), miscellaneous proteins (2%) and non-protein nitrogen (5%). This nitrogen distribution can be determined by the Rowland fractionation method:

  1. Precipitation at pH 4.6 - separates caseins from whey nitrogen
  2. Precipitation with sodium acetate and acetic acid (pH 5.0) - separates total proteins from whey NPN

Ninety-five percent of the nitrogen is associated with protein. The average concentration of proteins in milk is as follows (although there can be considerable natural variation): 

  grams/ litre % of total protein
Total Protein 33.0 100
Total Caseins  26.0 78.8
alpha s1-casein 10.7 32.4
alpha s2-casein 2.8 8.5
beta-casein 8.6 26.1
kappa-casein 3.1 9.4
gamma-casein 0.8 2.4
Total Whey Proteins 6.4 19.4
alpha lactalbumin 1.2 3.6
beta lactoglobulin 3.2 9.8
BSA 0.4 1.2
Immunoglobulins 0.8 2.4
Proteose peptone 0.8 2.4
Miscellaneous 0.6 1.8

Caseins, as well as their structural form - casein micelles, whey proteins, and milk enzymes will now be examined in further detail. 

Caseins

The casein content of milk represents about 80% of milk proteins. The principal casein fractions are alpha(s1) and alpha(s2)-caseins, ß -casein, and kappa-casein. The distinguishing property of all caseins is their low solubility at pH 4.6. The common compositional factor is that caseins are conjugated proteins, most with phosphate group(s) esterified to serine residues. These phosphate groups are important to the structure of the casein micelle. Calcium binding by the individual caseins is proportional to the phosphate content.

The conformation of caseins is much like that of denatured globular proteins. The high number of proline residues in caseins causes particular bending of the protein chain and inhibits the formation of close-packed, ordered secondary structures. Caseins contain no disulfide bonds. As well, the lack of tertiary structure accounts for the stability of caseins against heat denaturation because there is very little structure to unfold. Without a tertiary structure there is considerable exposure of hydrophobic residues. This results in strong association reactions of the caseins and renders them insoluble in water.

Within the group of caseins, there are several distinguishing features based on their charge distribution and sensitivity to calcium precipitation:

alpha(s1)-casein: (molecular weight 23,000; 199 residues, 17 proline residues)

Two hydrophobic regions, containing all the proline residues, separated by a polar region, which contains all but one of eight phosphate groups. It can be precipitated at very low levels of calcium.

alpha(s2)-casein: (molecular weight 25,000; 207 residues, 10 prolines)

Concentrated negative charges near N-terminus and positive charges near C-terminus. It can also be precipitated at very low levels of calcium.

ß -casein: (molecular weight 24,000; 209 residues, 35 prolines)

Highly charged N-terminal region and a hydrophobic C-terminal region. Very amphiphilic protein acts like a detergent molecule. Self association is temperature dependant; will form a large polymer at 20° C but not at 4° C. Less sensitive to calcium precipitation.

kappa-casein: (molecular weight 19,000; 169 residues, 20 prolines)

Very resistant to calcium precipitation, stabilizing other caseins. Rennet cleavage at the Phe105-Met106 bond eliminates the stabilizing ability, leaving a hydrophobic portion, para-kappa-casein, and a hydrophilic portion called kappa-casein glycomacropeptide (GMP), or more accurately, caseinomacropeptide (CMP).

Structure: The Casein Micelle

Most, but not all, of the casein proteins exist in a colloidal particle known as the casein micelle. Its biological function is to carry large amounts of highly insoluble CaP to mammalian young in liquid form and to form a clot in the stomach for more efficient nutrition. Besides casein protein, calcium and phosphate, the micelle also contains citrate, minor ions, lipase and plasmin enzymes, and entrapped milk serum. These micelles are rather porous structures, occupying about 4 mL/g and 6-12% of the total volume fraction of milk. Size ranges from 50-250 nm in diameter.

Micelle image

Casein micelle image from Dalgleish, D. G., P. Spagnuolo and H. D. Goff. 2004. A possible structure of the casein micelle based on high-resolution field-emission scanning electron microscopy. International Dairy Journal. 14: 1025-1031. This micelle is 120 nm in diameter.

There have been many models developed over the years to explain the structure of the casein micelle, based on all of the information available about its composition and reactivity. The casein sub-micelle model was prominent for many years, but there is sufficient evidence now to conclude that there is not a defined sub-micellar structure to the micelle at all. More recent models suggest a more open structure comprised of aggregates of protein around calcium phosphate nanoclusters. Each of the casein proteins has unique abilitites to either bind with CaP or with other caseins, which gie rise to the aggregates. The nanoclusters provide regions of more or less density. The structure is sufficiently porous to hold a considrable amount of water, and for the surface, and even part of the interior, to be reactive to other substances. All models agree that the k-casein is mostly present as a stabilizng layer around the exterior of the micelle. Please see any of the following references for great detail about micelle structures and models.

 

Selected References

Holt, C. & D. S. Horne. 1996. The hairy casein micelle: evolution of the concept and its implication for dairy technology. Neth. Milk Dairy J. 50: 85-111.

Horne, D. S. 1998. Casein interactions: casting light on the black boxes, the structure in dairy products. Internat. Dairy J. 8: 171-177.

Walstra, P. 1999. Casein sub-micelles: do they exist? Internat. Dairy J. 9: 189-192.

Horne, D. S. 2002. Casein structure, self-assembly and gelation. Current Opinion in Colloid and Interface Sci. 7: 456-461.

Dalgleish, D. G. 2011. On the structural models of bovine casein micelles - review and possible improvements. Soft Matter. 7: 2265-2272. 

Dalgleish, Douglas G. and Milena Corredig. 2012. The Structure of the Casein Micelle of Milk and Its Changes During Processing. Annual Reviews Food Sci. Technol. 3:449–67.

de Kruif , Cornelis G., Thom Huppertz, Volker S. Urban and Andrei V. Petukhov. 2012. Casein micelles and their internal structure. Advances in Colloid and Interface Science 171–172: 36–52.

Casein Micelle Stability

Colloidal calcium phosphate (CCP) acts as a cement between the hundreds or even thousands of submicelles that form the casein micelle. Binding may be covalent or electrostatic. Submicelles rich in kappa-casein occupy a surface position, whereas those with less are buried in the interior. The resulting hairy layer, at least 7 nm thick, acts to prohibit further aggregation of submicelles by steric repulsion. The casein micelles are not static; there are three dynamic equilibria between the micelle and its surroundings:

  • the free casein molecules and submicelles
  • the free submicelles and micelles
  • the dissoved colloidal calcium and phosphate

The following factors must be considered when assessing the stability of the casein micelle:

Role of Ca++:

More than 90% of the calcium content of skim milk is associated in some way or another with the casein micelle. The removal of Ca++ leads to reversible dissociation of ß -casein without micellular disintegration. The addition of Ca++ leads to aggregation.

H Bonding:

Some occurs between the individual caseins in the micelle but not much because there is no secondary structure in casein proteins.

Disulphide Bonds:

alpha(s1) and ß-caseins do not have any cysteine residues. If any S-S bonds occur within the micelle, they are not the driving force for stabilization.

Hydrophobic Interactions:

Caseins are among the most hydrophobic proteins and there is some evidence to suggest they play a role in the stability of the micelle. It must be remembered that hydrophobic interactions are very temperature sensitive.

Electrostatic Interactions:

Some of the subunit interactions may be the result of ionic bonding, but the overall micellar structure is very loose and open.

van der Waals Forces:

No success in relating these forces to micellular stability.

Steric stabilization:

As already noted, the hairy layer interferes with interparticle approach.

There are several factors that will affect the stability of the casein micelle system:

Salt content:

affects the calcium activity in the serum and calcium phosphate content of the micelles.

pH:

lowering the pH leads to dissolution of calcium phosphate until, at the isoelectric point (pH 4.6), all phosphate is dissolved and the caseins precipitate.

Temperature:

at 4° C, beta-casein begins to dissociate from the micelle, at 0° C, there is no micellar aggregation; freezing produces a precipitate called cryo-casein.

Heat Treatment:

whey proteins become adsorbed, altering the behaviour of the micelle.

Dehydration:

by ethanol, for example, leads to aggregation of the micelles.

When two or more of these factors are applied together, the effect can also be additive.

Casein micelle aggregation

Although the casein micelle is fairly stable, there are four major ways in which aggregation can be induced:

1. Enzymatic - chymosin (rennet) or other proteolytic enzymes as in Cheese manufacturing. 

Chymosin, or rennet, is most often used for enzyme coagulation. During the primary stage, rennet cleaves the Phe(105)-Met(106) linkage of kappa-casein resulting in the formation of the soluble CMP which diffuses away from the micelle and para-kappa-casein, a distinctly hydrophobic peptide that remains on the micelle. The patch or reactive site, as illustrated in the  image below, that is left on the micelles after enzymatic cleavage is necessary before aggregation of the paracasein micelles can begin.

During the secondary stage, the micelles aggregate, as illustrated on the right below. This is due to the loss of steric repulsion of the kappa-casein as well as the loss of electrostatic repulsion due to the decrease in pH. As the pH approaches its isoelectric point (pH 4.6), the caseins aggregate. The casein micelles also have a strong tendency to aggregate because of hydrophobic interactions. Calcium assists coagulation by creating isoelctric conditions and by acting as a bridge between micelles. The temperature at the time of coagulation is very important to both the primary and secondary stages. With an increase in temperature up to 40° C, the rate of the rennet reaction increases. During the secondary stage, increased temperatures increase the hydrophobic reaction. The tertiary stage of coagulation involves the rearrangement of micelles after a gel has formed. There is a loss of paracasein identity as the milk curd firms and syneresis begins.

 

The Schmidt Model

2. AcidAcidification causes the casein micelles to destabilize or aggregate by decreasing their electric charge to that of the isoelectric point. At the same time, the acidity of the medium increases the solubility of minerals so that organic calcium and phosphorus contained in the micelle gradually become soluble in the aqueous phase. Casein micelles disintegrate and casein precipitates. Aggregation occurs as a result of entropically driven hydrophobic interactions.

3. Heat. Milk is generally very stable to heat up to 90-95oC. At temperatures above the boiling point casein micelles will irreversibly aggregate. On heating, the buffer capacity of milk salts change, carbon dioxide is released, organic acids are produced, and tricalcium phophate and casein phosphate may be precipitated with the release of hydrogen ions.

4. Age gelation. Age gelation is an aggregation phenomenon that affects shelf-stable, sterilized dairy products, such as concentrated milk and UHT milk products. After weeks to months storage of these products, there is a sudden sharp increase in viscosity accompanied by visible gelation and irreversible aggregation of the micelles into long chains forming a three-dimensional network. The actual cause and mechanism is not yet clear, however, some theories exist:

  1. Proteolytic breakdown of the casein: bacterial or native plasmin enzymes that are resistant to heat treatment may lead to the formation of a slow gel forming over a long period of time.
  2. Chemical reactions: polymerization of casein and whey proteins due to Maillard type or other chemical reactions
  3. Formation of kappa-casein-ß -lactoglobulin complexes

An excellent source of information on casein micelle stability can be found in Walstra et al., 2006.

Whey Proteins

 The proteins appearing in the supernatant of milk after precipitation at pH 4.6 are collectively called whey proteins. These globular proteins are more water soluble than caseins and are subject to heat denaturation. Native whey proteins have good gelling and whipping properties. Denaturation increases their water holding capacity. The principle fractions are ß -lactoglobulin, alpha-lactalbumin, bovine serum albumin (BSA), and immunoglobulins (Ig).

ß -Lactoglobulins: (MW - 18,000; 162 residues) This group, including eight genetic variants, comprises approximately half the total whey proteins. ß -Lactoglobulin has two internal disulfide bonds and one free thiol group. The conformation includes considerable secondary structure and exists naturally as a noncovalent linked dimer. At the isoelectric point (pH 3.5 to 5.2), the dimers are further associated to octamers but at pH below 3.4, they are dissociated to monomers.

alpha-Lactalbumins: (MW - 14,000; 123 residues) These proteins contain eight cysteine groups, all involved in internal disulfide bonds, and four tryptophan residues. alpha-Lactalbumin has a highly ordered secondary structure, and a compact, spherical tertiary structure. Thermal denaturation and pH <4.0 results in the release of bound calcium.

Enzymes

 Enzymes are a group of proteins that have the ability to catalyze chemical reactions and the speed of such reactions. The action of enzymes is very specific. Milk contains both indigenous and exogenous enzymes. Exogenous enzymes mainly consist of heat-stable enzymes produced by psychrotrophic bacteria: lipases, and proteinases. There are many indigenous enzymes that have been isolated from milk. The most significant group are the hydrolases:

  • lipoprotein lipase
  • plasmin
  • alkaline phosphatase

Lipoprotein lipase (LPL): A lipase enzyme splits fats into glycerol and free fatty acids. This enzyme is found mainly in the plasma in association with casein micelles. The milkfat is protected from its action by the FGM. If the FGM has been damaged, or if certain cofactors (blood serum lipoproteins) are present, the LPL is able to attack the lipoproteins of the FGM. Lipolysis may be caused in this way.

Plasmin: Plasmin is a proteolytic enzyme; it splits proteins. Plasmin attacks both ß -casein and alpha(s2)-casein. It is very heat stable and responsible for the development of bitterness in pasteurized milk and UHT processed milk. It may also play a role in the ripening and flavour development of certain cheeses, such as Swiss cheese.

Alkaline phosphatase: Phosphatase enzymes are able to split specific phosporic acid esters into phosphoric acid and the related alcohols. Unlike most milk enzymes, it has a pH and temperature optima differing from physiological values; pH of 9.8. The enzyme is destroyed by minimum pasteurization temperatures, therefore, a phosphatase test can be done to ensure proper pasteurization.

Lactose

 Lactose is a disaccharide (2 sugars) made up of glucose and galactose (which are both monosaccharides). Because of the anomeric carbon on the right side of the structure below, lactose can exist as two isomers, alpha, as shown, or beta, in which the hydroxyl on the anomeric carbon would point up on the ring structure shown below.

Structure showing the glucose and galactose of lactose

It comprises 4.8 to 5.2% of milk, 52% of milk SNF, and 70% of whey solids. It is not as sweet as sucrose. When lactose is hydrolyzed by ß -D-galactosidase (lactase), an enzyme that splits these monosaccharides, the result is increased sweetness, and depressed freezing point.

One of its most important functions is its utilization as a fermentation substrate. Lactic acid bacteria produce lactic acid from lactose, which is the beginning of many fermented dairy products. Because of their ability to metabolize lactose, they have a competitive advantage over many pathogenic and spoilage organisms.

Some people suffer from lactose intolerance; they lack the lactase enzyme, hence they cannot digest lactose, or dairy products containing lactose.

Alpha-lactose crystalsThe picture shown on the left of alpha-lactose crystals shows the typical "tomahawk" morphology of lactose crystals when they form in dairy products. Lactose crystallization results in the defect called sandiness, due to the presence of these sharp crystals on the tongue. Lactose is relatively insoluble, which is a problem in many dairy products, especially ice cream and sweetened condensed milk.

In addition to lactose, fresh milk contains other carbohydrates in small amounts, including glucose, galactose, and oligosaccharides.

Vitamins

Vitamins are organic substances essential for many life processes. Milk includes fat soluble vitamins A , D, E, and K. Vitamin A is derived from retinol and ß -carotene. Because milk is an important source of dietary vitamin A, fat reduced products which have lost vitamin A with the fat, are required to supplement the product with vitamin A.

Milk is also an important source of dietary water soluble vitamins:

  • B1 - thiamine
  • B2 - riboflavin
  • B6 - pyridoxine
  • B12 - cyanocobalamin
  • niacin
  • pantothenic acid

There is also a small amount of vitamin C (ascorbic acid) present in raw milk but it is an insignificant amount relative to human needs and is quite heat-labile: about 20% is destroyed by pasteurization.

The vitamin content of fresh milk is given below: 

Vitamin Contents per litre
A (ug RE) 400
D (IU) 40
E (ug) 1000
K (ug) 50
B1 (ug) 450
B2 (ug) 1750
Niacin (ug) 900
B6 (ug) 500
Pantothenic acid (ug) 3500
Biotin (ug) 35
Folic acid (ug) 55
B12 (ug) 4.5
C (mg) 20

Minerals

 All 22 minerals considered to be essential to the human diet are present in milk. These include three families of salts:

  1. Sodium (Na), Potassium (K) and Chloride (Cl):These free ions are negatively correlated to lactose to maintain osmotic equilibrium of milk with blood.
  2. Calcium (Ca), Magnesium (Mg), Inorganic Phosphorous (P(i)), and Citrate: This group consists of 2/3 of the Ca, 1/3 of the Mg, 1/2 of the P(i), and less than 1/10 of the citrate in colloidal(nondiffusible) form and present in the casein micelle.
  3. Diffusible salts of Ca, Mg, citrate, and phosphate: These salts are very pH dependent and contribute to the overall acid-base equilibrium of milk.

The mineral content of fresh milk is given below. A wide range is shown, based on variability due to intrinsic (lactogenesis in the secretory cell of individual animals) and extrinsic factors (mainly feed and season/climate adaptation, but also contaminants). 

Mineral Content per litre
Sodium (mg) 250-640
Potassium (mg) 1100-1500
Chloride (mg) 800-1200
Calcium (mg) 1100-1300
Magnesium (mg) 70-140
Phosphorus (mg) 800-1000
Iron (ug) 100-700
Zinc (ug) 2500-7000
Copper (ug) 100-350
Manganese (ug) 10-50
Iodine (ug) 50-600
Fluoride (ug) 20-80
Selenium (ug) 20-40
Cobalt (ug) 0.5-1.3
Chromium (ug) 0.5-20
Molybdenum (ug) 20-100
Nickel (ug) 0-50
Arsenic (ug) 20-60
Aluminum (ug) 50-1600
Lead (ug) tr-20

This information is consolidated from C. D. Hunt and F. H. Nielsen, Chap. 10.1 in McSweeney, P.L.H. and P.F. Fox, 2009. Advanced Dairy Chemistry, Vol. 3, Lactose, water, salts and minor constituents, 3rd edn., Springer, pp. 392-8; and from, Cristina Sola-Larranaga; Inigo Navarro-Blasco. 2009. Chemometric analysis of minerals and trace elements in raw cow milk from the community of Navarra, Spain . Food Chemistry, 112 (1), 189-196.

Physical Properties of Milk

We will cover the following physical properties. More information can be found in Walstra's text.

  • Density
  • Viscosity
  • Freezing Point
  • Acid-base Equilibria
  • Optical Properties

Density

The density of milk and milk products is used for the following;

  • to convert volume into mass and vice versa
  • to estimate the solids content
  • to calculate other physical properties (e.g. kinematic viscosity)

Density, the mass of a certain quantity of material divided by its volume, is dependant on the following:

  • temperature at the time of measurement
  • temperature history of the material
  • composition of the material (especially the fat content)
  • inclusion of air (a complication with more viscous products)

With all of this in mind, the density of milk varies within the range of 1027 to 1033 kg /m3 at 20° C.

The following table gives the density of various fluid dairy products as a function of fat and solids-not-fat (SNF) composition: 

  Product Composition Density (kg/L) at:
Product Fat (%) SNF (%) 4.4oC 10oC 20oC 38.9oC
Producer milk 4.00 8.95 1.035 1.033 1.030 1.023
Homogenized milk 3.6 8.6 1.033 1.032 1.029 1.022
Skim milk, pkg 0.02 8.9 1.036 1.035 1.033 1.026
Fortified skim 0.02 10.15 1.041 1.040 1.038 1.031
Half and half 12.25 7.75 1.027 1.025 1.020 1.010
Half and half, fort. 11.30 8.9 1.031 1.030 1.024 1.014
Light cream 20.00 7.2 1.021 1.018 1.012 1.000
Heavy cream 36.60 5.55 1.008 1.005 0.994 0.978

Viscosity

Viscosity of milk and milk products is important in determining the following:

  • the rate of creaming
  • rates of mass and heat transfer
  • the flow conditions in dairy processes

Milk and skim milk, excepting cooled raw milk, exhibit Newtonian behavior, in which the viscosity is independent of the rate of shear. The viscosity of these products depends on the following:

  • Temperature:
    • cooler temperatures increase viscosity due to the increased voluminosity of casein micelles
    • temperatures above 65° C increase viscosity due to the denaturation of whey proteins
  • pH: an increase or decrease in pH of milk also causes an increase in casein micelle voluminosity

Cooled raw milk and cream exhibit non-Newtonian behavior in which the viscosity is dependant on the shear rate. Agitation may cause partial coalescence of the fat globules (partial churning) which increases viscocity. Fat globules that have under gone cold agglutination, may be dispersed due to agitation, causing a decrease in viscosity.

Freezing Point

Freezing point depression is a colligative property which is determined by the molarity of solutes rather than by the percentage by weight or volume. In the dairy industry, freezing point of milk is mainly used to determine added water but it can also been used to determine lactose content in milk, estimate whey powder contents in skim milk powder, and to determine water activity of cheese. The freezing point of milk is usually in the range of -0.512 to -0.550° C with an average of about -0.522° C.

Correct interpretation of freezing point data with respect to added water depends on a good understanding of the factors affecting freezing point depression. With respect to interpretation of freezing points for added water determination, the most significant variables are the nutritional status of the herd and the access to water. Under feeding causes increased freezing points. Large temporary increases in freezing point occur after consumption of large amounts of water because milk is iso-osmotic with blood. The primary sources of non-intentional added water in milk are residual rinse water and condensation in the milking system.

Acid-base equilibria

 Both titratable acidity and pH are used to measure milk acidity. The pH of milk at 25° C normally varies within a relatively narrow range of 6.5 to 6.7. The normal range for titratable acidity of herd milks is 13 to 20 mmol/L. Because of the large inherent variation, the measure of titratable acidity has little practical value except to measure changes in acidity (eg., during lactic fermentation) and even for this purpose, pH is a better measurement. 

There are many components in milk which provide a buffering action. The major buffering groups of milk are caseins and phosphate.

Optical properties

 Optical properties provide the basis for many rapid, indirect methods of analysis such as proximate analysis by infrared absorbency or light scattering. Optical properties also determine the appearance of milk and milk products. Light scattering by fat globules and casein micelles causes milk to appear turbid and opaque. Light scattering occurs when the wave length of light is near the same magnitude as the particle. Thus, smaller particles scatter light of shorter wavelengths. Skim milk appears slightly blue because casein micelles scatter the shorter wavelengths of visible light (blue) more than the red. The carotenoid precursor of vitamin A, ß -carotene, contained in milk fat, is responsible for the 'creamy' colour of milk. Riboflavin imparts a greenish colour to whey.

Refractive index (RI) is normally determined at 20° C with the D line of the sodium spectrum. The refractive index of milk is 1.3440 to 1.3485 and can be used to estimate total solids.

Dairy Microbiology

Basic Microbiology

Microorganisms

Microorganisms are living organisms that are individually too small to see with the naked eye. The unit of measurement used for microorganisms is the micrometer (µ m); 1 µ m = 0.001 millimeter; 1 nanometer (nm) = 0.001 µ m. Microorganisms are found everywhere (ubiquitous) and are essential to many of our planets life processes. With regards to the food industry, they can cause spoilage, prevent spoilage through fermentation, or can be the cause of human illness.

Scale showing the size of an animal cell, animal nucleus, yeast cell, virus, bacteria cell (rod) and bacteria cell (coccus)

There are several classes of microorganisms, of which bacteria and fungi (yeasts and moulds) will be discussed in some detail. Another type of microorganism, the bacterial viruses or bacteriophage, will be examined in a later section.

Bacteria

Bacteria are relatively simple single-celled organisms. One method of classification is by shape or morphology:

  • Cocci:
    • spherical shape
    • 0.4 - 1.5 µ m

Examples: staphylococci - form grape-like clusters; streptococci - form bead-like chains. 

  • Rods:
    • 0.25 - 1.0 µ m width by 0.5 - 6.0 µ m long

Examples: bacilli - straight rod; spirilla - spiral rod

There exists a bacterial system of taxonomy, or classification system, that is internationally recognized with family, genera and species divisions based on genetics.

Some bacteria have the ability to form resting cells known as endospores. The spore forms in times of environmental stress, such as lack of nutrients and moisture needed for growth, and thus is a survival strategy. Spores have no metabolism and can withstand adverse conditions such as heat, disinfectants, and ultraviolet light. When the environment becomes favourable, the spore germinates and giving rise to a single vegetative bacterial cell. Some examples of spore-formers important to the food industry are members of Bacillus and Clostridium generas.

Bacteria reproduce asexually by fission or simple division of the cell and its contents. The doubling time, or generation time, can be as short as 20-20 min. Since each cell grows and divides at the same rate as the parent cell, this could under favourable conditions translate to an increase from one to 10 million cells in 11 hours! However, bacterial growth in reality is limited by lack of nutrients, accumulation of toxins and metabolic wastes, unfavourable temperatures and desiccation. The maximum number of bacteria is approximately 1 X 10e9 CFU/g or ml.

Note: Bacterial populations are expressed as colony forming units (CFU) per gram or millilitre.

Hypothetical bacterial growth curveBacterial growth generally proceeds through a series of phases:

  • Lag phase: time for microorganisms to become accustomed to their new environment. There is little or no growth during this phase.
  • Log phase: bacteria logarithmic, or exponential, growth begins; the rate of multiplication is the most rapid and constant.
  • Stationary phase: the rate of multiplication slows down due to lack of nutrients and build-up of toxins. At the same time, bacteria are constantly dying so the numbers actually remain constant.
  • Death phase: cell numbers decrease as growth stops and existing cells die off.

The shape of the curve (shown on the right) varies with temperature, nutrient supply, and other growth factors. This exponential death curve is also used in modeling the heating destruction of microorganisms.

Yeasts

Yeasts are members of a higher group of microorganisms called fungi . They are single-cell organisms of spherical, elliptical or cylindrical shape. Their size varies greatly but are generally larger than bacterial cells. Yeasts may be divided into two groups according to their method of reproduction:

  1. budding: called Fungi Imperfecti or false yeasts
  2. budding and spore formation: called Ascomycetes or true yeasts

Unlike bacterial spores, yeast form spores as a method of reproduction.

Moulds

 Moulds are filamentous, multi-celled fungi with an average size larger than both bacteria and yeasts (10 X 40 µ m). Each filament is referred to as a hypha. The mass of hyphae that can quickly spread over a food substrate is called the mycelium. Moulds may reproduce either asexually or sexually, sometimes both within the same species.

Asexual Reproduction:

  • fragmentation - hyphae separate into individual cells called arthropsores
  • spore production - formed in the tip of a fruiting hyphae, called conidia, or in swollen structures called sporangium

Sexual Reproduction: sexual spores are produced by nuclear fission in times of unfavourable conditions to ensure survival.

Microbial Growth

 Hypothetical bacterial growth curveThere are a number of factors that affect the survival and growth of microorganisms in food. The parameters that are inherent to the food, or intrinsic factors, include the following:

  • nutrient content
  • moisture content
  • pH
  • available oxygen
  • biological structures
  • antimicrobial constituents

 

 

 

 

 

 

 

Nutrient Requirements

 While the nutrient requirements are quite organism specific, the microorganisms of importance in foods require the following:

  • water
  • energy source
  • carbon/nitrogen source
  • vitamins
  • minerals

Milk and dairy products are generally very rich in nutrients which provides an ideal growth environment for many microorganisms.

Moisture Content

 All microorganisms require water but the amount necessary for growth varies between species. The amount of water that is available in food is expressed in terms of water activity (aw), where the aw of pure water is 1.0. Each microorganism has a maximum, optimum, and minimum aw for growth and survival. Generally bacteria dominate in foods with high aw (minimum approximately 0.90 aw) while yeasts and moulds, which require less moisture, dominate in low aw foods ( minimum 0.70 aw). The water activity of fluid milk is approximately 0.98 aw.

pH

Most microorganisms have approximately a neutral pH optimum (pH 6-7.5). Yeasts are able to grow in a more acid environment compared to bacteria. Moulds can grow over a wide pH range but prefer only slightly acid conditions. Milk has a pH of 6.6 which is ideal for the growth of many microoorganisms.

Available Oxygen

Microorganisms can be classified according to their oxygen requirements necessary for growth and survival:

  • Obligate Aerobes: oxygen required
  • Facultative: grow in the presence or absence of oxygen
  • Microaerophilic: grow best at very low levels of oxygen
  • Aerotolerant Anaerobes: oxygen not required for growth but not harmful if present
  • Obligate Anaerobes: grow only in complete absence of oxygen; if present it can be lethal

Biological Structures

Physical barriers such as skin, rinds, feathers, etc. have provided protection to plants and animals against the invasion of microorganisms. Milk, however, is a fluid product with no barriers to the spreading of microorganisms throughout the product.

Antimicrobial Constituents

As part of the natural protection against microorganisms, many foods have antimicrobial factors. Milk has several nonimmunological proteins which inhibit the growth and metabolism of many microorganisms including the following most common:

  1. lactoperoxidase
  2. lactoferrin
  3. lysozyme
  4. xanthine

More information on these antimicrobials can be found in the dairy microbiology textbook by Marth and Steele. See also the discussion on lactoperoxidase in this series at https://www.uoguelph.ca/foodscience/book-page/effects-milk-handling-quality-and-hygiene

Where the intrinsic factors are related to the food properties, the extrinsic factors are related to the storage environment. These would include temperature, relative humidity, and gases that surround the food.

Temperature

As a group, microorganisms are capable of growth over an extremely wide temperature range. However, in any particular environment, the types and numbers of microorganisms will depend greatly on the temperature. According to temperature, microorganisms can be placed into one of three broad groups:

  • Psychrotrophs: optimum growth temperatures 20 to 30° capable of growth at temperatures less than 7° C. Psychrotrophic organisms are specifically important in the spoilage of refrigerated dairy products.
  • Mesophiles: optimum growth temperatures 30 to 40° C; do not grow at refrigeration temperatures
  • Thermophiles: optimum growth between 55 and 65° C

It is important to note that for each group, the growth rate increases as the temperature increases only up to an optimum, afterwhich it rapidly declines.

Detection and Enumeration of Microorganisms

There are several methods for detection and enumeration of microorganisms in food. The method that is used depends on the purpose of the testing.

Direct Enumeration:

Using direct microscopic counts (DMC), Coulter counter etc. allows a rapid estimation of all viable and nonviable cells. Identification through staining and observation of morphology also possible with DMC.

Viable Enumeration:

The use of standard plate counts, most probable number (MPN), membrane filtration, plate loop methos, spiral plating etc., allows the estimation of only viable cells. As with direct enumeration, these methods can be used in the food industry to enumerate fermentation, spoilage, pathogenic, and indicator organisms.

Metabolic Activity Measurement:

An estimation of metabolic activity of the total cell population is possible using dye reduction tests such as resazurin or methylene blue dye reduction (see below), acid production, electrical impedance etc. The level of bacterial activity can be used to assess the keeping quality and freshness of milk. Toxin levels can also be measured, indicating the presence of toxin producing pathogens.

Cellular Constituents Measurement:

Using the luciferase test to measure ATP is one example of the rapid and sensitive tests available that will indicate the presence of even one pathogenic bacterial cell.

Isolation of microorganisms is an important preliminary step in the identification of most food spoilage and pathogenic organisms. This can be done using a simple streak plate method.

Dye Reduction Tests: Methylene Blue and Resazurin

From: Atherton, H. V. and Newlander, J. A. 1977. Chemistry and Testing of Dairy Products. 4th Edn. AVI, Westport, CT.

Methylene Blue Reduction Test

The methylene blue reduction test is based on the fact that the color imparted to milk by the addition of a dye such as methylene blue will disappear more or less quickly. The removal of the oxygen from milk and the formation of reducing substances during bacterial metabolism causes the color to disappear. The agencies responsible for the oxygen consumption are the bacteria. Though certain species of bacteria have considerably more influence than others, it is generally assumed that the greater the number of bacteria in milk, the quicker will the oxygen be consumed, and in turn the sooner will the color disappear. Thus, the time of reduction is taken as a measure of the number of organisms in milk although actually it is likely that it is more truly a measure of the total metabolic reactions proceeding at the cell surface of the bacteria.

The methylene blue reduction test has lost much of its popularity because of its low correlation with other bacterial procedures. This is true particularly in those samples which show extensive multiplication of the psychrotropic species.

Apparatus.–The necessary equipment consists of test tubes with rubber stoppers, a pipette or dipper graduated to deliver 10 ml of milk and a water bath for maintaining the samples at 35o to 37oC. The bath should contain a volume of water sufficient to heat the samples to 35o C within 10 minutes after the tubes enter the water and should have some means of protecting the samples from light during the incubation period. If a hot-air chamber is used, the samples should be heated to 35o C in a water bath since warm air would heat the milk too slowly.

The dry tablets contain methylene blue thiocyanate and may be obtained from any of the usual laboratory supply houses. They should be certified by the Commission on Standardization of Biological Stains. The solution is prepared by autoclaving or momentarily boiling 200 ml of distilled water in a light resistant (amber) stoppered flask and then adding one methylene blue tablet to the flask of hot water. The tablet should be completely dissolved before the solution is cooled. The solution may be stored in the stoppered, amber flask or an amber bottle in the dark. Fresh solution should be prepared weekly.

Procedure in Testing.–The following procedures are recommended.

  1. Sterilize all glassware and rubber stoppers either in an autoclave or in boiling water. Be sure all glassware is chemically clean.
  2. Measure 1 ml of the methylene blue thiocyanate solution into a test tube.
  3. Add 10 ml of milk and stopper.
  4. Tubes may be placed in the water bath immediately or may be stored in the refrigerator at 0o to 4o C for a more convenient time of incubation. When ready to perform the test, the temperature of the samples should be brought to 35o C within 10 minutes.
  5. When temperature reaches 36o C, slowly invert tubes a few times to assure uniform creaming. Do not shake tubes. Record this time as the beginning of the incubation period. Cover to keep out light.
  6. Check samples for decolorization after 30 minutes of incubation. Make subsequent readings at hourly intervals thereafter.
  7. After each reading, remove decolorized tubes and then slowly make one complete inversion of remaining tubes.
  8. Record reduction time in whole hours between last inversion and decolorization. For example, if the sample were still blue after L 5 hours but was decolorized (white) at the 2.5 hour reading, the methylene blue reduction time would be recorded as 2 hours. Decolorization is considered complete when four-fifths of the color has disappeared.

Classification.–The suggested classification is listed.

Class 1. Excellent, not decolorized in 8 hours.

Class 2. Good, decolorized in less than 8 hours but not less than 6 hours.

Class 3. Fair, decolorized in less than 6 hours but not less than 2 hours.

Class 4. Poor, decolorized in less than 2 hours.

Factors Affecting the Test.–Many factors affect the methylene blue reduction test and therefore the steps of operation should be uniform.

Since the oxygen content must be used up before the color disappears, any manipulation that increases the oxygen affects the test. Cold milk holds more oxygen than warm milk; pouring milk back and forth from one container to another increases the amount, and at milking time much oxygen may be absorbed.

The kind of organisms affect the rate of reduction. The coliforms appear to be the most rapidly reducing organisms, closely followed by Streptococcus lactis, some of the faecal Streptococci, and certain micrococci. Thermoduric and psychrotrophic bacteria reduce methylene blue very slowly if at all. A large number of leucocytes affect the reduction time materially.

Light hastens reduction and therefore the tests should be kept covered. The concentration of the dye should be uniform as an increased concentration lengthens the time of reduction. Increasing the incubation temperature augments the activity of the bacteria and therefore shortens the reduction time.

The creaming of the test samples causes a number of organisms to be removed from the body of the milk and brought to the surface with the rising fat. This factor causes variations in the reduction time, since the bacteria are not evenly distributed. The accuracy of the test i s increased, reduction time shortened and decolorization more uniform if the samples are periodically inverted during incubation.

The Resazurin Test

The resazurin test is conducted similar to the methylene blue reduction test with the judgement of quality based either on the color produced after a stated period of incubation or on the time required to reduce the dye to a given end-point. Numerous modifications have been proposed. The two most commonly used are the "one-hour test" and the "triple-reading test" taken after one, two, and three hours of incubation. Other modifications have value in specific applications.

The procedure for making the resazurin test is as follows: Prepare resazurin solution by dissolving one resazurin tablet (dye content/ tablet, approximately 11 mg, certified by Biological Stain Commission) in 200 ml of hot distilled water as was done in the methylene blue test. Place one ml of dye solution in a sterile test tube, then add 10 ml of sample. Stopper the tube, place in the incubator and, when the temperature reaches 36o C, invert to mix the milk and dye. Incubate at 36o C. Tubes are examined and classified at the end of an hour in the "one-hour test" or at the end of three successive hourly intervals in the "triplereading test." The following relationships of color and quality are generally accepted:

Color of Sample: Quality of Milk

  1. Blue (no color change): Excellent
  2. Blue to deep mauve: Good
  3. Deep mauve to deep pink: Fair
  4. Deep pink to whitish pink: Poor
  5. White: Bad

The resazurin test may be a valuable time saving tool if properly conducted and intelligently interpreted, but should be supplemented by microscopic examination.

Results on the reliability of the resazurin tests are conflicting. One study in comparing the resazurin test with the Breed microscopic method on 235 samples found the test reliable. Other reports state that the resazurin test is an unreliable index of bacteriological quality in milk. A major criticism of the method is that the resazurin reduction time of refrigerated bottled milk at either 20o or 37o C is much too long to be of any value in evaluating bacteriological spoilage of stored milk.

Standard Methods notes that under no circumstances should results of either methylene blue or resazurin tests be reported in terms of bacterial numbers. The two dye reduction procedures are described in more detail in Chapter 15 of the Thirteenth Edition of Standard Methods compiled by the American Public Health Association.

Microorganisms in Milk

 Milk is sterile at secretion in the udder but is contaminated by bacteria even before it leaves the udder. Except in the case of mastisis, the bacteria at this point are harmless and few in number. Further infection of the milk by microorganisms can take place during milking, handling, storage, and other pre-processing activities.

Lactic acid bacteria: 

this group of bacteria are able to ferment lactose to lactic acid. They are normally present in the milk and are also used as starter cultures in the production of cultured dairy products such as yogurt. Note: many lactic acid bacteria have recently been reclassified; the older names will appear in brackets as you will still find the older names used for convenience sake in a lot of literature. Some examples in milk are:

  • lactococci
    • L. delbrueckii subsp. lactis (Streptococcus lactis )
    • Lactococcus lactis subsp. cremoris (Streptococcus cremoris )
  • lactobacilli
    • Lactobacillus casei
    • L.delbrueckii subsp. lactis (L. lactis )
    • L. delbrueckii subsp. bulgaricus

(Lactobacillus bulgaricus)

  • Leuconostoc

Coliforms: 

coliforms are facultative anaerobes with an optimum growth at 37° C. Coliforms are indicator organisms; they are closely associated with the presence of pathogens but not necessarily pathogenic themselves. They also can cause rapid spoilage of milk because they are able to ferment lactose with the production of acid and gas, and are able to degrade milk proteins. They are killed by HTST treatment, therefore, their presence after treatment is indicative of contamination.Escherichia coli is an example belonging to this group.

Significance of microorganisms in milk:

  • Information on the microbial content of milk can be used to judge its sanitary quality and the conditions of production
  • If permitted to multiply, bacteria in milk can cause spoilage of the product
  • Milk is potentially susceptible to contamination with pathogenic microorganisms. Precautions must be taken to minimize this possibility and to destroy pathogens that may gain entrance
  • Certain microorganisms produce chemical changes that are desirable in the production of dairy products such as cheese, yogurt.

Spoilage Microorganisms in Milk

 The microbial quality of raw milk is crucial for the production of quality dairy foods. Spoilage is a term used to describe the deterioration of a foods' texture, colour, odour or flavour to the point where it is unappetizing or unsuitable for human consumption. Microbial spoilage of food often involves the degradation of protein, carbohydrates, and fats by the microorganisms or their enzymes.

In milk, the microorganisms that are principally involved in spoilage are psychrotrophic organisms. Most psychrotrophs are destroyed by pasteurization temperatures, however, some like Pseudomonas fluorescens, Pseudomonas fragi can produce proteolytic and lipolytic extracellular enzymes which are heat stable and capable of causing spoilage.

Some species and strains of Bacillus, Clostridium, Cornebacterium, Arthrobacter, Lactobacillus, Microbacterium, Micrococcus, and Streptococcus can survive pasteurization and grow at refrigeration temperatures which can cause spoilage problems.

Pathogenic Microorganisms in Milk

Hygienic milk production practices, proper handling and storage of milk, and mandatory pasteurization has decreased the threat of milkborne diseases such as tuberculosis, brucellosis, and typhoid fever. There have been a number of foodborne illnesses resulting from the ingestion of raw milk, or dairy products made with milk that was not properly pasteurized or was poorly handled causing post-processing contamination. The following bacterial pathogens are still of concern today in raw milk and other dairy products:

  • Bacillus cereus
  • Listeria monocytogenes
  • Yersinia enterocolitica
  • Salmonella spp.
  • Escherichia coli O157:H7
  • Campylobacter jejuni

It should also be noted that moulds, mainly of species of Aspergillus, Fusarium, and Penicillium can grow in milk and dairy products. If the conditions permit, these moulds may produce mycotoxins which can be a health hazard.

HACCP

 Raw and end-products may be tested for the presence, level, or absence of microorganisms. Traditionally these practices were used to reduce manufacturing defects in dairy products and ensure compliance with specifications and regulations, however, they have many drawbacks:

  1. destructive and time consuming
  2. slow response
  3. small sample size
  4. delays in the release of the food

In the 1960's, the Pillsbury Company, the U.S. Army, and NASA introduced a system for assuring pathogen-free foods for the space program. This system, called Hazard Analysis and Critical Control Points (HACCP), is a focus on critical food safety areas as part of total quality programs. It involves a critical examination of the entire food manufacturing process to determine every step where there is a possibility of physical, chemical, or microbiological contamination of the food which would render it unsafe or unacceptable for human consumption. These identified points are the critical control points (CCP). There are seven prinicples to HACCP:

  1. analyze hazards
  2. determine CCPs
  3. establish critical limits
  4. establish monitoring procedures
  5. establish deviation procedures
  6. establish verification procedures
  7. establish record keeping procedures

Before these principles can be put into place, a prerequisite program and preliminary setup is necessary.

Prerequisite Program:

  • premise control
  • receiving and storage control
  • equipment performance and maintenance control
  • personnel training
  • sanitation
  • recall procedure

Preliminary Setup:

  • assemble team
  • describe the product
  • identify intended use
  • construct flow diagram and plant schematic
  • verify the diagram on-site

Food Safety Enhancement Program-FSEP is The Canadian Food Inspection Agency's HACCP initiative (see -> food -> safe food production systems). There is extensive information at their Web site regarding FSEP, including implementation manuals, HACCP curriculum guidelines, and generic models.

Starter Cultures

 Starter cultures are those microorganisms that are used in the production of cultured dairy products such as yogurt and cheese. The natural microflora of the milk is either inefficient, uncontrollable, and unpredictable, or is destroyed altogether by the heat treatments given to the milk. A starter culture can provide particular characteristics in a more controlled and predictable fermentation. The primary function of lactic starters is the production of lactic acid from lactose. Other functions of starter cultures may include the following:

  • flavour, aroma, and alcohol production
  • proteolytic and lipolytic activities
  • inhibition of undesirable organisms

There are two groups of lactic starter cultures:

  1. simple or defined: single strain, or more than one in which the number is known
  2. mixed or compound: more than one strain each providing its own specific characteristics

Starter cultures may be categorized as mesophilic, for example:

  • Lactococcus lactis subsp. cremoris
  • L. delbrueckii subsp. lactis
  • L. lactis subsp. lactis biovar diacetylactis
  • Leuconostoc mesenteroides subsp. cremoris 

or thermophilic:

  • Streptococcus salivarius subsp. thermophilus (S.thermophilus)
  • Lactobacillus delbrueckii subsp. bulgaricus
  • L. delbrueckii subsp. lactis
  • L. casei
  • L. helveticus
  • L. plantarum

Mixtures of mesophilic and thermophilic microorganisms can also be used as in the production of some cheeses.

Please see further details in our cheese technology section.

Bacteriophage

Bacteriophages are viruses that require bacteria host cells for growth and reproduction. Initially, the bacteriophage attaches itself to the bacteria cell wall and injects nuclear substance into the cell. Inside the cell, the nuclear substance produces shells, or phage coats, for the new bacteriophage which are quickly filled with nucleic acid. The bacterial cell ruptures and dies as the new bacteriophage are released. 

Bacteriophages are ubiquitous but generally enter the milk processing plant with the farm milk. They can be inactivated heat treatments of 30 min at 63 to 88° C, or by the use of chemical disinfectants.

Bacteriophages are of most concern in cheese making. They attack and destroy most of the lactic acid bacteria which prevents normal ripening known as slow or dead vat.

Starter Culture Preparation

Commercial manufacturers provide starter cultures in lyophilized (freeze-dryed), frozen or spray-dried forms. The dairy product manufacturers need to inoculate the culture into milk or other suitable substrate. There are a number of steps necessary for the propagation of starter culture ready for production:

  1. Commercial culture
  2. Mother culture - first inoculation; all cultures will originate from this preparation
  3. Intermediate culture - in preparation of larger volumes of prepared starter
  4. Bulk starter culture - this stage is used in dairy product production

Please see further details in our cheese technology section.

Dairy Processing

Clarification and Cream Separation

Centrifugation

Centrifugal separation is a process used quite often in the dairy industry. Some uses include:

  • clarification (removal of solid impurities from milk prior to pasteurization)
  • skimming (separation of cream from skim milk)
  • standardizing
  • whey separation (separation of whey cream (fat) from whey)
  • bactofuge treatment (separation of bacteria from milk)
  • quark separation (separation of quarg curd from whey)
  • butter oil purification (separation of serum phase from anhydrous milk fat)

Principles of Centrifugation

Centrifugation is based on Stoke's Law. The particle sedimentation velocity increases with:

  • increasing diameter
  • increasing difference in density between the two phases
  • decreasing viscosity of the continuous phase

If raw milk were allowed to stand, the fat globules would begin to rise to the surface in a phenomena called creaming. Raw milk in a rotating container also has centrifugal forces acting on it. This allows rapid separation of milk fat from the skim milk portion and removal of solid impurities from the milk.

Clarification

 Separation and clarification can be done at the same time in one centrifuge. Particles, which are more dense than the continuous milk phase, are thrown back to the perimeter. The solids that collect in the centrifuge consist of dirt, epithelial cells, leucocytes, corpuscles, bacteria sediment and sludge. The amount of solids that collect will vary, however, it must be removed from the centrifuge.

More modern centrifuges are self-cleaning allowing a continuous separation/clarification process. This type of centrifuge consists of a specially constructed bowl with peripheral discharge slots. These slots are kept closed under pressure. With a momentary release of pressure, for about 0.15 s, the contents of sediment space are evacuated. This can mean anywhere from 8 to 25 L are ejected at intervals of 60 min. For one dairy, self-cleaning translated to a loss of 50 L/hr of milk.

The following image is a schematic of both a clarifier and a separator.

Diagram of Clarification and Separation

Separation

Centrifuges can be used to separate the cream from the skim milk. The centrifuge consists of up to 120 discs stacked together at a 45 to 60 degree angle and separated by a 0.4 to 2.0 mm gap or separation channel. Milk is introduced towards the inner edge of the disc stack. The stack of discs has vertically aligned distribution holes into which the milk is introduced.

Under the influence of centrifugal force the fat globules, which are less dense than the skim milk, move inwards through the separation channels toward the axis of rotation. Some skim is needed to carry the fat globules out of the separation, and the combination of fat globules in a much-reduced volume of skim milk is called "cream", i.e., creamis skim milk enriched in fat globules. The skim milk, now devoid of fat globules, will move outwards and leaves through a separate outlet.

A centrifuge

Standardization

Diagram explaining the automatic standardization process

The streams of skim and cream after separation must be recombined to a specified fat content. This can be done by adjusting the throttling valve of the cream outlet; if the valve is completely closed, all milk will be discharged through the skim milk outlet. If the valve is very slightly opened, you get most of the fat globules in a very small volume of skim, so cream with a high fat content. As the valve is progressively opened further, larger volumes of skim push the fat globules out, so you get larger volumes of cream but with diminishing fat contents (%) being discharged from the cream outlet. With direct standardization the cream and skim are automatically remixed at the separator to provide the desired fat content (see the diagram above to explain the automatic standardization process) . Some basic standardization problems including mass balance and Pearson square approach can be viewed on the following page. 

Standardization Problems

Here are the two examples of how to do milk standardization calculations:

1. If a dairy has 160 kg of 40% fat cream and wishes to standardize it to 32% fat cream, how much skimmilk (0% fat) must be added?

Mass Balance Approach

let x = kg skimmilk

y = kg of 32% cream

Mass Balance Equation (total mass into the process = total mass out of the process) 160 + x = y

Component Balance for Fat (fat into the process = fat out of the process)

.40 (160) = .32 (y) which says 40% of 160 kg comes in and 32% of y goes out
.40 (160) = .32(160+ x), substituting our equation for y
.32x = 64 - 51.2
x = 40 kg skimmilk

2. How much cream testing 35% fat must be added to 500 kg of milk testing 4% fat to obtain cream testing 10% fat?

Mass Balance Approach

let x = kg 35% cream

y = kg of 10% cream

Mass Balance x + 500 = y (mass in = mass out)

Component Balance for Fat

x (.35) + 500 (.04) = y (.10) again, fat in = fat out
x (.35) + 20 = x(.10) + 50
.25 x = 30
x = 120 kg of 35% cream

Note: if your algebra skills are not very good, I can highly recommend this website - I Do Maths.

 

Pearson square approach to above problem

A shortcut for 2-component mass balances
Place desired percentage in the centre of a rectangle
Place percentage composition of two available streams in left corners of rectangle
Cross subtract lower from higher numbers for right corners of triangle (higher number - lower number)
Use right corners as ratios of two streams

Using the above problem:

Component Mass
35     6
   10
4     25        500
      ___     ____
       31 

35% cream = (500 x 6)/25 = 120 kg
This will produce 620 kg cream at 10% fat. 

Check yur work - (620 x 0.10) = (120 x .35) + (500 x .04) - correct!

 

Pasteurization

Introduction

The process of pasteurization was named after Louis Pasteur who discovered that spoilage organisms could be inactivated in wine by applying heat at temperatures below its boiling point. The process was later applied to milk and remains the most important operation in the processing of milk.

Definition:

The heating of every particle of milk or milk product to a specific temperature for a specified period of time without allowing recontamination of that milk or milk product during the heat treatment process.

Purpose There are two distinct purposes for the process of milk pasteurization:

  1. Public Health Aspect - to make milk and milk products safe for human consumption by destroying all bacteria that may be harmful to health (pathogens) 
  2. Keeping Quality Aspect - to improve the keeping quality of milk and milk products. Pasteurization can destroy some undesirable enzymes and many spoilage bacteria. Shelf life can be 7, 10, 14 or up to 16 days.

The extent of microorganism inactivation depends on the combination of temperature and holding time. Minimum temperature and time requirements for milk pasteurization are based on thermal death time studies for the most heat resistant pathogen found in milk, Coxelliae burnettiiThermal lethality determinations require the applications of microbiology to appropriate processing determinations. An overview can be found on the next page.

To ensure destruction of all pathogenic microorganisms, time and temperature combinations of the pasteurization process are highly regulated: 

Ontario Pasteurization Regulations

Milk: 
63° C for not less than 30 min.,
72° C for not less than 16 sec.,
or equivalent destruction of pathogens and the enzyme phosphatase as permitted by Ontario Provincial Government authorities. Milk is deemed pasteurized if it tests negative for alkaline phosphatase.

Frozen dairy dessert mix (ice cream or ice milk, egg nog):
at least 69° C for not less than 30 min;
at least 80° C for not less than 25 sec;
other time temperature combinations must be approved (e.g. 83° C/16 sec).

Milk based products- with 10% mf or higher, or added sugar (cream, chocolate milk, etc)
66° C/30 min, 75° C/16 sec

There has also been some progress with low temperature pasteurization methods using membrane processing technology.

Thermal Destruction of Microorganisms

Heat is lethal to microorganisms, but each species has its own particular heat tolerance. During a thermal destruction process, such as pasteurization, the rate of destruction is logarithmic, as is their rate of growth. Thus bacteria subjected to heat are killed at a rate that is proportional to the number of organisms present. The process is dependent both on the temperature of exposure and the time required at this temperature to accomplish to desired rate of destruction. Thermal calculations thus involve the need for knowledge of the concentration of microorganisms to be destroyed, the acceptable concentration of microorganisms that can remain behind (spoilage organisms, for example, but not pathogens), the thermal resistance of the target microorganisms (the most heat tolerant ones), and the temperature time relationship required for destruction of the target organisms.

The extent of the pasteurization treatment required is determined by the heat resistance of the most heat-resistant enzyme or microorganism in the food. For example, milk pasteurization historically was based on Mycobacterium tuberculosis and Coxiella burnetti, but with the recognition of each new pathogen, the required time temperature relationships are continuously being examined.

A thermal death curve for this process is shown below. It is a logarithmic process, meaning that in a given time interval and at a given temperature, the same percentage of the bacterial population will be destroyed regardless of the population present. For example, if the time required to destroy one log cycle or 90% is known, and the desired thermal reduction has been decided (for example, 12 log cycles), then the time required can be calculated. If the number of microorganisms in the food increases, the heating time required to process the product will also be increased to bring the population down to an acceptable level. The heat process for pasteurization is usually based on a 12 D concept, or a 12 log cycle reduction in the numbers of this organism.

Thermal Death Time Curve for Coxiella burnetti, z=4oCSeveral parameters help us to do thermal calculations and define the rate of thermal lethality. The D value is a measure of the heat resistance of a microorganism. It is the time in minutes at a given temperature required to destroy 1 log cycle (90%) of the target microorganism. (Of course, in an actual process, all others that are less heat tolerant are destroyed to a greater extent). For example, a D value at 72°C of 1 minute means that for each minute of processing at 72°C the bacteria population of the target microorganism will be reduced by 90%. In the illustration below, the D value is 14 minutes (40-26) and would be representative of a process at 72°C.

Log 10 of living bacteriaThe Z value reflects the temperature dependence of the reaction. It is defined as the temperature change required to change the D value by a factor of 10. In the illustration below the Z value is 10°C.

Graph of D-value (minutes) and Temperature (Celsius)Reactions that have small Z values are highly temperature dependent, whereas those with large Z values require larger changes in temperature to reduce the time. A Z value of 10°C is typical for a spore forming bacterium. Heat induced chemical changes have much larger Z values that microorganisms, as shown below.

Bacteria Z (°C) 5-10 D121 (min) 1-5

enzymes Z (°C) 30-40 D121 (min) 1-5

vitamins Z (°C) 20-25 D121 (min) 150-200

pigments Z (°C) 40-70 D121 (min) 15-50

The figure below (which is schematic and not to scale)  illustrates the relative changes in time temperature profiles for the destruction of microorganisms. Above and to the right of each line the microorganisms or quality factors would be destroyed, whereas below and to the left of each line, the microorganisms or quality factors would not be destroyed. Due to the differences in Z values, it is apparent that at higher temperatures for shorter times, a region exists (shaded area) where pathogens can be destroyed while vitamins can be maintained. The same holds true for other quality factors such as colour and flavour components. Thus in UHT milk processing, very high temperatures for very short times (e.g., 140oC for 1-2 s) are favoured compared to a lower temperature longer time processes since it results in bacterial spore elimination with a lower loss of vitamins and better sensory quality.

Graph of Heating time (minutes) and Temperature (Celsius)

Alkaline phosphatase is a naturally-occurring enzyme in raw milk which has a similar Z value to heat-resistant pathogens. Since the direct estimation of pathogen numbers by microbial methods is expensive and time consuming, a simple test for phosphatase activity is routinely used. If activity is found, it is assumed that either the heat treatment was inadequate or that unpasteurized milk has contaminated the pasteurized product.

A working example of how to use D and Z values in pasteurization calculations:

Pooled raw milk at the processing plant has bacterial population of 4x10exp5/mL. It is to be processed at 79°C for 21 seconds. The average D value at 65°C for the mixed population is 7 min. The Z value is 7°C. How many organisms will be left after pasteurization? What time would be required at 65°C to accomplish the same degree of lethality?

Answer:

At 79°C, the D value has been reduced by two log cycles from that at 65°C since the Z value is 7°C. Hence it is now 0.07 min. The milk is processed for 21/60=0.35 min, so that would accomplish 5 log cycle reductions to 4 organisms/mL. At 65°C, you would need 35 minutes to accomplish a 5D reduction.

Methods of Pasteurization

Batch method

The batch method uses a vat pasteurizer which consists of a jacketed vat surrounded by either circulating water with added steam or heating coils of hot water or direct steam.

Batch Pasteurizer

In the vat the milk is heated and held throughout the holding period while being agitated. The milk may be cooled in the vat or removed hot after the holding time is completed for every particle. As a modification, the milk may be partially heated in tubular or plate heater before entering the vat. This method has very little use for milk but some use for milk by-products (e.g. creams, chocolate) and special batches. The vat pasteurizer is used extensively in the ice cream industry as it allows for dissolution and blending of ingredients during the heating stage.

Continuous Method

Continuous process method has several advantages over the vat method, the most important being time and energy saving. For most continuous processing, a high temperature short time (HTST) pasteurizer is used. The heat treatment is accomplished using a plate heat exchanger. This piece of equipment consists of a stack of corrugated stainless steel plates clamped together in a frame. There are several flow patterns that can be used. Gaskets are used to define the boundaries of the channels and to prevent leakage. The heating medium can be vacuum steam or hot water.

Diagram of a single plate and a Flow Pattern in Series of Plates

HTST Milk Flow Overview

 This overview is meant as an introduction and a summary. Each piece of HTST equipment will be discussed in further detail later. Please refer to the two diagrams below when reading this section - hopefully the flow of milk through the HTST pasteurized will then make sense.

Cold raw milk at 4° C in a constant level tank is drawn into the regenerator section of pasteurizer. Here it is warmed to approximately 57° C - 68° C by heat given up by hot pasteurized milk flowing in a counter current direction on the opposite side of thin, stainless steel plates. The raw milk, still under suction, passes through a positive displacement timing pump which delivers it under positive pressure through the rest of the HTST system.

The raw milk is forced through the heater section where hot water on opposite sides of the plates heat milk to a temperature of at least 72° C. The milk, at pasteurization temperature and under pressure, flows through the holding tube where it is held for at least 16 sec. The maximum velocity is governed by the speed of the timing pump, diameter and length of the holding tube, and surface friction. After passing temperature sensors of an indicating thermometer and a recorder-controller at the end of the holding tube, milk passes into the flow diversion device (FDD). The FDD assumes a forward-flow position if the milk passes the recorder-controller at the preset cut-in temperature (>72° C). The FDD remains in normal position which is in diverted-flow if milk has not achieved preset cut-in temperature. The improperly heated milk flows through the diverted flow line of the FDD back to the raw milk constant level tank. Properly heated milk flows through the forward flow part of the FDD to the pasteurized milk regenerator section where it gives up heat to the raw product and in turn is cooled to approximately 32° C - 9° C.

The warm milk passes through the cooling section where it is cooled to 4° C or below by coolant on the opposite sides of the thin, stainless steel plates. The cold, pasteurized milk passes through a vacuum breaker at least 12 inches above the highest raw milk in the HTST system then on to a storage tank filler for packaging.

Diagram of HTST flow

HTST Continuous Plate Pasteurizer

The diagram below illustrates HTST milk pasteurization equipment and flow diagram when incorporating a centrifugal booster pump (to help fill the regenerator on large systems) and pressure differential switch/back pressure controller on the regenerator (to maintain the pressure differential due to the action of the booster pump on the raw side). It also shows the position of the homogenizer when included in-line in the HTST system.

HTST booster diagram

Holding Time

When fluids move through a pipe, either of two distinct types of flow can be observed. The first is known as turbulent flow which occurs at high velocity and in which eddies are present moving in all directions and at all angles to the normal line of flow. The second type is streamline, or laminar flow which occurs at low velocities and shows no eddy currents. The Reynolds number, is used to predict whether laminar or turbulent flow will exist in a pipe:

Re < 2100 laminar
Re > 4000 fully developed turbulent flow

There is an impact of these flow patterns on holding time calculations and the assessment of proper holding tube lengths.

The holding time is determined by timing the interval for an added trace substance (salt) to pass through the holder. The time interval of the fastest particle of milk is desired. Thus the results found with water are converted to the milk flow time by formulation since a pump may not deliver the same amount of milk as it does water.

Note: the formulation assumes flow patterns are the same for milk and water. If they are not, how would this affect the efficiency of the pasteurization process?

Pressure Differential

For continuous pasteurizing, it is important to maintain a higher pressure on the pasteurized side of the heat exchanger. By keeping the pasteurized milk at least 1 psi higher than raw milk in regenerator, it prevents contamination of pasteurized milk with raw milk in event that a pin-hole leak develops in thin stainless steel plates. This pressure differential is maintained using a timing pump in simple systems, and differential pressure controllers and back pressure flow regulators at the chilled pasteurization outlet in more complex systems. The position of the timing pump is crucial so that there is suction on the raw regenerator side and pushes milk under pressure through pasteurized regenerator. There are several other factors involved in maintaining the pressure differential: 

  • The balance tank overflow level must be less than the level of lowest milk passage in the regenerator
  • Properly installed booster pump is all that is permitted between balance tank and raw regenerator
  • No pump after pasteurized milk outlet to vacuum breaker
  • There must be greater than a 12 inch vertical rise to the vacuum breaker
  • The raw regenerator drains freely to balance tank at shut-down

Basic Component Equipment of HTST Pasteurizer

Balance Tank

The balance, or constant level tank provides a constant supply of milk. It is equipped with a float valve assembly which controls the liquid level nearly constant ensuring uniform head pressure on the product leaving the tank. The overflow level must always be below the level of lowest milk passage in regenerator. It, therefore, helps to maintain a higher pressure on the pasteurized side of the heat exchanger. The balance tank also prevents air from entering the pasteurizer by placing the top of the outlet pipe lower than the lowest point in the tank and creating downward slopes of at least 2%. The balance tank provides a means for recirculation of diverted or pasteurized milk.

Diagram of a balance tank

Regenerator

 Heating and cooling energy can be saved by using a regenerator which utilizes the heat content of the pasteurized milk to warm the incoming cold milk. Its efficiency may be calculated as follows:

% regeneration = temp. increase due to regenerator/total temp. increase

For example: Cold milk entering system at 4° C, after regeneration at 65° C, and final temperature of 72° C would have an 89.7% regeneration:

65 - 4
______ = 89.7
72 - 4

Timing pump

The timing pump draws product through the raw regenerator and pushes milk under pressure through pasteurized regenerator. It governs the rate of flow through the holding tube. It must be a positive displacement pump equipped with variable speed drive that can be legally sealed at the maximum rate to give minimum holding time in holding tubes. It also must be interwired so it only operates when FDD is fully forward or fully diverted, and must be "fail-safe". A centrifugal pump with magnetic flow meter and controller may also be used (see below).

Holding tube

 Must slope upwards 1/4"/ft. in direction of flow to eliminate air entrapment so nothing flows faster at air pocket restrictions.

Indicating thermometer

The indicating thermometer is considered the most accurate temperature measurement. It is the official temperature to which the safety thermal limit recorder (STLR) is adjusted. The probe should sit as close as possible to STLR probe and be located not greater than 18 inches upstream of the flow diversion device.

Recorder-controller (STLR)

The STLR records the temperature of the milk and the time of day. It monitors, controls and records the position of the flow diversion device (FDD) and supplies power to the FDD during forward flow. There are both pneumatic and electronic types of controllers. The operator is responsible for recording the date, shift, equipment, ID, product and amount, indicating thermometer temperature, cleansing cycles, cut in and cut out temperatures, any connects for unusual circumstances, and his/her signature.

Flow Diversion Device (FDD)

Also called the flow diversion valve (FDV), it is located at the downstream end of the upward sloping holding tube. It is essentially a 3-way valve, which, at temperatures greater than 72° C,opens to forward flow. This step requires power. At temperatures less than 72° C, the valve recloses to the normal position and diverts the milk back to the balance tank. It is important to note that the FDD operates on the measured temperature, not time, at the end of the holding period. There are two types of FDD:

single stem - an older valve system that has the disadvantage that it can't be cleaned in place. 

dual stem - consists of 2 valves in series for additional fail safe systems. This FDD can be cleaned in place and is more suited for automation.

Diagram of a single stem FDD and a Dual Stem FDD

Vacuum Breaker

At the pasteurized product discharge is a vacuum breaker which breaks to atmospheric pressure. It must be located greater than 12 inches above the highest point of raw product in system. It ensures that nothing downstream is creating suction on the pasteurized side.

Auxiliary Equipment

Booster Pump

It is centrifugal "stuffing" pump which supplies raw milk to the raw regenerator for the balance tank. It must be used in conjunction with pressure differential controlling device and shall operate only when timing pump is operating, proper pressures are achieved in regenerator, and system is in forward flow. 

Homogenizer 

The homogenizer may be used as timing pump. It is a positive pressure pump; if not, then it cannot supplement flow. Free circulation from outlet to inlet is required and the speed of the homogenizer must be greater than the rate of flow of the timing pump. 

Magnetic flow meterMagnetic flow meter and centrifugal pump arrangements 

Magnetic flow meters (shown on the right) can be used to measure the flow rate. It is essentially a short piece of tubing (approximately 25 cm long) surrounded by a housing, inside of which are located coils that generate a magnetic field. When milk passes through the magnetic field, it causes a voltage to be induced, and the generated signal is directly proportional to velocity. Application of the magnetic flow meter in the dairy industry has centered around its replacing the positive displacement timing pump as the metering device in HTST pasteurizing systems, where with certain products the timing pump rotors reportedly wear out in a relatively short period of time. In operation, the electrical signal is sent by the magnetic flow meter to the flow controller, which determines what the actual flow is compared to the flow rate set by the operator. Since the magnetic flow meter continuously senses flow rate, it will signal the electronic controller if the actual flow exceeds the set flow rate for any reason. If the flow rate is exceeded for any reason, the flow diversion device is put into diverted flow. A significant difference from the normal HTST system (with timing pump) comes into focus at this point. This system can be operated at a flow rate greater than (residence time less than) the legal limit. However, it will be in diverted flow and never in forward flow.
Another magnetic flow meter based system with an AC variable frequency motor control drive on a centrifugal pump is also possible in lieu of a positive displacement metering pump on a HTST pasteurizer.  This system does not use a control valve but rather the signal from the magnetic flow meter is transmitted to the AC variable frequency control to vary the speed of the centrifugal pump. The pump, then controls the flow rate of product through the system and its holding time in the holding tube.

Automated Public Health Controllers

 These systems are used for time and temperature control of HTST systems. There are concerns that with sequential control, the critical control points (CCP's) are not monitored all the time; if during the sequence it got held up, the CCP's would not be monitored. With operator control, changes can be made to the program which might affect CCP's; the system is not easily sealed. No computer program can be written completely error free in large systems; as complexity increases, so too do errors.

This gives rise to a need for specific regulations or computer controlled CCP's of public health significance:

  1. dedicated computer - no other assignments, monitor all CCP's at least once/sec
  2. not under control of any other computer system or override system, i.e., network
  3. separate computer on each pasteurizer
  4. I/O bus for outputs only, to other computers no inputs from other computers
  5. on loss of power - public health computers should revert to fail safe position (e.g. divert)
  6. last state switches during power up must be fail safe position
  7. programs in ROM - tapes/disks not acceptable
  8. inputs must be sealed, modem must be sealed, program sealed
  9. no operator override switches
  10. proper calibration procedure during that printing - Public health computer must not leave public health control for > 1 sec and upon return must complete 1 full cycle before returning to printing
  11. FDV position must be monitored and temperature in holding tube recorded during change in FDV position
  12. download from ROM to RAM upon startup
  13. integrated with CIP computer which can be programmed e.g., FDV, booster pump controllable by CIP computer when in CIP made only

UHT Treatment

Introduction

While pasteurization conditions effectively eliminate potential pathogenic microorganisms, it is not sufficient to inactivate the thermoresistant spores in milk. The term sterilization refers to the complete elimination of all microorganisms. The food industry uses the more realistic term "commercial sterilization"; a product is not necessarily free of all microorganisms, but those that survive the sterilization process are unlikely to grow during storage and cause product spoilage.

In canning we need to ensure the "cold spot" has reached the desired temperature for the desired time. With most canned products, there is a low rate of heat penetration to the thermal centre. This leads to overprocessing of some portions, and damage to nutritional and sensory characteristics, especially near the walls of the container. This implies long processing times at lower temperatures.

Milk can be made commercially sterile by subjecting it to temperatures in excess of 100° C, and packaging it in air-tight containers. The milk may be packaged either before or after sterilization. The basis of UHT, or ultra-high temperature, is the sterilization of food before packaging, then filling into pre-sterilized containers in a sterile atmosphere. Milk that is processed in this way using temperatures exceeding 135° C, permits a decrease in the necessary holding time (to 2-5 s) enabling a continuous flow operation.

Some examples of food products processed with UHT are:

  • liquid products - milk, juices, cream, yoghurt, wine, salad dressings
  • foods with discrete particles - baby foods; tomato products; fruits and vegetables juices; soups
  • larger particles - stews

Advantages of UHT

High quality:

The D and Z valves are higher for quality factors than microorganisms. The reduction in process time due to higher temperature (UHTST) and the minimal come-up and cool-down time leads to a higher quality product.

Long shelf life:

Greater than 6 months, without refrigeration, can be expected.

Packaging size:

Processing conditions are independent of container size, thus allowing for the filling of large containers for food-service or sale to food manufacturers (aseptic fruit purees in stainless steel totes).

Cheaper packaging:

Both cost of package and storage and transportation costs; laminated packaging allows for use of extensive graphics.

Difficulties with UHT

Sterility:

Complexity of equipment and plant are needed to maintain sterile atmosphere between processing and packaging (packaging materials, pipework, tanks, pumps); higher skilled operators; sterility must be maintained through aseptic packaging

Particle Size:

With larger particulates there is a danger of overcooking of surfaces and need to transport material - both limits particle size.

Equipment:

There is a lack of equipment for particulate sterilization, due especially to settling of solids and thus overprocessing

Keeping Quality:

Heat stable lipases or proteases can lead to flavour deterioration, age gelation of the milk over time - nothing lasts forever! There is also a more pronounced cooked flavour to UHT milk.

UHT Methods

 There are two principal methods of UHT treatment:

  1. Direct Heating
  2. Indirect Heating

Direct heating systems

The product is heated by direct contact with steam of potable or culinary quality. The main advantage of direct heating is that the product is held at the elevated temperature for a shorter period of time. For a heat-sensitive product such as milk, this means less damage.

Graph of Direct and Indirect Continuous Sterilization

There are two methods of direct heating (please see these schematic diagrams of the equipment;

  1. injection
  2. infusion

Injection 

High pressure steam is injected into pre-heated liquid by a steam injector leading to a rapid rise in temperature. After holding, the product is flash-cooled in a vacuum to remove water equivalent to amount of condensed steam used. This method allows fast heating and cooling, and volatile removal, but is only suitable for some products. It is energy intensive and because the product comes in contact with hot equipment, there is potential for flavour damage.

Infusion

The liquid product stream is pumped through a distributing nozzle into a chamber of high pressure steam. This system is characterized by a large steam volume and a small product volume, distributed in a large surface area of product. Product temperature is accurately controlled via pressure. Additional holding time may be accomplished through the use of plate or tubular heat exchangers, followed by flash cooling in vacuum chamber. This method has several advantages:

  • instantaneous heating and rapid cooling
  • no localized overheating or burn-on
  • suitable for low and higher viscosity products

Indirect heating systems

The heating medium and product are not in direct contact, but separated by equipment contact surfaces. Several types of heat exchangers are applicable:

  • plate
  • tubular
  • scraped surface

Plate Heat Exchangers

Similar to that used in HTST but operating pressures are limited by gaskets. Liquid velocities are low which could lead to uneven heating and burn-on. This method is economical in floor space, easily inspected, and allows for potential regeneration.

Tubular Heat Exchangers

There are several types:

  • shell and tube
  • shell and coil
  • double tube
  • triple tube

All of these tubular heat exchangers have fewer seals involved than with plates. This allows for higher pressures, thus higher flow rates and higher temperatures. The heating is more uniform but difficult to inspect.

Scraped Surface Heat Exchangers

The product flows through a jacketed tube, which contains the heating medium, and is scraped from the sides with a rotating knife. This method is suitable for viscous products and particulates (< 1 cm) such as fruit sauces, and can be adjusted for different products by changing configuration of rotor. There is a problem with larger particulates; the long process time for particulates would mean long holding sections which are impractical. This may lead to damaged solids and overprocessing of sauce.

Packaging for Aseptic Processing

The most important point to remember is that it must be sterile! All handling of product post-process must be within the sterile environment.

There are 5 basic types of aseptic packaging lines:

  1. Fill and seal: preformed containers made of thermoformed plastic, glass or metal are sterilized, filled in aseptic environment, and sealed
  2. Form, fill and seal: roll of material is sterilized, formed in sterile environment, filled, sealed e.g. tetrapak
  3. Erect, fill and seal: using knocked-down blanks, erected, sterilized, filled, sealed. e.g. gable-top cartons, cambri-bloc
  4. Thermoform, fill, sealed roll stock sterilized, thermoformed, filled, sealed aseptically. e.g. creamers, plastic soup cans
  5. Blow mold, fill, seal:

There are several different package forms that are used in aseptic UHT processing:

  • cans
  • paperboard/plastic/foil/plastic laminates
  • flexible pouches
  • thermoformed plastic containers
  • flow molded containers
  • bag-in-box
  • bulk totes

It is also worth mentioning that many products that are UHT heat treated are not aseptically packaged. This gives them the advantage of a longer shelf life at refrigeration temperatures compared to pasteurization, but it does not produce a shelf-stable product at ambient temperatures, due to the possibility of recontamination post-processing.

Homogenization of Milk and Milk Products

 The following topics will be covered in this section:

  • Introduction
  • Homogenization Mechanisms
    • turbulence
    • cavitation
  • Effect Of Homogenization
    • fat globule properties
    • surface layers

Introduction

Milk is an oil-in-water emulsion, with the fat globules dispersed in a continuous skimmilk phase. If raw milk were left to stand, however, the fat would rise and form a cream layer. Homogenization is a mechanical treatment of the fat globules in milk brought about by passing milk under high pressure through a tiny orifice, which results in a decrease in the average diameter and an increase in number and surface area, of the fat globules. The net result, from a practical view, is a much reduced tendency for creaming of fat globules. Three factors contribute to this enhanced stability of homogenized milk: a decrease in the mean diameter of the fat globules (a factor in Stokes Law), a decrease in the size distribution of the fat globules (causing the speed of rise to be similar for the majority of globules such that they don't tend to cluster during creaming), and an increase in density of the globules (bringing them closer to the continuous phase) owing to the adsorption of a protein membrane. In addition, heat pasteurization breaks down the cryo-globulin complex, which tends to cluster fat globules causing them to rise.

Diagram of raw milk, cold raw milk after 1 hour, and homogenized milk during storage

Homogenization Mechanism

 

 

Auguste Gaulin's patent in 1899 consisted of a 3 piston pump in which product was forced through one or more hair like tubes under pressure. It was discovered that the size of fat globules produced were 500 to 600 times smaller than tubes. There have been over 100 patents since, all designed to produce smaller average particle size with expenditure of as little energy as possible. The homogenizer consists of a 3 cylinder positive piston pump (operates similar to car engine) and homogenizing valve. The pump is turned by electric motor through connecting rods and crankshaft.

To understand the mechanism, consider a conventional homogenizing valve processing an emulsion such as milk at a flow rate of 20,000 l/hr. at 14 MPa (2100 psig). As it first enters the valve, liquid velocity is about 4 to 6 m/s. It then moves into the gap (see below) between the valve and the valve seat and its velocity is increased to 120 meter/sec in about 0.2 millisec. The liquid then moves across the face of the valve seat (the land) and exits in about 50 microsec. The homogenization phenomena is completed before the fluid leaves the area between the valve and the seat, and therefore emulsification is initiated and completed in less than 50 microsec. The whole process occurs between 2 pieces of steel in a steel valve assembly. The product may then pass through a second stage valve similar to the first stage. While most of the fat globule reduction takes place in the first stage, there is a tendency for clumping or clustering of the reduced fat globules. The second stage valve permits the separation of those clusters into individual fat globules. 

Diagram of the effects of 2-stage homogenization on fat globule size distribution as seen under the light microscope

 

Diagram of a Homogenizer

 

It is most likely that a combination of two theories, turbulence and cavitation, explains the reduction in size of the fat globules during the homogenization process.

Turbulence 

Energy, dissipating in the liquid going through the homogenizer valve, generates intense turbulent eddies of the same size as the average globule diameter. Globules are thus torn apart by these eddy currents reducing their average size.

Cavitation 

Considerable pressure drop with change of velocity of fluid. Liquid cavitates because its vapor pressure is attained. Cavitation generates further eddies that would produce disruption of the fat globules. The high velocity gives liquid a high kinetic energy which is disrupted in a very short period of time. Increased pressure increases velocity. Dissipation of this energy leads to a high energy density (energy per volume and time). Resulting diameter is a function of energy density.

In summary, the homogenization variables are:

  • type of valve
  • pressure
  • single or two-stage
  • fat content
  • surfactant type and content
  • viscosity
  • temperature

Also to be considered are the droplet diameter (the smaller, the more difficult to disrupt), and the log diameter which decreases linearly with log P and levels off at high pressures.

Effect of Homogenization

  No Homogenization 15 MPa (2500 psig)
Av. diam. (µ m) 3.3 0.4
Max. diam. (µ m) 10 2
Surf. area (m2/ml of milk) 0.08 0.75
Number of globules (µ m-3) 0.02 12

Surface layers  The milk fat globule has a native membrane, picked up at the time of secretion, made of amphiphilic molecules with both hydrophilic and hydrophobic sections. This membrane lowers the interfacial tension resulting in a more stable emulsion. During homogenization, there is a tremendous increase in surface area and the native milk fat globule membrane (MFGM) is lost. However, there are many amphiphilic molecules present from the milk plasma that readily adsorb: casein micelles (partly spread) and whey proteins. The interfacial tension of raw milk is 1-2 mN/m, immediately after homogenization it is unstable at 15 mN/m, and shortly becomes stable (3-4 mN/m) as a result of the adsorption of protein. The transport of proteins is not by diffusion but mainly by convection. Rapid coverage is achieved in less than 10 sec but is subject to some rearrangement.

Surface excess is a measure of how much protein is adsorbed; for example 10 mg/m2 translates to a thickness of adsorbed layer of approximately 15 nm.

Membrane Processing

 Membrane processing is a technique that permits concentration and separation without the use of heat. Particles are separated on the basis of their molecular size and shape with the use of pressure and specially designed semi-permeable membranes. There are some fairly new developments in terms of commercial reality and is gaining readily in its applications:

  • proteins can be separated in whey for the production of whey protein concentrate (WPC)
  • milk can be concentrated prior to cheesemaking at the farm level
  • apple juice and wine can be clarified
  • waste treatment and product recovery is possible in edible oil, fat, potato, and fish processing
  • fermentation broths can be clarified and separated
  • whole egg and egg white ultrafiltration as a preconcentration prior to spray drying

The following topics will be covered in this section:

Principle of Operation

When a solution and water are separated by a semi-permeable membrane, the water will move into the solution to equilibrate the system. This is known as osmotic pressure. If a mechanical force is applied to exceed the osmotic pressure (up to 700 psi), the water is forced to move down the concentration gradient i.e. from low to high concentration. Permeate designates the liquid passing through the membrane, and retentate (concentrate) designates the fraction not passing through the membrane.

Diagram of membrane processing

Reverse Osmosis, Ultra- and Diafiltration and Microfiltration

Reverse osmosis (RO) is a membrane separation process, driven by a pressure gradient, in which the membrane separates the solvent (generally water) from other components of a solution. The membrane configuration is usually cross-flow. With reverse osmosis, the membrane pore size is very small allowing only water and perhaps very small amounts of very low molecular weight solutes to pass through the membranes. It is a concentration process using a 100 MW cutoff, 700 psig, temperatures less than 40°C with cellulose acetate membranes and 70-80°C with composite membranes. Hyperfiltration is the same as RO.

Ultrafiltration (UF) is a membrane separation process, driven by a pressure gradient, in which the membrane fractionates dissolved and dispersed components of a liquid as a function of their solvated size and structure. The membrane configuration is usually cross-flow. In UF, the membrane pore size is larger than RO, thus allowing some components to pass through the pores with the water. It is a separation/ fractionation process using a 10,000 MW cutoff, 40 psig, and temperatures of 50-60°C with polysulfone membranes. In UF of skim milk, lactose and minerals are not fractionated; for example, in the retentate would be 100% of the protein but the same % of lactose and free minerals in solution (in the water phase) as existed in the skim.

Diagram of reverse osmosis and ultrafiltration

This can be visualized with another schematic, as follows, which may be more informative:

Diagram of reverse osmosis, ultrafiltration and microfiltration

Diafiltration is a specialized type of ultrafiltration process in which the retentate is diluted with water and re-ultrafiltered, to reduce the concentration of soluble permeate components and increase further the concentration of retained components. This schematic shows the process of diaflitration, as a step in ultrafiltration.

Diagram of diafiltration

Microfiltration (MF) (see diagram above) is a membrane separation process similar to UF but with even larger membrane pore size allowing particles in the range of 0.2 to 2 micrometers to pass through. The pressure used is generally lower than that of UF process. The membrane configuration is usually cross-flow. MF is used in the dairy industry for making low-heat sterile milk as proteins may pass through but bacteria do not. The permeate of skim milk is used as "bacteria-free" skim (although thee is no fail-safe guarntee as there could be pin-holes in the membrane) since all of the milk components will pass through the membrane. In that case, the retentate, skim enriched in bacteria, is high-heat treated. MF skim can then be stadardized for fat with high heat-treated cream. 

Hardware Design

Open Tubular:

Tubes of membrane with a diameter of 1/2 to 1 inch and length to 12 ft. are encased in reinforced fibreglass or enclosed inside a rigid PVC or stainless steel shell. As the feed solution flows through the membrane core, the permeate passes through the membrane and is collected in the tubular housing. Imagine 12 ft long straws!

Hollow Fibre:

Similar to open tubular, but the cartridges contain several hundred very small (1 mm diam) hollow membrane tubes or fibres. As the feed solution flows through the open cores of the fibres, the permeate is collected in the cartridge area surrounding the fibres. 

Plate and Frame:

This system is set up like a plate heat exchanger with the retentate on one side and the permeate on the other. The permeate is collected through a central collection tube. 

Spiral Wound:

This design tries to maximize surface area in a minimum amount of space. It consists of consecutive layers of large membrane and support material in an envelope type design rolled up around a perforated steel tube.

Electrodialysis

Electrodialysis is used for demineralization of milk products and whey for infant formula and special dietary products. Also used for desalination of water.

Principles of operation:

Under the influence of an electric field, ions move in an aqueous solution. The ionic mobility is directly proportioned to specific conductivity and inversely proportioned to number of molecules in solution. ~3-6 x 102 mm/sec.

Charged ions can be removed from a solution by synthetic polymer membranes containing ion exchange groups. Anion exchange membranes carry cationic groups which repel cations and are permeable to anions, and cation exchange membranes contain anionic groups and are permeable only to cations.

Schematic diagram to show amion and cation exchange membranesElectrodialysis membranes are comprised of polymer chains - styrene-divinyl benzene made anionic with quaternary ammonium groups and made cationic with sulphonic groups. 1-2V is then applied across each pair of membranes.

Electrodialysis process:

Amion and cation exchange membranes are arranged alternately in parallel between an anode and a cathode (see this schematic diagram to the right). The distance between the membranes is 1mm or less. A plate and frame arrangement similar to a plate heat exchanger or a plate filter is used. The solution to be demineralized flows through gaps between the two types of membranes. Each type of membrane is permeable to only one type of ion. Thus, the anions leave the gap in the direction of the anode and cations leave in the direction of the cathode. Both are then taken up by a concentrating stream. 

Problems:

Concentration polarization. Deposits on membrane surfaces, e.g. proteins - pH control is important. Prior concentration of whey, to 20% TS, is necessary before electrodialysis.

Ion Exchange

Ion exchange is not a membrane process but I have included it here anyway because it is used for product of protein isolates of higher concentration than obtainable by membrane concentration.

Fractionation may also be accomplished using ion exchange processing. It relies on inert resins (cellulose or silica based) that can adsorb charged particles at either end of the pH scale. The design can be a batch type, stirred tank or continuous column. The column is more suitable for selective fractionation. Whey protein isolate (WPI), with a 95% protein content, can be produced by this method. Following adsorption and draining of the deproteined whey, the pH or charge properties are altered and proteins are eluted. Protein is recovered from the dilute stream through UF and drying. Selective resins may be used for fractionated protein products or enriched in fraction allow tailoring of ingredients.

Evaporation and Dehydration

The removal of water from foods provides microbiological stability, reduces deteriorative chemical reactions, and reduces transportation and storage costs. Both evaporation and dehydration are methods used in the dairy industry for this purpose. The following topics will be addressed here:

Evaporation

  • Principle of Operation
  • Evaporator designs
    • Batch Pan
    • Rising Film
    • Falling Film
    • Multiple Effect Evaporators
    • Thermo compression
    • Mechanical Vapour Recompression 

Dehydration

  • Spray Drying Process
  • Powder Recovery
    • Bag filters
    • Cyclone collector
    • Wet scrubber
  • Two and Three Stage Spray Driers
    • Principles of Fluid Beds
    • Process
  • Agglomeration and Instantizing

Evaporation

Evaporation refers to the process of heating liquid to the boiling point to remove water as vapour.

Principle of operation:

The driving force for heat transfer is the difference in temperature between the steam in the coils and the product in the pan. The steam is produced in large boilers, generally tube and chest heat exchangers. The steam temperature is a function of the steam pressure. Water boils at 100° C at 1 atm., but at other pressures the boiling point changes. At its boiling point, the steam condenses in the coils and gives up its latent heat. If the steam temperature is too high, burn-on/fouling increases so there are limits to how high steam temperatures can go. The product is also at its boiling point. The boiling point can be elevated with an increase in solute concentration. This boiling point elevation works on the same principles as freezing point depression.

Operating design:

Because milk is heat sensitive, heat damage can be minimized by evaporation under vacuum to reduce the boiling point. The basic components of this process consist of: 

  • heat-exchanger
  • vacuum
  • vapour separator
  • condenser 

The heat exchanger is enclosed in a large chamber and transfers heat from the heating medium, usually low pressure steam, to the product usually via indirect contact surfaces. The vacuum keeps the product temperature low and the difference in temperatures high. The vapour separator removes entrained solids from the vapours, channelling solids back to the heat exchanger and the vapours out to the condenser. It is sometimes a part of the actual heat exchanger, especially in older vacuum pans, but more likely a separate unit in newer installations. The condenser condenses the vapours from inside the heat exchanger and may act as the vacuum source.

Diagram of an evaporator

Evaporator Designs

Types of single effect evaporators:

  • Batch Pan
  • Rising film
  • Falling film
  • Plate evaporators
  • Scraped surface 

Batch pan evaporators are the simplest and oldest. They consist of spherical shaped, steam jacketed vessels. The heat transfer per unit volume is small requiring long residence times. The heating is due only to natural convection, therefore, the heat transfer characteristics are poor. Batch plants are of historical significance; modern evaporation plants are far-removed from this basic idea. The vapours are a tremendous source of low pressure steam and must be reused. 

Rising film evaporators consist of a heat exchanger isolated from the vapour separator. The heat exchanger, or calandria, consists of 10 to 15 meter long tubes in a tube chest which is heated with steam. The liquid rises by percolation from the vapours formed near the bottom of the heating tubes. The thin liquid film moves rapidly upwards. The product may be recycled if necessary to arrive at the desired final concentration. This development of this type of modern evaporator has given way to the falling film evaporator.

The falling film evaporators are the most widely used in the food industry. They are similar in components to the rising film type except that the thin liquid film moves downward under gravity in the tubes. A uniform film distribution at the feed inlet is much more difficult to obtain. This is the reason why this development came slowly and it is only within the last decade that falling film has superceded all other designs. Specially designed nozzles or spray distributors at the feed inlet permit it to handle more viscous products. The residence time is 20-30 sec. as opposed to 3-4 min. in the rising film type. The vapour separator is at the bottom which decreases the product hold-up during shut down. The tubes are 8-12 meters long and 30-50 mm in diameter. 

Multiple Effect Evaporators

multiple effect evaporatorTwo or more evaporator units can be run in sequence to produce a multiple effect evaporator (shown on the right). Each effect would consist a heat transfer surface, a vapour separator, as well as a vacuum source and a condenser. The vapours from the preceding effect are used as the heat source in the next effect. There are two advantages to multiple effect evaporators: 

  • economy - they evaporate more water per kg steam by re-using vapours as heat sources in subsequent effects
  • improve heat transfer - due to the viscous effects of the products as they become more concentrated

Each effect operates at a lower pressure and temperature than the effect preceding it so as to maintain a temperature difference and continue the evaporation procedure. The vapours are removed from the preceding effect at the boiling temperature of the product at that effect so that no temperature difference would exist if the vacuum were not increased. The operating costs of evaporation are relative to the number of effects and the temperature at which they operate. The boiling milk creates vapours which can be recompressed for high steam economy. This can be done by adding energy to the vapour in the form of a steam jet, thermo compression or by a mechanical compressor, mechanical vapour recompression.

Thermo Compression (TC) 

Involves the use of a steam-jet booster to recompress part of the exit vapours from the first effect. Through recompression, the pressure and temperature of the vapours are increased. As the vapours exit from the first effect, they are mixed with very high pressure steam. The steam entering the first effect calandria is at slightly less pressure than the supply steam. There is usually more vapours from the first effect than the second effect can use; usually only the first effect is coupled with multiple effect evaporators. 

Mechanical Vapour Recompression (MVR) 

Whereas only part of the vapour is recompressed using TC, all the vapour is recompressed in an MVR evaporator. Vapours are mechanically compressed by radial compressors or simple fans using electrical energy.

There are several variations; in single effect, all the vapours are recompressed therefore no condensing water is needed; in multiple effect, can have MVR on first effect, followed by two or more traditional effects; or can recompress vapours from all effects. 

Dehydration

Dehydration refers to the nearly complete removal of water from foods to a level of less than 5%. Although there are many types of driers, spray driers are the most widely used type of air convection drier. It turns out more tonnage of dehydrated products than all other types of driers combined. It is limited to food that can be atomized, i.e. liquids, low viscosity pastes, and purees. Drying takes place within a matter of seconds at temperatures approximately 200° C. Evaporative cooling maintains low product temperatures, however, prompt removal of the product is still necessary.

Spray Drying - Process Summary

The liquid food is generally preconcentrated by evaporation to economically reduce the water content. The concentrate is then introduced as a fine spray or mist into a tower or chamber with heated air. As the small droplets make intimate contact with the heated air, they flash off their moisture, become small particles, and drop to the bottom of the tower and are removed. The advantages of spray drying include a low heat and short time combination which leads to a better quality product.

Diagram of a 2 stage dryer

Diagram of a spray dryer

Principal components include: 

  • a high pressure pump for introducing liquid into the tower
  • a device for atomizing the feed stream
  • a heated air source with blower
  • a secondary collection vessel for removing the dried food from the airstream
  • means for exhausting the moist air
  • usually includes a preconcentration step i.e. MVR evaporation

Atomizing devices are the distinguishing characteristic of spray drying. They provide a large surface area for exposure to drying forces:

1 litre = 12 billion particles = >300 ft2 (30m2)

The exit air temperature is an important parameter to monitor because it responds readily to changes in the process and reflects the quality of the product. Generally, we want it high enough to yield desired moisture without heat damage. There are two controls that may be used to adjust the exit air temperature:

  • altering feed flow rate
  • altering inlet temperature

If heat damage occurs before the product is dried, the particle size must be reduced; smaller particle dries faster, therefore, less heat damage. This can be accomplished in three ways: 

  • smaller orifice
  • increase atomizing pressure
  • reduce viscosity - by increasing feed temperature or reducing solids

Powder Recovery

It is essential for both economic and environmental reasons that as much powder as possible be recovered from the air stream. Three systems are available, however wet scrubbers usually act as a secondary collection system following a cyclone.

Bag Filters

Bag filters are very efficient (99.9%), but not as popular due to labor costs, sanitation, and possible heat damage because of the long residence time. They are not recommended in the case of handling high moisture loads or hygroscopic particles.

Cyclone Separators

Cyclones are not as efficient (99.5%) as bag filters but several can be placed in series. Air enters at tangent at high velocity into a cylinder or cone which has a much larger cross section. Air velocity is decreased in the cone permitting settling of solids by gravity. Centrifugal force is important in removing particles from the air stream. High air velocity is needed to separate small diameter and light materials from air; velocities may approach 100 ft/sec (70 MPH). Higher centrifugal force can be obtained by using small diameter cyclones, several of which may be placed in parallel; losses may range from 0.5-2%. A rotary airlock is used to remove powder from the cyclone. (An example of a rotary airlock is a revolving door at a hotel lobby which is intended to break the outside and inside environments).

Wet Scrubbers

 Wet scrubbers are the most economical outlet air cleaner. The principle of a wet scrubber is to dissolve any dust powder left in the airstream into either water or the feed stream by spraying the wash stream through the air. This also recovers heat from the exiting air and evaporates some of the water in the feed stream (if used as the wash water).

Diagram of wet scrubbers

Wet scrubbers not only recover most of what would be lost product, but also recover approximately 90% of the potential drying energy normally lost in exit air. The exit air picks up moisture which increases evaporative capacity by 8% (concentration of feed). Cyclone separators are probably the best primary powder separator system because they are hygienic, easy to operate, and versatile, however, high losses may occur. Wet scrubbers are designed for a secondary air cleaning system in conjunction with the cyclone. Either feed stream or water can be used as scrubbing liquor. Also, there are heat recovery systems available.

Two and Three Stage Spray Drying With a Fluidized Bed

Principles of Fluid Beds

Air is blown up through a wire mesh belt on porous plate that supports and conveys the product. A slight vibration motion is imparted to the food particles. When the air velocity is increased to the point where it just exceeds the velocity of free fall (gravity) of the particles, fluidization occurs. The dancing/boiling motion subdivides the product and provides intimate contact of each particle with the air, but keeps clusters from forming.

Diagram of a fluid bed

With products that are particularly difficult to fluidize, a vibrating motion of the drier itself is used to aid fluidization; it is called vibro-fluidizer which is on springs. The fluidized solid particles then behave in an analogous manner to a liquid., i.e. they can be conveyed. Air velocities will vary with particle size and density, but are in the range of 0.3 - 0.75 m/s. They can be used not only for drying but also for cooling. If the velocity is too high, the particles will be carried away in the gas stream, therefore, gravitational forces need to be only slightly exceeded.

Two- and three-stage drying processes

 In standard, single stage spray drying, the rate of evaporation is particularly high in the first part of the process, and it gradually decreases because of the falling moisture content of the particle surfaces. In order to complete the drying in one stage, a relatively high outlet temperature is required during the final drying phase. Of course the outlet temperature is reflective of the particle temperature and thus heat damage .

Consequently the two stage drying process was introduced which proved to be superior to the traditional single stage drying in terms of product quality and cost of production.

The two stage drier consists of a spray drier with an external vibrating fluid bed placed below the drying chamber. The product can be removed from the drying chamber with a higher moisture content, and the final drying takes place in the external fluid bed where the residence time of the product is longer and the temperature of the drying air lower than in the spray dryer.

This principle forms the basis of the development of the three stage drier. The second stage is a fluid bed built into the cone of the spray drying chamber. Thus it is possible to achieve an even higher moisture content in the first drying stage and a lower outlet air temperature from the spray drier. This fluid bed is called the integrated fluid bed. The inlet air temperature can be raised resulting in a larger temperature difference and improved efficiency in the drying process. The exhaust heat from the chamber is used to preheat the feed stream. The third stage is again the external fluid bed, which can be static or vibrating, for final drying and/or cooling the powder. The results are as follows:

  • higher quality powders with much better rehydrating properties directly from the drier
  • lower energy consumption
  • increased range of products which can be spray dried i.e., non density, non hygroscopic
  • smaller space requirements

Agglomerating and Instantizing

These processes have allowed the manufacturing of milk powders with better reconstitution properties, such as instantized skim milk powder. 

Agglomeration Mechanism: Powder is wetted with water or steam. The surface must be uniformly wetted but not excessively. The powder is held wet over a selected period of time to give moisture stability to the clusters which have formed. The clusters are dried to the desired moisture content and then cooled (e.g., fluid bed). Dried clusters are screened and sized to reduce excessively large particles and remove excessively small ones. The agglomeration process causes an increase in the amount of air incorporated between powder particles. More incorporated air is replaced with more water when the powder is reconstituted, which immediately wet the powder particles. 

Agglomerating Techniques

Rewet Methods:

This method uses powder as feed stock. An example is the ARCS Instantizer. Humidified air moistens powder, which causes it to cluster. It is re-dried and wetted. The clustered powder is then exposed to heated, filtered, high-velocity air. The dried clusters are then exposed to cooled air on a vibrating belt. It is then sized (pelleted to uniform size) and the fines are removed. 

Straight Thru Process:

A multi-stage drying process produces powders with much better solubility characteristics similar to instantized powder. This method uses a low outlet temperature which allows higher moisture in powder as it is taken from spray drier with excess moisture removed in the fluid bed. The powder fines are reintroduced to the atomizing cloud in the drying chamber.

More information on specific evaporated and dehydrated products is located in the Dairy Products section.

Production and Utilization of Steam and Refrigeration

This section will describe the production and utilization of both steam and refrigeration, two utilities absolutely essential to the operation of a dairy processing plant. 

Steam Production and Utilization

Understanding Steam

The diagram below helps to explain the various principles involved in the thermodynamics of steam. It shows the relationship between temperature and enthalpy (energy or heat content) of water as it passes through its phase change.

Graph showing the relationship between temperature and enthalpy (energy or heat content) of water as it passes through its phase change

The reference point for enthalpy of water and steam is 0oC, at which point an enthalpy value of 0 kJ/kg is given to it (but of course water at 0o has a lot of energy in it, which is given up as it freezes - it's not until 0K, absolute zero, when it truly has no enthalpy!). As we increase the temperature of water, its enthalpy increases by 4.18 kJ/kg oC until we hit its boiling point (which is a function of its pressure - the boiling point of water is 100oC ONLY at 1 atm. pressure). At this point, a large input of enthalpy causes no temperature change but a phase change, latent heat is added and steam is produced. Once all the water has vaporized, the temperature again increases with the addition of heat (sensible heat of the vapour).

Steam Production and Distribution

 Steam is produced in large tube and chest heat exchangers, called water tube boilers if the water is in the tubes, surrounded by the flame, or fire tube boilers if the opposite is true. The pressure inside a boiler is usually high, 300-800 kPa. The steam temperature is a function of this pressure. The steam, usually saturated or of very high quality, is then distributed to the heat exchanger where it is to be used, and it provides heat by condensing back to water (called condensate) and giving up its latent heat. The temperature desired at the heat exchanger can be adjusted by a pressure reducing valve, which lowers the pressure to that corresponding to the desired temperature. After the steam condenses in the heat exchanger, it passes through a steam trap (which only allows water to pass through and hence holds the steam in the heat exchanger) and then the condensate (hot water) is returned to the boiler so it can be reused. The following image is a schematic of a steam production and distribution cycle.

schematic of a steam production and distribution cycle

Refrigeration

The following image is a schematic of a refrigeration cycle. It is described in detail below, so you may want to go back and forth between the diagram and the description.

schematic of a refrigeration cycle

Mechanical refrigerators have four basic elements: an evaporator, a compressor, a condenser, and a refrigerant flow control (expansion valve). A refrigerant circulates among the four elements changing from liquid to gas and back to liquid.

In the evaporator, the liquid refrigerant evaporates (boils) under reduced pressure and in doing so absorbs latent heat of vaporization and cools the surroundings. The evaporator is at the lowest temperature in the system and heat flows to it. This heat is used to vaporize the refrigerant. The temperature at which this occurs is a function of the pressure on the refrigerant: for example if ammonia is the refrigerant, at -18oC the ammonia pressure required is 1.1 kg/sq. cm. The part of the process described thus far is the useful part of the refrigeration cycle; the remainder of the process is necessary only so that the refrigerant may be returned to the evaporator to continue the cycle.

The refrigerant vapour is sucked into a compressor, a pump that increases the pressure and then exhausts it at a higher pressure to the condenser. For ammonia, this is approx. 10 kg/sq. cm. To complete the cycle, the refrigerant must be condensed back to liquid and in doing this it gives up its latent heat of vaporization to some cooling medium such as water or air. The condensing temperature of ammonia is 29oC, so that cooling water at about 21oC could be used. In home refrigerators, the compressed gas (not ammonia) is sent through the pipes at the back, which are cooled by circulating air around them. Often fins are added to these tubes to increase the cooling area. The gas had to be compressed so that it could be condensed at these higher temperatures, using free cooling from water or air.

The refrigerant is now ready to enter the evaporator to be used again. It passes through an expansion valve to enter into the region of lower pressure, which causes it to boil and absorb more heat from the load. By adjusting the high and low pressures, the condensing and evaporating temperatures can be adjusted as required.

Dairy Products

Overview and Fluid Milk Products

Overview of the Range of Products that can be Manufactured from Milk as a Starting Point

Diary Tree showing the range of products that can be manufactured from milk

Fluid Milk Products and Processing

Beverage Milks

The production of beverage milks combines the unit operations of clarification, separation (for the production of lower fat milks), pasteurization, and homogenization. The process is simple, as indicated in the flow chart. While the fat content of most raw milk is 4% or higher, the fat content in most beverage milks has been reduced to 3.4%. Lower fat alternatives, such as 2% fat, 1% fat, or skim milk (<0.1% fat) or also available in most markets. These products are either produced by partially skimming the whole milk, or by completely skimming it and then adding an appropriate amount of cream back to achieve the desired final fat content.

Vitamins may be added to both full fat and reduced fat milks. Vitamins A and D (the fat soluble ones) are often supplemented in the form of a water soluble emulsion to offset that quantity lost in the fat separation process.

Diagram of the process of clarification, separation, pasteurization, and homogenization for producing milk

Creams

During the separation of whole milk, two streams are produced: the fat-depleted stream, which produces the beverage milks as described above or skim milk for evaporation and possibly for subsequent drying, and the fat-rich stream, the cream. This usually comes off the separator with fat contents in the 35-45% range. Cream is used for further processing in the dairy industry for the production of ice cream or butter, or can be sold to other food processing industries. These industrial products normally have higher fat contents than creams for retail sale, normally in the range of 45-50% fat. A product known as "plastic" cream can be produced from certain types of milk separators. This product has a fat content approaching 80% fat, but it remains as an oil-in-water emulsion (the fat is still in the form of globules and the skim milk is the continuous phase of the emulsion), unlike butter which also has a fat content of 80% but which has been churned so that the fat occupies the continuous phase and the skim milk is dispersed throughout in the form of tiny droplets (a water-in-oil emulsion).

For retail cream products, the fat is normally standardized to 35% (heavy cream for whipping), 18% or 10% (cream for coffee or cereal). Higher fat creams have also been produced for retail sale, a product known as double cream has a fat content of 55% and is quite thick. Creams for packaging and sale in the retail market must be pasteurized to ensure freedom from pathogenic bacteria. Whipping cream is not normally homogenized, as the high fat content will lead to extensive fat globule aggregation and clustering, which leads to excessive viscosity and a loss of whipping ability. This phenomena has been used, however, to produce a spoonable cream product to be used as a dessert topping. Lower fat creams (10% or 18%) can be homogenized, usually at lower pressure than whole milk.

Whipped Cream Structure

The structure of whipped cream is very similar to the fat and air structure that exists in ice cream. Cream is an emulsion with a fat content of 35-40%. When you whip a bowl of heavy cream, the agitation and the air bubbles that are added cause the fat globules to begin to partially coalesce in chains and clusters and adsorb to and spread around the air bubbles.

Diagram of whipped cream structureAs the fat partially coalesces, it causes one fat-stabilized air bubble to be linked to the next, and so on. The whipped cream soon starts to become stiff and dry appearing and takes on a smooth texture. This results from the formation of this partially coalesced fat structure stabilizing the air bubbles. The water, lactose and proteins are trapped in the spaces around the fat-stabilized air bubbles. The crystalline fat content is essential (hence whipping of cream is very temperature dependent) so that the fat globules partially coalesce into a 3-dimensional structure rather than fully coalesce into larger and larger globules that are not capable of structure-building. This is caused by the crystals within the globules that cause them to stick together into chains and clusters, but still retain the individual identity of the globules. Please see a further description of this process for details. If whipped cream is whipped too far, the fat will begin to churn and butter particles will form.

Partially-crystalline Fat Globule, Partially-coalesced

Below are scanning electron micrographs image of whipped cream. If you compare the schematics above with the "real thing" below, you should be able to fully understand whipped cream structure.

The structure of whipped cream as determined by scanning electron microscopy. A. Overview showing the relative size and prevalence of air bubbles (a) and fat globules (f); bar = 30 um. B. Internal structure of the air bubble, showing the layer of partially coalesced fat which has stabilized the bubble; bar = 5 um. C. Details of the partially coalesced fat layer, showing the interaction of the individual fat globules. Bar = 3 um. 

The structure of whipped cream as determined by scanning electron microscopyFat partial coalescence as it affects things like whipped cream and ice cream structure is an active area of our research here at the University of Guelph. Please see my publications for more details of our research.

Recombined Milk

Beverage milks can also be prepared by recombining skim milk powder and butter with water. This is often done in countries where there is not enough milk production to meet the demand for beverage milk consumption. The concept is simple. Skim milk powder is dispersed in water and allowed to hydrate. Butter is then emulsified into this mixture by either blending melted butter into the liquid mixture while hot, or by dispersing solid butter into the liquid through a high shear blender device. In some cases, a non-dairy fat source may also be used. The recombined milk product is then pasteurized, homogenized and packaged as in regular milk production. The final composition is similar to that of whole milk, approximately 9% milk solids-not-fat, and either 2% or 3.4% fat. The water source must be of excellent quality. The milk powder used for recombining must be of high quality and good flavour. Care must be taken to ensure adequate blending of the ingredients to prevent aggregation or lumping of the powder. Its dispersal in water is the key to success.

Chocolate Milk

An industry standard for the production of chocolate milk consists of:

  • 93% milk
  • 6.3% sugar
  • 0.65% cocoa powder
  • 0.05% carrageenan

The final product is usually standardized to either 2% fat or 1% fat (meaning, 2.15% or 1.1% fat in the milk before addition of other ingredients). The sugar, cocoa powder and carrageenan are dry blended, and added to cold milk with vigorous agitation, and then pasteurized.

Concentrated and Dried Dairy Products

Fluid milk contains approximately 88% water. Concentrated milk products are obtained through partial water removal. Dried dairy products have even greater amounts of water removed to usually less than 4%. The benefits of both these processes include an increased shelf-life, convenience, product flexibility, decreased transportation costs, and storage.

The following products will be discussed here:

Concentrated Dairy Products

  • Evaporated Skim or Whole Milk
  • Sweetened Condensed Milk
  • Condensed Buttermilk
  • Condensed Whey

Dried Dairy Products

  • Milk Powder
  • Whey Powder
  • Whey Protein Concentrates

The principles of evaporation and dehydration can be found in the Dairy Processing section.

Concentrated Dairy Products

Evaporated Skim or Whole Milk

After the raw milk is clarified and standardized, it is given a pre-heating treatment of 93-100° C for 10 to 25 min or 115-128° C for 1 to 6 min.. There are several benefits to this treatment:

  • increases the concentrated milk stability during sterilization; decreases the chance of coagulation taking place during storage
  • decreases the initial microbial load
  • modifies the viscosity of the final product
  • milk enters the evaporator already hot

Milk is then concentrated at low temperatures by vacuum evaporation. This process is based on the physical law that the boiling point of a liquid is lowered when the liquid is exposed to a pressure below atmospheric pressure. In this case, the boiling point is lowered to approximately 40-45° C. This results in little to no cooked flavour. The milk is concentrated to 30-40% total solids.

The evaporated milk is then homogenized to improve the milkfat emulsion stability. There are other benefits particular to this type of product:

  • increased white colour
  • increased viscosity
  • decreased coagulation ability

A second standardization is done at this time to ensure the proper salt balance is present. The ability of milk to withstand intensive heat treatment depends to a great degree on its salt balance.

The product at this point is quite perishable. The fat is easily oxidized and the microbial load, although decreased, is still a threat. The evaporated milk at this stage is often shipped by the tanker for use in other products.

In order to extend the shelf life, evaporated milk can be packaged in cans and then sterilized in an autoclave. Continuous flow sterilization followed by packaging under aseptic conditions is also done. While the sterilization process produces a light brown colouration, the product can be successfully stored for up to a year.

Sweetened Condensed Milk

Where evaporated milk uses sterilization to extend its shelf-life, sweetened condensed milk has an extended shelf-life due to the addition of sugar. Sucrose, in the form of crystals or solution, increases the osmotic pressure of the liquid. This in turn, prevents the growth of microorganisms.

The only real heat treatment (85-90° C for several seconds) this product receives is after the raw milk has been clarified and standardized. The benefits of this treatment include totally destroying osmophilic and thermophilic microorganisms, inactivating lipases and proteases, decreases fat separation and inhibits oxidative changes. Unfortunately it also affects the final product viscosity and may promote the defect age gelation.

The milk is evaporated in a manner similar to the evaporated milk. Although sugar may be added before evaporation, post evaporation addition is recommended to avoid undesirable viscosity changes during storage. Enough sugar is added so that the final concentration of sugar is approximately 45%.

The sweetened evaporated milk is then cooled and lactose crystallization is induced. The milk is inoculated, or seeded, with powdered lactose crystals, then rapidly cooled while being agitated. The lactose can crystalize without the seeding but there is the danger of forming crystals that are too large. This would result in a texture defect similar in ice cream called sandiness, which affects the mouthfeel. By seeding, the number of crystals increases and the size of those crystals decreases.

The product is packaged in smaller containers, such as cans, for retail sales and bulk containers for industrial sales.

Condensed Buttermilk

Buttermilk is a by-product of the butter industry. It can be evaporated on its own or it can be blended with skimmilk and dried to produce skimmilk powder. This blended product may oxidise readily due to the higher fat content. Condensed buttermilk is perishable and, therefore, the supply must be fresh and it must be stored cool.

Condensed Whey

In the process of cheesemaking, there is a lot of whey that needs to be disposed of. One of the ways of utilizing cheese whey is to condense it. The whey contains fat, lactose, ß -lactoglobulin, alpha-lactalbumin, and water. The fat is generally removed by centrifugation and churned as whey cream or used in ice cream. Evaporation is the first step in producing whey powder.

Dried Dairy Products

Milk Powder

Milk used in the production of milk powders is first clarified, standardized and then given a heat treatment. This heat treatment is usually more severe than that required for pasteurization. Besides destroying all the pathogenic and most of the spoilage microorganisms, it also inactivates the enzyme lipase which could cause lipolysis during storage.

The milk is then evaporated prior to drying for the following reasons:

  • less occluded air and longer shelf life for the powder
  • viscosity increase leads to larger powder particles
  • less energy required to remove part of water by evaporation; more economical

Homogenization may be applied to decrease the free fat content. Spray drying is the most used method for producing milk powders. After drying, the powder must be packaged in containers able to provide protection from moisture, air, light, etc. Whole milk powder can then be stored for long periods (up to about 6 months) of time at ambient temperatures.

Skim milk powder (SMP) processing is similar to that described above except for the following points:

  1. contains less milkfat (0.05-0.10%)
  2. heat treatment prior to evaporation can be more or less severe
  3. homogenization not required
  4. maximum shelf life extended to approximately 3 years

Low-heat SMP is given a pasteurization heat treatment and is used in the production of cheese, baby foods etc. High-heat SMP requires a more intense heat treatment in addition to pasteurization. This product is used in the bakery industry, chocolate industry, and other foods where a high degree of protein denaturation is required.

Instant milk powder is produced by partially rehydrating the dried milk powder particles causing them to become sticky and agglomerate. The water is then removed by drying resulting in an increased amount of air incorporated between the powder particles.

Whey Powder

Whey is the by-product in the manufacturing of cheese and casein. Disposing of this whey has long been a problem. For environmental reasons it cannot be discharged into lakes and rivers; for economical reasons it is not desirable to simply dump it to waste treatment facilities. Converting whey into powder has led to a number products that it can be incorporated into. It is most desirable, if and where possible, to use it for human food, as it contains a small but valuable protein component. It is also feasible to use it as animal feed. Between the pet food industry and animal feed mixers, hundred's of millions of pounds are sold every year. The feed industry may be the largest consumer of dried whey and whey products.

Whey powder is essentially produced by the same method as other milk powders. Reverse osmosis can be used to partially concentrate the whey prior to vacuum evaporation. Before the whey concentrate is spray dried, lactose crystallization is induced to decrease the hygroscopicity. This is accomplished by quick cooling in flash coolers after evaporation. Crystallization continues in agitated tanks for 4 to 24 h.

fluidized bed may be used to produce large agglomerated particles with free-flowing, non-hygroscopic, no caking characteristics.

Whey Protein Concentrates

Both whey disposal problems and high-quality animal protein shortages have increased world-wide interest in whey protein concentrates. After clarification and pasteurization, the whey is cooled and held to stabilize the calcium phosphate complex, which later decreases membrane fouling. The whey is commonly processed using ultrafiltration, although reverse osmosis, microfiltration, and demineralization methods can be used. During ultrafiltration, the low molecular weight compounds such as lactose, minerals, vitamins and nonprotein nitrogen are removed in the permeate while the proteins become concentrated in the retentate. After ultrafiltration, the retentate is pasteurized, may be evaporated, then dried. Drying, usually spray drying, is done at lower temperatures than for milk in order that large amounts of protein denaturation may be avoided.

Cultured Dairy Products and Cheese

Cheese - the short version

Traditionally, cheese was made as a way of preserving the nutrients of milk. In a simple definition, cheese is the fresh or ripened product obtained after coagulation and whey separation of milk, cream or partly skimmed milk, buttermilk or a mixture of these products. It is essentially the product of selective concentration of milk. Thousands of varieties of cheeses have evolved that are characteristic of various regions of the world.

Some common cheesemaking steps will be outlined here. Also included is a document entitled "Making Cheese at Home", which includes some helpful references, several simple cheese making procedures and information about sourcing cheese making supplies.

Please refer to the extended version, Cheese Making Technology, for further details.

Treatment of Milk for Cheesemaking

Like most dairy products, cheesemilk must first be clarifiedseparated and standardized. The milk may then be subjected to a sub-pasteurization treatment of 63-65° C for 15 to 16 sec. This thermization treatment results in a reduction of high initial bacteria counts before storage. It must be followed by proper pasteurization. While HTST pasteurization (72° C for 16 sec) is often used, an alternative heat treatment of 60° C for 16 sec may also be used. This less severe heat treatment is thought to result in a better final flavour cheese by preserving some of the natural flora. If used, the cheese must be stored for 60 days prior to sale, which is similar to the regulations for raw milk cheese.

Homogenization is not usually done for most cheesemilk. It disrupts the fat globules and increases the fat surface area where casein particles adsorb. This reults in a soft, weak curd at renneting and increased hydrolytic rancidity.

Additives

The following may all be added to the cheese milk:

  • Calcium chloride
  • nitrates
  • colour
  • hydrogen peroxide
  • lipases

Calcium chloride is added to replace calcium redistributed during pasteurization. Milk coagulation by rennet during cheese making requires an optimum balance among ionic calcium and both soluble insoluble calcium phosphate salts. Because calcium phosphates have reverse solubility with respect to temperature, the heat treatment from pasteurization causes the equilibrium to shift towards insoluble forms and depletes both soluble calcium phosphates and ionic calcium. Near normal equilibrium is restored during 24 - 48 hours of cold storage, but cheese makers can't wait that long, so CaCl2 is added to restore ionic calcium and improve rennetability. The calcium assists in coagulation and reduces the amount of rennet required.

Sodium or potassium nitrate is added to the milk to control the undesirable effects of Clostridium tyrobutyricum in cheeses such as Edam, Gouda, and Swiss.

Because milk colour varies from season to season, colour may added to standardize the colour of the cheese throughout the year. Annato, Beta-carotene, and paprika are used.

The addition of hydrogen peroxide is sometimes used as an alternative treatment for full pasteurization.

Lipases, normally present in raw milk, are inactivated during pasteurization. The addition of kid goat lipases are common to ensure proper flavour development through fat hydrolysis.

Inoculation and Milk Ripening

The basis of cheesemaking relies on the fermentation of lactose by lactic acid bacteria (LAB). LAB produce lactic acid which lowers the pH and in turn assists coagulation, promotes syneresis, helps prevent spoilage and pathogenic bacteria from growing, contributes to cheese texture, flavour and keeping quality. LAB also produce growth factors which encourages the growth of non-starter organisms, and provides lipases and proteases necessary for flavour development during curing. Further information on LAB and starter cultures can be found in the microbiology section.

After innoculation with the starter culture, the milk is held for 45 to 60 min at 25 to 30° C to ensure the bacteria are active, growing and have developed acidity. This stage is called ripening the milk and is done prior to renneting.

Milk Coagulation

Coagulation is essentially the formation of a gel by destabilizing the casein micelles causing them to aggregate and form a network which partially immobilizes the water and traps the fat globules in the newly formed matrix. This may be accomplished with:

  • enzymes
  • acid treatment
  • heat-acid treatment

Enzymes

Chymosin, or rennet, is most often used for enzyme coagulation.

Acid Treatment

Lowering the pH of the milk results in casein micelle destabilization or aggregation. Acid curd is more fragile than rennet curd due to the loss of calcium. Acid coagulation can be achieved naturally with the starter culture, or artificially with the addition of gluconodeltalactone. Acid coagulated fresh cheeses may include Cottage cheese, Quark, and Cream cheese.

Heat-Acid Treatment

Heat causes denaturation of the whey proteins. The denatured proteins then interact with the caseins. With the addition of acid, the caseins precipitate with the whey proteins. In rennet coagulation, only 76-78% of the protein is recovered, while in heat-acid coagulation, 90% of protein can be recovered. Examples of cheeses made by this method include Paneer, Ricotta and Queso Blanco.

Curd Treatment

After the milk has gel has been allowed to reach the desired firmness, it is carefully cut into small pieces with knife blades or wires. This shortens the distance and increases the available area for whey to be released. The curd pieces immediately begin to shrink and expel the greenish liquid called whey. This syneresis process is further driven by a cooking stage. The increase in temperature causes the protein matrix to shrink due to increased hydrophobic interactions, and also increases the rate of fermentation of lactose to lactic acid. The increased acidity also contributes to shrinkage of the curd particles. The final moisture content is dependant on the time and temperature of the cook stage. This is important to monitor carefully because the final moisture content of the curd determines the residual amount of fermentable lactose and thus the final pH of the cheese after curing.

When the curds have reached the desired moisture and acidity they are separated from the whey. The whey may be removed from the top or drained by gravity. The curd-whey mixture may also be placed in moulds for draining. Some cheese varieties, such as Colby, Gouda, and Brine Brick include a curd washing which increases the moisture content, reduces the lactose content and final acidity, decreases firmness, and increases openness of texture.

Curd handling from this point on is very specific for each cheese variety. Salting may be achieved through brine as with Gouda, surface salt as with Feta, or vat salt as with Cheddar. To achieve the characteristics of Cheddar, a cheddaring stage (curd manipulation), milling (cut into shreds), and pressing at high pressure are crucial.

Cheese Ripening

Except for fresh cheese, the curd is ripened, or matured, at various temperatures and times until the characteristic flavour, body and texture profile is achieved. During ripening, degradation of lactose, proteins and fat are carried out by ripening agents. The ripening agents in cheese are:

  • bacteria and enzymes of the milk
  • lactic culture
  • rennet
  • lipases
  • added moulds or yeasts
  • environmental contaminants

Thus the microbiological content of the curd, the biochemical composition of the curd, as well as temperature and humidity affect the final product. This final stage varies from weeks to years according to the cheese variety.

Making Cheese at Home

by Dr. A.R. Hill

Department of Food Science

University of Guelph, ON N1G 2W1

Email: Dr. A.R. Hill

Cheese is made from the milk of goats, sheep, buffalo, reindeer, camel, llama, and yak but is usually made from cow's milk. Cow's milk is about 88% water and the remainder is fat, protein, sugar, minerals and vitamins. In the process of cheese-making, most of the protein, fat and some minerals and vitamins are concentrated and separated as a solid. The remaining liquid, called 'whey', contains most of the sugar and water and some protein, minerals and vitamins. Whey is utilized in foods and feeds or disposed of as waste.

There are two principal agents which bring about the concentration and separation of protein and fat to make cheese, namely, bacterial culture and coagulating enzyme.

Bacterial culture

Bacteria are often responsible for food spoilage but there are also many useful types. During the manufacture of cheese and other cultured dairy products lactic acid bacteria change the milk sugar to lactic acid. The acid acts as a preservative by inhibiting undesirable types of bacteria, helps remove water from the curd (formation of curd is described in the next section) and is important to the development of cheese texture. The lactic acid bacteria and other microorganisms which happen to be present in the cheese contribute enzymes which break down fats, proteins and sugar during aging to produce flavours characteristic of particular cheese varieties. Lactic acid bacteria are naturally present in milk, and cheese can be made by holding fresh milk in a warm environment. However, this process is slow and cheese quality tends to be inconsistent. It is recommended that the milk be pasteurized by heating at 60-62C (140-144F) for 30 min . This heat treatment will destroy most lactic acid bacteria in the milk and will also destroy pathogenic bacteria which may cause food illness. Note that over pasteurization will prevent proper coagulation. Most store bought milk is unsuitable for cheese making because it has received too much heat treatment.

After pasteurization the milk is cooled to 32-37C (89.6-98.6F) and lactic acid bacteria are added to the milk. The suspension of bacteria is called a 'culture' and the process of adding the culture to the milk is called 'inoculation'. The culture may be a frozen or freeze-dried concentrate of bacterial cells or it could be cultured milk (milk in which lactic acid bacteria have been allowed to grow). Different bacterial cultures are recommended for specific types of cheese but most types can be made using fresh, plain yoghurt or buttermilk as a culture. If yoghurt is used, the milk should be inoculated at 37C. Buttermilk contains gas forming bacteria and may cause the development of small eyes in some cheese. In addition to bacteria, some types of cheese such as 'blue' and 'camembert' are inoculated with mould to develop characteristic appearance and flavour. 

Coagulating enzymes 

Proteins can be thought of as long microscopic chains. Various food products such as jello, jams and cheese depend on the ability of protein chains to intertwine and form a mesh-like network. The formation of this network is called 'coagulation'. When proteins coagulate in water, they trap water in the network and change the liquid to a semisolid gel. In cheese-making gelation is caused by an enzyme, 'rennet'. When rennet is added to warm milk, the liquid milk is transformed into a soft gel. When the gel is firm enough, it is cut into small pieces, 0.5-1.0 cm square (1/4-3/8 inch) called 'curds'. 

 Exceptions

Certain types of cheese such as some types of Queso Blanco (Latin American countries) and Paneer (India) are made without bacterial cultures and without rennet. In these types, curd is formed by adding vinegar (or other acid juices) to hot milk. A procedure for heat-acid precipitated Queso Blanco is included in this booklet because it is one of the most simple varieties to make and has the advantage that all the milk proteins including proteins normally lost in the whey are included in the cheese. Some fresh cheese (i.e. cheese which are eaten immediately after manufacture) such as Cottage cheese and quark are made with little or no rennet. In these cheese, coagulation is caused by high acid development by the bacterial culture. A procedure for fresh cheese or European style Cottage cheese is included.

Cheese-making supplies and training

For the home cheese maker, a start up set of supplies should include: a pasteuriser, cheese mould, cheese press, dairy thermometer or any food grade thermometer for the range of 0 to 100C, and cheese cloth. Bacterial cultures and rennet can sometimes be purchased in natural food stores.

Small scale cheese making equipment and other supplies, including literature, can be obtained from New England Cheese Making Supply Company, 85 Main St., Ashfield, MA 01330 (413-628-3808; Fax: 413-628-4061).

Cheese making supplies and one day courses in cheese making are available from Glengarry Cheesemaking and Dairy Supplies, RR#2,Alexandria, Ontario, K0C 1A0 Phone: (613) 525-3133, Fax: (613) 525-3394, glengarrycheesemaking.on.ca

Cultures, rennet, cheesemaking equipment and other supplies are available from Danlac, 466 Summerwood Place, Airdrie, Alberta, T4B 1W5, Phone 403-948-4644, Fax 403-948-4643, www.danlac.com, e-mail Egon Skovmose

Freeze dried cultures and rennet in tablet form are available in large orders from Chr. Hansens Laboratories Ltd., 1146 Aerowood Drive, Mississauga, L4N 1Y5, 905-625-8157, and from Rhodia Canada Inc., 2000 Argentia Road, Plaza 3, Suite 400, Mississauga, Ontario, L5N 1V9, Phone 905-821-4450, Fax 905-821-9339. Call and ask about retail distributors closest to you.

Some References

 Alfa-Laval. Dairy Handbook. Alfa-Laval, Food Engineering AB. P.O. Box 65, S-221 00 Lund, Sweden. [Well illustrated text. Excellent introduction to dairy technology].

American Public Health Association, Standard Methods for the examination of dairy products. 1015 Eighteenth St. NW, Washington, D.C.

Berger, W., Klostermeyer, H., Merkenich, K. and Uhlmann, G. 1989. Processed Cheese Manufacture, A JOHA guide. BK Ladenburg, Ladenburg.

Carroll, R. and Carroll, R. 1982. Cheese making made easy. Storey Communications Inc., Ponnal, Vermont. [Well illustrated manual for small and home cheese making operations]

Kosikowski, F.V. and Mistry, V.V. 1997. Cheese and Fermented Milk Foods, 3rd Edition, F.V. Kosikowski and Associates, Brooktondale, NY.

Law, B. 1999. Technology of cheese making Sheffield Academic Press, Sheffield, UK.

Scott, R., Robinson, R.K. and Wilbey, R.A. 1998. Cheese making Practice. 3rd Edition. Applied Science. Publ. Ltd., London.

Troller, J.A. 1993. Sanitation in Food Processing. 2nd Edition. Academic Press. New York.

Walstra, P., Geurts, T.J., Noomen, A., Jellema, A. and van Boekel, M.A.. 1999. Dairy Technology. Marcel Dekker Inc. New York, NY. 

Websites  

Food Science University of Guelph: http://www.uoguelph.ca/foodscience/content/dairy-education-series

Centre For Dairy Research, Madison, WI. www.cdr.wisc.edu/

Canadian Dairy Information Centre, www.dairyinfo.gc.ca

CheeseNet , cheesenet.wgx.com/

 

SWEET CURD BRICK CHEESE

 Brick cheese is a semi-soft ripened cheese. Its texture and flavour is derived from the action of bacteria which grow on the surface of the cheese. It is usually formed in the shape of a loaf.

Procedure  

  1. Pasteurize whole milk by heating at 62C for 30 min. Do not over pasteurize.
  2. Cool milk to 30C and add 25 ml of low temperature (sometimes called mesophyllic cheese starter and 2 ml of rennet per 10 kg of milk. (Note: a bacterial smear should develop spontaneously during ripening in the wet room (Step 12), however, you can increase the success rate and uniformity by adding a smear culture with the lactic culture. Suitable cultures are available from many culture suppliers)
  3. When the milk gel breaks cleanly on a knife (about 25 minutes after adding rennet), cut the gel into 1/4" cubes.
  4. Stir gently for 10 minutes.
  5. Begin cooking. Slowly raise the temperature to 36C. This should take 20 minutes.
  6. Remove most of the whey but leave enough to cover the curd.
  7. Add water at 36C to wash the curd. Add the equivalent of half the weight of the milk and agitate gently for 20 minutes.
  8. Drain most of the whey but leave enough to cover the curd.
  9. Pour the curd and remaining wash water into the hoops.
  10. Turn the cheese after the first 30 minutes and then every hour for 4 hours (5 turns in all).
  11. Rub salt over the entire surface of the cheese.
  12. Store cheese in a wet room (95% humidity) at 12-15C to develop a smear (bacterial growth on surface) for about 2 weeks. Turn the cheese every second or third day and wash with 4% brine. In the absence of a wet room you can put the cheese in a covered but not sealed container. The interior must remain moist and have some air exchange.
  13. Wash cheese to remove smear, dry and vacuum package or coat with paraffin. Store at 5C for further ripening. Flavour should be optimum after about 4 weeks of ripening.

  

EUROPEAN STYLE COTTAGE CHEESE

European style cottage cheese has small curds and is often heavily creamed. The milk is coagulated by a lactic culture without rennet or other coagulating enzyme. 

Procedure   

  1. Skim as much cream as possible from fresh milk.
  2. Pasteurize the skim milk at 62C for 30 minutes and the cream at 70C for 30 minutes.
  3. Cool the skim milk to 32C.
  4. Add a low temperature cheese starter at the rate of 5%, i.e. 0.5 kg starter for every 10 kg of milk. Let milk set for 4-6 hours until a soft gel is formed. When broken with a knife or a blunt object the curd should break cleanly and the broken portion should fill up with clear whey. Alternatively, 1% of culture may be used with a setting time of 12-18 hours.
  5. Stir gently and heat slowly to 52C. Hold at this temperature until curd is firm, about 30 minutes.
  6. Drain most of the whey, replace it with cold water and agitate gently for 15 minutes to leach the acid flavour from the curd. Washing may be omitted if you prefer an acid cheese.
  7. Drain the remaining whey and wash water.
  8. Add cream or cream dressing to the curd according to taste.  

 Note: It may be convenient to drain the curd in a cloth bag, in which case, it could be washed by soaking the whole bag in cold water for 15 minutes.

 

NO-RENNET QUESO BLANCO (LATIN AMERICAN WHITE CHEESE)

Heat-acid or no-rennet Queso Blanco is a white, semi-hard cheese made without culture or rennet. It is eaten fresh and may be flavoured with peppers, caraway, onions, etc. It belongs to a family of "frying cheeses" which do not melt and may be deep fried or barbecued to a golden brown for a tasty snack. Deep fried Queso Blanco may be steeped in a sugar syrup for a dessert dish or added to soup as croutons. The procedure given here is similar to the manufacture of Indian Paneer and Channa which is made by adding acid to hot milk. Ricotta cheese is also made by heat-acid precipitation of proteins from blends of milk and whey. Latin American white cheese is also made by renneting whole milk with little or no bacterial culture. Rennet Queso Blanco is also useful as a frying cheese because its lack of acidity gives it low meltability.

Procedure  

  1. Heat milk to 80C for 20 minutes.
  2. Add vinegar (5% acetic acid) at the rate of about 175 ml per 5 kg of milk. Vinegar should be diluted in two equal volumes of water and then added slowly to the hot milk until the whey is semi-clear and the curd particles begin to mat together and become slightly stretchy. You should be able to stretch a piece of curd about 1 cm before it breaks. It may not be necessary to add all of the vinegar.
  3. Separate the curd by filtering through a cloth bag until free whey is removed.
  4. Work in salt (about 1%) and spices to taste.
  5. Press the curd (high pressure is not required).
  6. Package curd in boilable bags (vacuum package if possible) and place in boiling water for 5 minutes to sterilize the surface and prevent mould growth.
  7. Queso Blanco may keep for several weeks if properly packed but should be eaten as fresh as possible.

 

RICOTTA CHEESE RECIPE

  1. Heat fresh whey to 85C. Heating must begin immediately after the whey is removed from the curd to prevent further acidification by the lactic acid bacteria. Some small curd particles will form.
  2. Slowly add about 10 ml of vinegar per litre of whey with gentle agitation. You will see more curd particles forming and the whey will become less 'milky'.
  3. Pour into a cloth to separate the curds. After the curd is dripped dry it is ready to eat. Use it in lasagna or eat as a side dish along with the main course or use it like cottage cheese in salads. 

Notes 

Before heating the whey, you can add up to 10% whole milk (that is, 100 ml of milk in 1 litre of whey). Addition of milk will help form larger curds which are easier to separate and the cheese will have a better texture. You also have to add more vinegar depending on the amount of milk. Continue adding vinegar until the whey is quite clear. By adding the vinegar slowly over a time period of about 5 minutes you will obtain better quality curd and it will be easier to know when to stop.

Cheese - the long version

Please jump to the top page of our extensive Cheese Making Technology section.

Yogurt and Fermented Beverages

Yogurt (also spelled yogourt or yoghurt) is a semi-solid fermented milk product that originated centuries ago and has evolved from many traditional Eastern European (e.g., Turkish and Bulgarian) products. The word is from the Turkish Yogen, meaning thick. It's popularity has grown and is now consumed in most parts of the world. Although the consistency, flavour and aroma may vary from one region to another, the basic ingredients and manufacturing are essentially consistent:

Ingredients

Although milk of various animals has been used for yogurt production in various parts of the world, most of the industrialized yogurt production uses cow's milk. Whole milk, partially skimmed milk, skim milk or whole milk enriched with cream may be used, to lower or raise the fat content as desired. In order to ensure the development of the yogurt culture the following criteria for the raw milk must be met:

  • low bacteria count
  • free from antibiotics, sanitizing chemicals, mastitis milk, colostrum, and rancid milk
  • no contamination by bacteriophages

Other yogurt ingredients may include some or all of the following:
Other Dairy Products: concentrated skim milk, nonfat dry milk, whey, lactose. These products are often used to increase the nonfat solids content. Reconstitution of these milk solids ingredients with water can also be used to standardize the solids-not-fat content, if permitted based on regulations of the legal jurisdiction.
Sweeteners: glucose or sucrose, high-intensity sweeteners (e.g. aspartame)
Stabilizers: gelatin, carboxymethyl cellulose, locust bean gum, guar, alginates, carrageenans, whey protein concentrate
Flavours
Fruit Preparations
: including natural and artificial flavouring, colour

Starter culture

The starter culture for most yogurt production in North America is a symbiotic blend of Streptococcus thermophilus (ST) and Lactobacillus delbrueckii subsp. bulgaricus (LB). Although they can grow independently, the rate of acid production is much higher when used together than either of the two organisms grown individually. ST grows faster and produces both acid and carbon dioxide. The formate and carbon dioxide produced stimulates LB growth. On the other hand, the proteolytic activity of LB produces stimulatory peptides and amino acids for use by ST. These microorganisms are ultimately responsible for the formation of typical yogurt flavour and texture. The yogurt mixture coagulates during fermentation due to the drop in pH. The streptococci are responsible for the initial pH drop of the yogurt mix to approximately 5.0. The lactobacilli are responsible for a further decrease to pH 4.5. The following fermentation products contribute to flavour:

  • lactic acid
  • acetaldehyde
  • acetic acid
  • diacetyl

Manufacturing Method

The milk is clarified and separated into cream and skim milk, then standardized with other dairy ingredients to achieve the desired fat and milk solids-not-fat content. The various ingredients are then blended together in a mix tank equipped with a powder funnel and an agitation system. The mixture is then pasteurized using a continuous plate heat exchanger for 30 min at 85° C or 10 min at 95° C. These heat treatments, which are much more severe than fluid milk pasteurization, are necessary to achieve the following:

  • produce a relatively sterile and conducive environment for the starter culture
  • denature and coagulate whey proteins to enhance the viscosity and texture; this effect results from modification of the surface of the casein micelle so that milk thickens in a structurally-different manner than it would in a non-heated acid gel

The mix is then homogenized using high pressures of 2000-2500 psi. Besides thoroughly mixing the stabilizers and other ingredients, homogenization also prevents creaming and wheying off during incubation and storage. Stability, consistency and body are enhanced by homogenization. Once the homogenized mix has cooled to an optimum growth temperature, the yogurt starter culture is added.

A ratio of 1:1, ST to LB, inoculation is added to the jacketed fermentation tank. A temperature of 43° C is maintained for 2-2.5 h under quiescent (no agitation) conditions. This temperature is a compromise between the optimums for the two microorganisms (ST 39° C; LB 45° C). The titratable acidity is carefully monitored until the TA is 0.85 to 0.90% (pH 4.5). At this time the jacket is replaced with cool water and agitation begins, both of which stop the fermentation. The coagulated product is cooled to 5-22° C, depending on the product. Fruit and flavour may be incorporated at this time, then packaged. The product is now cooled and stored at refrigeration temperatures (5° C) to slow down the physical, chemical and microbiological degradation.

Yogurt Products

There are two types of plain yogurt:

  • Stirred style yogurt
  • Set style yogurt - The above description is essentially the manufacturing procedures for stirred style. In set style, the yogurt is packaged immediately after inoculation with the starter and is incubated in the packages.

Other yogurt products include:

  • Sweetened stirred style yogurt with fruit preparation
  • Fruit-on-the-bottom set style: - fruit mixture is layered at the bottom followed by inoculated yogurt, incubation occurs in the sealed cups
  • Soft-serve and Hard Pack frozen yogurt (see Frozen desserts section)
  • Probiotic yogourts: it has become quite common to add probiotic bacterial strains to yogourt (those with proven health-promoting benefits, in addition to ST and LB. These could include Lactobacillus acidophilus, Lactobacilus casei, or Bifidobacterium spp. When probiotics are added, it has also become common to add ingredients known as prebiotics, such as inulin, which will, after digestion, aid in the growth of the probiotics in the colon. Inulin, for example, is a polymer of fructose (fructo-oligosaccharide) that is indigestible in the small intestine because we do not have sufficient enzymes to cleave the fructose bonds. However, in the colon, bacterial enzymes can easily release free fructose, which has been shown to positively affect the growth of the probiotic organisms. 

Yogurt Beverages

Drinking yogurt is essentially stirred yogurt that has a sufficiently low total solids content to achieve a liquid or pourable consistency and which has undergone homogenization to further reduce the viscosity. Fat and solids-not-fat can both be standardized. If the desired snf level in the product is lower than it is in whole milk or skimmed milk, then dilution with water of fruit juices may be used, depending on the requirements of the legal jurisdiction. Sweeteners, flavouring and colouring are invariably added. Heat treatment may be applied to extend the storage life, although this would reduce or eliminate the viable yogourt culture organisms. HTST pasteurization with aseptic processing will give a shelf life of several weeks at 2-4°C, while UHT processes with aseptic packaging will give a shelf life of several weeks at room temperature.

Other Fermented Milk Beverages

Cultured Buttermilk

This product was originally the fermented byproduct of butter manufacture, but today it is more common to produce cultured buttermilks from skim or whole milk. The culture most frequently used in Loctococcus lactis, perhaps also subsp. cremoris or diacetylactis. Milk is usually heated to 95°C and cooled to 20-25°C before the addition of the starter culture. Starter is added at 1-2% and the fermentation is allowed to proceed for 16-20 hours, to an acidity of 0.9% lactic acid. This product is frequently used as an ingredient in the baking industry, in addition to being packaged for sale in the retail trade.

Acidophilus milk

Acidophilus milk is a traditional milk fermented with Lactobacillus acidophilus (LA), which has been thought to have therapeutic benefits in the gastrointestinal tract. Skim or whole milk may be used. The milk is heated to high temperature, e.g., 95°C for 1 hour, to reduce the microbial load and favour the slow growing LA culture. Milk is inoculated at a level of 2-5% and incubated at 37°C until coagulated. Some acidophilus milk has an acidity as high as 1% lactic acid, but for therapeutic purposes 0.6-0.7% is more common.

Another variation has been the introduction of a sweet acidophilus milk, one in which the LA culture has been added but there has been no incubation. It is thought that the culture will reach the GI tract where its therapeutic effects will be realized, but the milk has no fermented qualities, thus delivering the benefits without the high acidity and flavour, considered undesirable by some people. 

Sour Cream

Cultured cream usually has a fat content between 12-30%, depending on the required properties. The starter is similar to that used for cultured buttermilk. The cream after standardization is usually heated to 75-80°C and is homogenized at >13 MPa to improve the texture. Inoculation and fermentation conditions are also similar to those for cultured buttermilk, but the fermentation is stopped at an acidity of 0.6%. 

Others

There are a great many other fermented dairy products, including kefir, koumiss, beverages based on bulgaricus or bifidus strains, labneh, and a host of others. Many of these have developed in regional areas and, depending on the starter organisms used, have various flavours, textures, and components from the fermentation process, such as gas or ethanol.

Butter Manufacture

Butter is essentially the fat of the milk. It is usually made from sweet cream and is salted. However, it can also be made from acidulated or bacteriologically soured cream and saltless (sweet) butters are also available. Well into the 19th century butter was still made from cream that had been allowed to stand and sour naturally. The cream was then skimmed from the top of the milk and poured into a wooden tub. Buttermaking was done by hand in butter churns. The natural souring process is, however, a very sensitive one and infection by foreign micro-organisms often spoiled the result. Today's commercial buttermaking is a product of the knowledge and experience gained over the years in such matters as hygiene, bacterial acidifying and heat treatment, as well as the rapid technical development that has led to the advanced machinery now used. The commercial cream separator was introduced at the end of the 19th century, the continuous churn had been commercialized by the middle of the 20th century.

Definitions and Standards

Milkfat

the lipid components of milk, as produced by the cow, and found in commercial milk and milk-derived products, mostly comprised of triglyceride.

Butterfat

almost synonymous with milkfat; all of the fat components in milk that are separable by churning.

Anhydrous Milkfat (AMF) 

the commercially- prepared extraction of cow's milkfat, found in bulk or concentrated form (comprised of 100% fat, but not necessarily all of the lipid components of milk).

Butteroil 

synonymous with anhydrous milkfat; (conventional terminology in the fats and oils field differentiates an oil from a fat based on whether it is liquid at room temp. or solid, but very arbitrary).

Butter 

a water-in-oil emulsion, comprised of >80% milkfat, but also containing water in the form of tiny droplets, perhaps some milk solids-not-fat, with or without salt (sweet butter); texture is a result of working/kneading during processing at appropriate temperatures, to establish fat crystalline network that results in desired smoothness (compare butter with melted and recrystallized butter); used as a spread, a cooking fat, or a baking ingredient.

The principal constituents of a normal salted butter are fat (80 - 82%), water (15.6 - 17.6%), salt (about 1.2%) as well as protein, calcium and phosphorous (about 1.2%). Butter also contains fat-soluble vitamins A, D and E.

Butter should have a uniform colour, be dense and taste clean. The water content should be dispersed in fine droplets so that the butter looks dry. The consistency should be smooth so that the butter is easy to spread and melts readily on the tongue.

Overview of the Buttermaking Process

Buttermaking Process Flowchart

The buttermaking process involves quite a number of stages. The continuous buttermaker has become the most common type of equipment used.

The cream can be either supplied by a fluid milk dairy or separated from whole milk by the butter manufacturer. The cream should be sweet (pH >6.6, TA = 0.10 - 0.12%), not rancid and not oxidized.

If the cream is separated by the butter manufacturer, the whole milk is preheated to the required temperature in a milk pasteurizer before being passed through a separator. The cream is cooled and led to a storage tank where the fat content is analyzed and adjusted to the desired value, if necessary. The skim milk from the separator is pasteurized and cooled before being pumped to storage. It is usually destined for concentration and drying.

From the intermediate storage tanks, the cream goes to pasteurization at a temperature of 95oC or more. The high temperature is needed to destroy enzymes and micro-organisms that would impair the keeping quality of the butter.

If ripening is desired for the production of cultured butter, mixed cultures of S. cremoris, S. lactis diacetyl lactis, Leuconostocs, are used and the cream is ripened to pH 5.5 at 21oC and then pH 4.6 at 13oC. Most flavour development occurs between pH 5.5 - 4.6. The colder the temperature during ripening the more the flavour development relative to acid production. Ripened butter is usually not washed or salted.

In the aging tank, the cream is subjected to a program of controlled cooling designed to give the fat the required crystalline structure. The program is chosen to accord with factors such as the composition of the butterfat, expressed, for example, in terms of the iodine value which is a measure of the unsaturated fat content. The treatment can even be modified to obtain butter with good consistency despite a low iodine value, i.e. when the unsaturated proportion of the fat is low.

As a rule, aging takes 12 - 15 hours. From the aging tank, the cream is pumped to the churn or continuous buttermaker via a plate heat exchanger which brings it to the requisite temperature. In the churning process the cream is violently agitated to break down the fat globules, causing the fat to coagulate into butter grains, while the fat content of the remaining liquid, the buttermilk, decreases.

Thus the cream is split into two fractions: butter grains and buttermilk. In traditional churning, the machine stops when the grains have reached a certain size, whereupon the buttermilk is drained off. With the continuous buttermaker the draining of the buttermilk is also continuous.

After draining, the butter is worked to a continuous fat phase containing a finely dispersed water phase. It used to be common practice to wash the butter after churning to remove any residual buttermilk and milk solids but this is rarely done today.

Salt is used to improve the flavour and the shelf-life, as it acts as a preservative. If the butter is to be salted, salt (1-3%) is spread over its surface, in the case of batch production. In the continuous buttermaker, a salt slurry is added to the butter. The salt is all dissolved in the aqueous phase, so the effective salt concentration is approximately 10% in the water.

After salting, the butter must be worked vigorously to ensure even distribution of the salt. The working of the butter also influences the characteristics by which the product is judged - aroma, taste, keeping quality, appearance and colour. Working is required to obtain a homogenous blend of butter granules, water and salt. During working, fat moves from globular to free fat. Water droplets decrease in size during working and should not be visible in properly worked butter. Overworked butter will be too brittle or greasy depending on whether the fat is hard or soft. Some water may be added to standardize the moisture content. Precise control of composition is essential for maximum yield.

The finished butter is discharged into the packaging unit, and from there to cold storage.

The background science of butter churning

The fat globule

Milk fat is comprised mostly of triglycerides, with small amounts of mono- and diglycerides, phospholipids, glycolipids, and lipo-proteins. The trigylcerides (98% of milkfat) are of diverse composition with respect to their component fatty acids, approximately 40% of which are unsaturated fat firmness varies with chain length, degree of unsaturation, and position of the fatty acids on the glycerol. Fat globules vary from 0.1 - 10 micron in diameter. The fat globule membrane is comprised of surface active materials: phospholipids and lipoproteins.

Fat globules typically aggregate in three ways:

  • flocculation
  • coalescence
  • partial coalescence

Whipping and Churning

Many milk products foam easily. Skim milk foams copiously with the amount of foam being very dependent on the amount of residual fat - fat depresses foaming. The foaming agents are proteins, the amount of proteins in the foam are proportional to their contents in milk. Foaming is decreased in heat treated milk, possibly because denaturated whey proteins produce a more brittle protein layer at the interface. Fats tend to spread over the air-water interface and destabilize the foam; very small amounts of fats (including phospholipids) can destabilize a foam.

During the interaction of fat globules with air bubbles the globule may also be disrupted (this is the only way that fat globules can be disrupted without considerable energy input). Disruption of the fat globule by interaction between the fat globule and air bubbles is rare except in the case of newly formed air bubbles where the air-water interfacial layer is still thin. If part of the fat globule is solid, churning will result, hence the term "flotation churning" -from repeated rupturing of air bubbles and resulting coalescence of the adsorbed fat.

In spite of the above comments on the destabilization of foams by fat, milk fat is essential for the formation of stable whipped products which depend on the interaction between fat globules, air bubbles and plasma components (esp. proteins).

When cream is beaten air cells form more slowly partly because of higher viscosity and partly because the presence of fat causes immediate collapse of most of the larger bubbles. If most of the fat is liquid (high temperature) the fat globule membrane is not readily punctured and churning does not occur -at cold temperature where solid fat is present, churning (clumping) of the fat globule takes place. Clumps of globules begin to associate with air bubbles so that a network of air bubbles and fat clumps and globules form entrapping all the liquid and producing a stable foam. If beating continues the fat clumps increase in size until they become too large and too few to enclose the air cells, hence air bubbles coalesce, the foam begins to "leak" and ultimately butter and butter milk remain.

Crystallizing of the milkfat during aging

Before churning, cream is subjected to a program of cooling designed to control the crystallization of the fat so that the resultant butter has the right consistency. The consistency of butter is one of its most important quality-related characteristics, both directly and indirectly, since it affects the other characteristics - chiefly taste and aroma. Consistency is a complicated concept and involves properties such as hardness, viscosity, plasticity and spreading ability.

The relative amounts of fatty acids with high melting point determine whether the fat will be hard or soft. Soft fat has a high content of low-melting fatty acids and at room temperature this fat has a large continuous fat phase with a low solid phase, i.e. crystallized, high-melting fat. On the other hand, in a hard fat, the solid phase of high-melting fat is much larger than the continuous fat phase of low-melting fatty acids.

In buttermaking, if the cream is always subjected to the same heat treatment it will be the chemical composition of the milk fat that determines the butter's consistency. A soft milk fat will make a soft and greasy butter, whereas butter from hard milk fat will be hard and stiff. If, however, the heat treatment is modified to suit the iodine value of the fat, the consistency of the butter can be optimized. For the heat treatment regulates the size of the fat crystals, and the relative amounts of solid fat and the continuous phase - the factors that determine the consistency of the butter.

Pasteurization causes the fat in the fat globules to liquefy. And when the cream is subsequently cooled a proportion of the fat will crystallize. If cooling is rapid, the crystals will be many and small; if gradual the yield will be fewer but larger crystals. The more violent the cooling process, the more will be the fat that will crystallize to form the solid phase, and the less the liquid fat that can be squeezed out of the fat globules during churning and working.

The crystals bind the liquid fat to their surface by adsorption. Since the total surface area is much greater if the crystals are many and small, more liquid fat will be adsorbed than if the crystals were larger and fewer. In the former case, churning and working will press only a small proportion of the liquid fat from the fat globules. The continuous fat phase will consequently be small and the butter firm. In the latter case, the opposite applies. A larger amount of liquid fat will be pressed out; the continuous phase will be large and the butter soft.

So by modifying the cooling program for the cream, it is possible to regulate the size of the crystals in the fat globules and in this way influence both the magnitude and the nature of the important continuous fat phase.

 

Tempering Treatment of Hard Fat. For optimum consistency where the iodine value is low, i.e. the butterfat is hard, as much as possible of the hardest fat must be converted to as few crystals as possible, so that little of the liquid fat is bound to the crystals. The liquid fat phase in the fat globules will thereby be maximized and much of it can be pressed out during churning and working, resulting in butter with a relatively large continuous phase of liquid fat and with the hard fat concentrated to the solid phase.

The program of treatment necessary to achieve this result comprises the following stages:

  • rapid cooling to about 8oC and storage for about 2 hours at this temperature;
  • heating gently to 20 - 21oC and storage at this temperature for at least 2 hours (water at 27 - 29oC is used for heating);
  • cooling to about 16oC.

Cooling to about 8oC causes the formation of a large number of small crystals that bind fat from the liquid continuous phase to their surface.

When the cream is gently heated to 20 - 21oC the bulk of the crystals melt, leaving only the hard fat crystals which, during the storage period at 20 - 21oC, grow larger.

After 1 - 2 hours most of the hard fat has crystallized, binding little of the liquid fat. By dropping the temperature now to about 16oC, the hardest portion of the fat will be fixed in crystal form while the rest is liquefied. During the holding period at 16oC, fat with a melting point of 16oC or higher will be added to the crystals. The treatment has thus caused the high-melting fat to collect in large crystals with little adsorption of the low-melting liquid fat, so that a large proportion of the butter oil can be pressed out during churning and working.

 

Tempering Treatment of Medium Hard Fat. With an increase in the iodine value, the heating temperature is accordingly reduced from 20-21oC. Consequently a larger number of fat crystals will form and more liquid fat will be adsorbed than is the case with the hard fat program. For iodine values up to 39, the heating temperature can be as low as 15oC.

 

Tempering Treatment of Very Soft Fat. Where the iodine value is greater than 39-40 the "summer method" of treatment is used. After pasteurization the cream is cooled to 20oC. If the iodine value is around 39 - 40 the cream is cooled to about 8oC, and if 41 or greater to 6oC. It is generally held that aging temperatures below the 20o level will give a soft butter..

 

 

 

Butter structure

Diagram of Butter Structure

It should now be obvious from the discussions regarding the background science of churning and the crystallization processes that the structure of butter is quite complicated. The size and extent of crystal networks both within the globules and within the non-globular phases is controlled to a large extent by milkfat's variable composition and by the aging process. The extent of globular versus non-globular fat is controlled to a large extent also by the amount of physical working applied to the butter post-churning.
 

Continuous Buttermaking

There are essentially four types of buttermaking processes:

  • traditional batch churning from 25- 35% mf. cream;
  • continuous flotation churning from 30-50% mf. cream;
  • the concentration process whereby "plastic" cream at 82% mf. is separated from 35% mf. cream at 55oC and then this oil-in-water emulsion cream is inverted to a water-in-oil emulsion butter with no further draining of buttermilk;
  • the anhydrous milkfat process whereby water, SNF, and salt are emulsified into butter oil in a process very similar to margarine manufacture.

An optimum churning temperature must be determined for each type of process but is mainly dependent on the mean melting point and melting range of the lipids, as discussed above, i.e., 7-10oC in summer and 10 - 13oC in winter. If churning temperature is too warm or if the thermal cream aging cycle permits too much liquid fat, then a soft greasy texture results; if too cold or too much solid fat, then butter becomes too brittle.

Continuous Flotation Churns

Continuous butter churn diagram

The cream is first fed into a churning cylinder fitted with beaters that are driven by a variable speed motor.

Rapid conversion takes place in the cylinder and, when finished, the butter grains and buttermilk pass on to a draining section. The first washing of the butter grains sometimes takes place en route - either with water or recirculated chilled buttermilk. The working of the butter commences in the draining section by means of a screw, which also conveys it to the next stage.

On leaving the working section the butter passes through a conical channel to remove any remaining buttermilk. Immediately afterwards, the butter may be given its second washing, this time by two rows of adjustable high-pressure nozzles. The water pressure is so high that the ribbon of butter is broken down into grains and consequently any residual milk solids are effectively removed. Following this stage, salt may be added through a high-pressure injector.

The third section in the working cylinder is connected to a vacuum pump. Here it is possible to reduce the air content of the butter to the same level as conventionally churned butter.

In the final or mixing section the butter passes a series of perforated disks and star wheels. There is also an injector for final adjustment of the water content. Once regulated, the water content of the butter deviates less than +/- 0.1%, provided the characteristics of the cream remain the same.

The finished butter is discharged in a continuous ribbon from the end nozzle of the machine and then into the packaging unit.

Concentration Method

  • 30% fat cream pasteurized at 90oC
  • degassed in a vacuum
  • cooled to 45-70oC
  • separated to 82% fat ("plastic" cream)
  • the concentrate, still an O/W emulsion, is cooled to 8-13oC
  • fat crystals forming in the tightly packed globules perforate the membranes, cause liquid fat leakage and rapid phase inversion
  • contrast to mayonnaise, also a o/w emulsion at 82% fat but is winterized to prevent crystallization
  • butter from this method contains all membrane material, therefore, more phospholipids
  • no butter milk produced
  • after phase inversion the butter is worked and salted.

Phase Separation

 Butter from anhydrous milkfat:

  • prepare "plastic" cream (>80% fat)
  • heat with agitation to destabilize emulsion
  • separate oil from aqueous phase: 82 to 98% butter fat
  • this butter oil is then blended with water, salt and milk solids in an emulsion pump and transferred to a scraped surface heat exchanger for cooling and to initiate crystallization
  • further worked to develop crystal structure and texture
  • process similar to margarine manufacture
  • margarine has advantage of fat composition control to modify physical properties
  • butter produced by phase separation contains few phospholipids.

Butter Yield Calculations

Technological limits to yield efficiency are defined by separation efficiency, churning efficiency, composition overrun, and package over fill.

Separation efficiency (Es)

 - represents fat transferred from milk to cream 

Es = 1 - fs/fm 
where fs = skim fat as percent w/w
 fm = milk fat as percent w/w

Separation efficiency depends on initial milk fat content and residual fat in the skim. Assuming optimum operation of the separator, the principal determining factor of fat loss to the skim is fat globule size. Modern separators should achieve a skim fat content of 0.04 - 0.07%.

Churning Efficiency (Ec)

 - represents fat transferred from cream to butter

Ec = 1 - fbm/fc 
where fbm = buttermilk fat as percent w/w
fc = cream fat as percent w/w

Maximum acceptable fat loss in buttermilk is about 0.7% of churned fat corresponding to a churning efficiency of 99.3% of cream fat recovered in the butter. Churning efficiency is highest in the winter months and lowest in the summer months. Fat losses are higher in ripened butter due to a restructuring of the FGM (possibly involving crystallization of high melting triglycerides on the surface of the globules). If churning temperature is too high, churning occurs more quickly but fat loss in buttermilk increases. For continuous churns assuming 45% cream, churning efficiency should be 99.61 - 99.42%.

Composition Overrun

% Churn Overrun 

= (Kg butter made - Kg fat churned)/Kg fat churned x 100 %

% Composition Overrun 
= (100 - % fat in butter)/% fat x 100 %

Package Fill Control

 = (actual wt. - nominal wt.)/nominal wt. x 100%

An acceptable range for 25 kg butter blocks is 0.2 - 0.4% overfill. Overfill on 454 g prints is about 0.6%.

Other factors affecting yield

  • shrinkage due to leaky butter (improperly worked).
  • shrinkage due to moisture loss; avoided by aluminum wrap.
  • loss of butter remnants on processing equipment; % loss minimal in large scale continuous processing.

Plant Overrun

Plant efficiency or plant overrun is the sum of separation, churning, composition overrun and package fill efficiencies. In summary the theoretical maximum efficiency values are:

Separation Efficiency 98.85 
Churning Efficiency 99.60 
Composition overrun (% fat) 23.30 
Package overfill 0.20

These values can be used to predict the expected yield of butter per kg of milk or kg of milk fat received.

Example

3.6% m.f. milk

0.05% m.f. in skim
40% m.f. in cream
0.3% m.f. in buttermilk
81.5% m.f. in butter

Es = 1 - .05/3.6 = 98.6
Ec = 1 - .3/40 = 99.25
% Composition Overrun = (100-81.5)/81.5 = 22.7%
If 100 kg of milk was used, 8.9 kg of cream would be produced (from a Pearson Square mass balance) and 4.35 kg butter would be produced from that. This is the theoretical yield based on no losses. The mass balance of fat shows that 98.3% of the fat ended up in the butter, 0.4% of the fat ended up in the buttermilk and 1.3% of the fat ended up in the skim.
The % Churn Overrun = (4.35 - 3.6)/3.6 = 20.8%

Whipped Butter

Whipped butter is typically used in foodservice situations. The main advantage of whipped butter is increased spreadability even at refrigeration temperatures, thus providing great advantage for the restaurant industry. The volume increase is usually 25 - 30%. Whipping is achieved by injecting an inert gas (nitrogen) into the butter after churning. In the phase separation process, whipping can be achieved by injecting nitrogen in the crystallizer as is done in the production of whipped margarine.

Anhydrous Milkfat ("butter oil")

Anhydrous milk fat, butter oil, can be manufactured from either butter or from cream. For the manufacture from butter, non-salted butter from sweet cream is normally used, and the process works better if the butter is at least a few weeks old. Melted butter is passed through a centrifuge, to concentrate the fat to 99.5% of greater. This oil is heated again to 90-95oC and vacuum cooled before packaging.

The processes for the production of anhydrous fat, using cream as the raw material, are based on the emulsion splitting principle. In brief, the processes consist of the cream first being concentrated to 75% fat or greater, in two stages. In both of these stages, the fat is concentrated in a hermetic solids-ejecting separator. The fat globules are then broken down mechanically, so that phase inversion occurs and the fat is liberated. This forms a continuous fat phase containing dispersed water droplets, which can be separated from the fat phase by centrifugation. This is similar to the concentration method for buttermaking, with the addition of the mechanical rupture of the emulsion and additional separator for removal of the residual water phase.

One of the key machines in the system is the mechanical device for phase inversion. This can be in the form of a centrifugal separator equipped with a serrated disc. The disc breaks down the emulsion, so that the liquid leaving the machine is a continuous oil phase, with dispersed water droplets and buttermilk. Larger equipment could be equipped with a motor-driven serrated disc or with a homogenizer. After phase inversion, the fat is concentrated to 99.5% or greater in a hermetic separator.

Fractionation of anhydrous milk fat

Milk fat is a complicated mixture of triglycerides that contain numerous fatty acids of varying carbon chain lengths and degrees of saturation. The proportions of the various fatty acids present will also vary depending on the conditions surrounding the production of milk.

One method of milkfat fraction is by thermal treatment. The mixture can be separated into fractions on the basis of their melting point. The technique consists of melting the entire quantity of fat and then cooling it down to a predetermined temperature. The triglycerides with the higher melting point will then crystallize and settle out.

In the modern thermal fractionation method, sedimentation by gravity is replaced by centrifugal separation. Since a modern separator generates a force that is thousands of times greater than the force of gravity and since the sedimentation distances are very short, the process is incomparably faster. The crystallizing stage can also be accelerated, since the crystals need not be large if centrifugal separation is employed.

Fractionation of milkfat can also be accomplished by supercritical fluid extraction techniques.
Some of this material has been condensed from the Alfa-Laval Dairy Handbook, with permission.

Ice Cream

Please jump to the top page of our extensive Ice Cream Technology section.

Milk Ingredients

Various milk ingredients, including concentrated and dried milk powdersmilk and whey protein concentratescheesesmilkfat ingredients, etc., are used in many food applications, including bakery, meats, sauces, fabricated foods, etc.

For a website devoted to discussion of milk ingredients, including Canadian suppliers, please go to: 

www.milkingredients.ca

Structure of Milk and Dairy Products

This is a link to an absolutely marvellous website developed by my good friend Dr. Miloslav Kalab, entitled "Foods Under the Microscope", with many high-quality images of the structure of milk and dairy products obtained during Dr. Kalab's long and outstanding career as a food microscopist with Agriculture and Agri-Food Canada in Ottawa. He had a section on his site for guest microscopists, which I have contributed to, and I have copied my contributions here. My first contribution is a general discussion on ice cream structure as viewed by scanning electron microscopy and my second one focuses on the use of cryo-fixation and TEM for visualization of fat and air structures in ice cream. One of my graduate students, Alejandra Regand, also made a contribution, based on her M.Sc. thesis work, focussing on the structure of polysaccharides in frozen solutions.

Dr. Kalab has recently published a review of the structure of milk and dairy products entitled "The Beauty of Milk at High Magnification" available from the Royal Microscopical Society. Dr. Kalab was awarded the Isaac Heertje Lifetime Achievement Award by the Food Structure and Fnctionality Forum and the International dairy Federation at a joint meeting in Montreal in 2018. His lecture, "Milk and Microscopy",  following the presentation of his award is available on YouTube - https://www.youtube.com/watch?v=WhXY3YbY-08. 

From 1982-1994, Dr. Kalab was an editor of the Food Structure Journal, which became defunct in 1994. Dr. Kalab has written a history of the journal on his website and there is also now a Wikipedia entry on the journal. On the next page, I have included a list of the papers published in the journal from the University of Guelph. As a service to future Food Microcopists, Dr. Kalab and I have reproduced the Tables of Contents of all Food Structure journal issues on the page below. In addition, all the papers from this journal are now being scanned and made available digitally at Utah State University Digital Commons, thanks to Prof. Donald McMahon, Utah State University.  

Food Structure journal - Contributions from the University of Guelph

Food Structure journal: 1982-1993

The news about a new Food Structure journal established in 2013 by Elsevier Publishers comes almost exactly 20 years after the demise of a journal of that name, which represented a specialized publication forum for food scientists for 12 years.

An American organization in Chicago, Scanning Electron Microscopy, Inc., included food science in 1979 for the first time in its program of international annual conferences. This success led to a decision to start publishing a dedicated food structure journal under the title "Food Microstructure", which appeared semiannually starting in 1982. The Editor-in-Chief was a Canadian scientist, M. Kalab (Agriculture and Agri-Food Canada) and the first editors were 3 American scientists, S.H. Cohen, Eugenia Davis, and D.N. Holcomb. Om Johari of Scanning Electron Microscopy, Inc., (later renamed Scanning Microscopy International) was managing editor and publisher. W.J. Wolf (USDA) joined the journal as an additional American editor. Additional editors - I. Heertje of Unilever Research in the Netherlands and K. Sato of Hiroshima University in Japan took editorial care of manuscripts arriving from Europe and Asia, respectively. In addition to the editors, there was an Editorial Board, the members of which helped to review manuscripts. A total of 19 countries were represented in the Board over the existence of the journal. In 1990, it was renamed "Food Structure" at the suggestion of I. Heertje.

As a co-founder of the journal and its editor-in-chief, M. Kalab has published his reminiscences on the beginnings as well as the demise of the journal. He has included statistical data on the papers published as well as the representation of individual foods. A total of 386 papers were published (see Tables of Contents of all journals here). Milk and dairy products were the subjects of 79 papers followed by 61 papers on meat. The following 15 papers were published from the University of Guelph:

  • H. D. Geissinger, D. W. Stanley. 1981. Preparation of muscle samples for electron microscopy (Tutorial paper). Studies of Food Microstrucure 61-72 (SEM/1981/III:415-426, 414).
  • J. M. deMan. 1982. Microscopy in the study of fats and emulsions (Review paper). Food Microstructure 1(2):209-222.
  • D. W. Stanley. 1983. A review of the muscle cell cytoskeleton and its possible relation to meat texture and sarcolemma emptying.  (Review paper). Food Microstructure 2(1):99-110.
  • J. M. deMan, A. N. Mostafa, A. K. Smith. 1985. Thermal analysis microscopy for the study of phase changes in fats. Food Microstructure 4(2):233-240.
  • J. M. deMan, L. deMan, S. Gupta. 1986. Texture and microstructure of soybean curd (tofu) as affected by different coagulants.  Food Microstructure 5(1):83-90.
  • Goff, H.D., M. Liboff, W.K. Jordan, and J.E. Kinsella. 1987. The effects of Polysorbate 80 on the fat emulsion in ice cream mix : evidence from transmission electron microscopy studies. Food Microstructure 6(2):193-198.
  • Liboff, M., H.D. Goff, Z. Haque, W.K. Jordan, and J.E. Kinsella. 1988. Changes in the ultrastructure of emulsions as a result of electron microscopy preparation procedures. Food Microstructure 7(1):67-74.
  • A. Gordon, S. Barbut. 1989. The effect of chloride salts on the texture, microstructure and stability of meat batters. Food Microstructure 8(2):271-283.
  • A. Gordon, S. Barbut. 1990. The role of the interfacial protein film in meat batter stabilization. Food Structure 9(2):77-90.
  • A. Gordon, S. Barbut. 1990. The microstructure of raw meat batters prepared with monovalent and divalent chloride salts. Food Structure 9(4):279-296.
  • P. Chawla, J. M. deMan, A. K. Smith. 1990. Crystal morphology of shortenings and margarines (Tutorial paper). Food Structure 9(4):329-336.
  • A. Gordon, S. Barbut. 1991. Effect of chemical modification on the microstructure of raw meat batters. Food Structure 10(3):241-254.
  • Caldwell, K. B., H. D Goff, and D. W. Stanley. 1992. A low-temperature scanning electron microscopy study of ice cream. I. Techniques and general microstructure. Food Structure. 11(1):1-9 .
  • Caldwell, K. B., H. D Goff, and D. W. Stanley. 1992. A low-temperature scanning electron microscopy study of ice cream. II. Influence of selected ingredients and processes. Food Structure 11(1):11-23.
  • A. Gordon, S. Barbut. 1992. Effect of chemical modifications on the stability, texture and microstructure of cooked meat batters. (Review paper). Food Structure 11(2):133-146.

Food Structure journal - Tables of Contents from 1982-1994

FOOD STRUCTURE
(formerly FOOD MICROSTRUCTURE)
Tables of Contents for 1982-1994

Image of the Food Structure JournalBeginning in 1979, programs on food microstructure took place at annual Scanning Electron Microscopy (SEM) Meetings. Thirty six papers published in various SEM volumes have been compiled to form a book entitled Studies of Food Microstructure. The papers have been divided into 4 general areas:

1. general applications
2. meat foods
3. milk products
4. foods of plant origin

The table of contents is at the end of this page. Publication of the book preceded establishing the new journal, Food Microstructure
FOOD MICROSTRUCTURE was established as an international scientific journal on the microstructure and microanalysis of foods, feeds, and their ingredients in 1982 in the USA by food scientists:

  • S. H. Cohen (Sci. & Adv. Tech. Lab., US Army Natick R&D, Natick, MA)
  • E. A. Davis (Dept. Food Sci. & Nutr., Univ. of Minnesota, St. Paul, MN)
  • D. N. Holcomb (Kraft Inc., Glenview, IL)
  • M. Kaláb (Food Res. Inst., Agriculture Canada, Ottawa, Ont., Canada)

The journal was published initially twice a year by Scanning Electron Microscopy, Inc., AMF O'Hare (Chicago), IL, under the direction of Om Johari, Managing Editor. In 1990, the title of the journal was changed to FOOD STRUCTURE and the publication frequency was increased to 4 issues per annum. W. J. Wolf (USDA Northern Regional Research Center, Peoria, IL) joined the journal as another editor in 1991. I. Heertje (Unilever Research Laboratorium, Vlaardingen, the Netherlands) served as the European editor and K. Sato (Department of Applied Biology, Hiroshima University, Shitami, Japan) took editorial care of manuscripts arriving from Asian countries. Food structure studies flourished. Irrespective of this success, however, Om Johari terminated publishing the journal in 1994 without giving any reason. This means that Vol. 12 is the last one. Manuscripts submitted for publication in FOOD STRUCTURE were later published in FOOD SCIENCE & TECHNOLOGY (LWT) by Academic Press.

All manuscripts were peer-reviewed. Almost all authors responded in writing to reviewers' comments in the form of a 'discussion with reviewers'. Alternatively they revised their manuscripts. In addition to critically evaluating former achievements in the particular fields, all review papers also contain new information provided by the authors.

The table of contents published in Food Microstructure and later in Food Structure is being provided to assist food microscopists in their search for topics of interest. Journal issues may be found in food science libraries at various universities under ISSN 0730-5419, CODEN - FMICDK (Food Microstructure, 1982-1990) and ISSN 1046-705X, CODEN - FSTUE2 (Food Structure - 1991-1993). Thanks to Prof. Donald McMahon, Utah State University, this journal collection is now being scanned and made available digitally at the USU Digital Commons.

 

Vol. 1 - 1982 - Vol. 2 - 1983 - Vol. 3 - 1984 - Vol. 4 - 1985 
Vol. 5 - 1986 - Vol. 6 - 1987 - Vol. 7 - 1988 - Vol. 8 - 1989 
Vol. 9 - 1990 - Vol. 10. - 1991 - Vol. 11 - 1992 - Vol. 12 - 1993

Food Microstructure Vol. 1, Number 1, 1982

  • Food microstructure: An integrative approach. E. A. Davis, J. Gordon (Review paper) pp. 1-12.
  • Correlation of microscopic structure of corn starch granules with rheological properties of cooked pastes. D. D. Christianson, F. L. Baker, A. R. Loffredo, E. B. Bagley pp. 13-24.
  • Structure and properties of the particulate constituents of human milk: A review. M. Rüegg, B. Blanc (Review paper) pp. 25-48.
  • Detection of buttermilk solids in meat binders by electron microscopy. M. Kaláb, F. Comer pp. 49-54.
  • Some effects of lipids on the structure of foods. K. Larsson (Review paper) pp. 55-62.
  • Protein bodies in dormant, imbibed and germinated sunflower cotyledons. R. D. Allen, H. J. Arnott pp. 63-73.
  • Effect of high hydrostatic pressure on meat microstructure. E. A. Elgasim, W. H. Kennick pp. 75-82.
  • Ultrastructural studies of milk digestion in the suckling rat. P. B. Berendsen(Review paper) pp. 83-90.
  • Freeze-induced fibre formation in protein extracts from residues of mechanically separated poultry. R. A. Lawrence, P. Jelen pp. 91-97.
  • Instrumental and sensory analysis of the action of cathepsin enzymes on flaked and formed beef. S. H. Cohen, R. A. Segars, A. Cardello, J. Smith, F. M. Robbins pp. 99-105.

Food Microstructure Vol. 1, Number 2, 1982

  • Grain structure and end-use properties. Y. Pomeranz (Review paper) pp. 107-124.
  • Scanning electron microscopy of the pericarp and testa of several sorghum varieties. C. F. Earp, L. W. Rooney pp. 125-134.
  • The microscopic structure and chemistry of rapeseed and its products. S. H. Yiu, H. Poon, R. G. Fulcher, I. Altosaar pp. 135-144.
  • Light microscopy preparation techniques for starch and lipid containing snack foods. F. O. Flint pp. 145-150.
  • Electron microscopy of milk and milk products: Problems and possibilities. D. G. Schmidt pp. 151-166.
  • Fluorescence microscopy of cereals. R. G. Fulcher (Review paper) pp. 167-176.
  • Freeze-etch of emulsified cake batters during baking. J. D. Cloke, J. Gordon, E. A. Davis pp. 177-188.
  • Aspects of sample preparation for freeze-fracture/freeze-etch studies of proteins and lipids in food systems. W. Buchheim (Review paper) pp.189-208.
  • Microscopy in the study of fats and emulsions. J. M. deMan (Review paper) pp. 209-222.
  • Morphological and textural comparisons of soybean Mozzarella cheese analogs prepared with different hydrocolloids. C. S. Yang, M. V. Taranto pp. 223-232.
  • Electron microscopic localization of solvent-extractable fat in agglomerated spray-dried whole milk powder particles. W. Buchheim pp. 233-238.

Food Microstructure Vol. 2, Number 1, 1983

  • Ultrastructure studies of pasta. A review. P. Resmini, M. A. Pagani (Review paper) pp. 1-12.
  • Endosperm degradation in barley kernels that synthesize alpha-amylase in the absence of embryos and exogenous gibberellic acid. A. W. MacGregor, P. B. Nicholls, L. Dushnicky pp. 13-22.
  • An alternative to critical point drying for preparing meat emulsions for scanning electron microscopy. E. J. Basgall, P. J. Bechtel, F. K. McKeith pp. 23-26.
  • Image analysis of morphological changes in wiener batters during chopping and cooking. A. G. Kempton, S. Trupp (Review paper) pp. 27-42.
  • Composition and microstructure of soft brine cheese made from instant whole milk powder. M. M. Omar, W. Buchheim pp. 43-50.
  • Development of microstructure in set-style nonfat yoghurt - a review. M. Kaláb, P. Allan-Wojtas, B. E. Phipps-Todd (Review paper) pp. 51-66.
  • Field spectroscopy in the food production chain. E. J. Brach (Review paper) pp. 67-80.
  • The structure of fresh and desiccated coconut. J. F. Heathcock, J. A. Chapmanpp. 81-90.
  • Effect of prerigor pressurization on bovine lysosomal enzyme activity. E. A. Elgasim, W. H. Kennick, A. F. Anglemier, M. Koohmaraie, E. A. Elkhalifa pp. 91-98.
  • A review of the muscle cell cytoskeleton and its possible relation to meat texture and sarcolemma emptying. D. W. Stanley (Review paper) pp. 99-110.
  • Myofibrillar characteristics of porcine stress syndrome. P. K. Basrur, S. Frombach, W. N. McDonell, H. D. Geissinger pp. 111-118.

Food Microstructure Vol. 2, Number 2, 1983

  • Sensory and instrumental texture properties of flaked and formed beef. A. V. Cardello, R. A. Segars, J. Secrist, J. Smith, S. H. Cohen, R. Rosenkrans pp. 119-134.
  • Morphometry of meat by scanning light microscopy. H. J. Swatland pp. 135-142.
  • Infection of oriental mustard by Nematospora: A fluorescence and scanning electron microscope study. R. A. Holley, B. E. Phipps-Todd, S. H. Yiu pp. 143-152.
  • Evaluation of selected properties of chlorinated wheat flours in a lean cake formulation. J. Grider, E. A. Davis, J. Gordon pp.153-160.
  • Stranded structure development in thermally produced whey protein concentrate gel. T. Beveridge, L. Jones, M. A. Tung pp. 161-164.
  • The effect of commercial processing on the structure and microchemical organization of rapeseed. S. H. Yiu, I. Altosaar, R. G. Fulcher pp. 165-174.
  • Microstructure of winged beans. K. Saio, Y. Nakano, S. Uemoto pp. 175-182.
  • Beta-glucans in the caryopsis of Sorghum bicolor (L.) Moench. C. F. Earp, C. A. Doherty, R. G. Fulcher, L. W. Rooney pp. 183-188.

Food Microstructure Vol. 3, Number 1, 1984

  • An analysis of microstructural factors which influence the use of muscle as food. R. G. Cassens, C. E. Carpenter, T. J. Eddinger (Review paper) pp. 1-8.
  • Studies on the microdistribution of aerobic enzymes and myoglobin in pork.H. J. Swatland pp. 9-16.
  • The role of gap filaments in muscle and in meat. R. H. Locker (Review paper) pp. 17-32.
  • Processing effects on meat product microstructure. G. R. Schmidt (Review paper) pp. 33-40.
  • X-Ray microanalysis of hollow heart potatoes. W. P. Mohr, A. R. Spurr, P. Fenn, H. Timm pp. 41-48.
  • Determination of element concentration in fresh and processed vegetables by quantitative X-ray microanalysis. M. Grote, H. G. Fromme pp. 49-54.
  • Electron microscopic investigations of the cell structure in fresh and processed vegetables (carrots and green bean pods). M. Grote, H. G. Fromme pp. 55-64
  • Microstructural changes in winged bean and soybean during fermentation into miso. K. Saio, H. Suzuki, T. Kobayashi, M. Namikawa pp. 65-72.
  • The effect of microwave energy and convection heating on wheat starch granule transformation. N. K. Goebel, J. Grider, E. A. Davis, J. Gordon pp.73-82.
  • Microstructure of set-style yoghurt manufactured from cow's milk fortified by various methods. A. Y. Tamime, M. Kaláb, G. Davies (Review paper) pp. 83-92.
  • Transportation of fragile food specimens such as milk gels destined for electron microscopy. P. Allan-Wojtas pp. 93.

Food Microstructure Vol. 3, Number 2, 1984

  • Artefacts in conventional scanning electron microscopy of some milk products. M. Kaláb pp. 95-112.
  • The effect of salt and pyrophosphate on the structure of meat. C. A. Voyle, P. D. Jolley, G. W. Offer pp. 113-126.
  • Some factors influencing ante-mortem changes in muscle: A brief review. S. C. Seideman, S. H. Cohen, J. V. Schollmeyer (Tutorial paper) pp. 127-132.
  • Preliminary evaluation of lectins as fluorescent probes of seed structure and composition. S. Shea Miller, S. H. Yiu, R. G. Fulcher, I. Altosaar pp. 133-140.
  • Morphological development in sorghum grain. C. W. Glennie, N. v. d. W. Liebenberg, T. J. van Tonder pp. 141-148.
  • Mineral migration in the wheat kernel during mill conditioning. A. Al Saleh, B. Bouchet, D. J. Gallant pp. 149-158.
  • Light and scanning electron microscopy of wheat and rye-bread crumb. Interpretation of specimens prepared by various methods. Y. Pomeranz, D. Meyer pp. 159-164.
  • Ultrastructure of quinoa fruit (Chenopodium quinoa Willd). E. Varriano-Marston, A. DeFrancisco pp. 165-174.
  • Ultrastructural aspects of spun pea and fababean proteins. D. J. Gallant, B. Bouchet, J. Culioli pp. 175-184.
  • Effect of environment on the physical structure of the peanut (Arachis hypogaea L.). C. T. Young, W. E. Schadel pp. 185-190.
  • Ultrastructural studies of raw and processed tissue of the major cultivated mushroom, Agaricus bisporus. E. M. Jasinski, B. Stembereger, R. Walsh, A. Kilara pp. 191-196.
  • A simple procedure for the preparation of stirred yoghurt for electron microscopy. P. Allan-Wojtas, M. Kaláb pp. 197-198.

Food Microstructure Vol. 4, Number 1, 1985

  • The size distribution of bovine casein micelles: A review. C. Holt (Review paper) pp. 1-10.
  • Scanning electron microscopic study of rockfish preserved at either ambient temperature or by isothermal freeze-fixation. L. E. Lampila, V. Mohr, D. S. Reidpp. 11-16.
  • Characterisation of milk proteins in confectionery products. J. F. Heathcockpp. 17-28.
  • The light microscopy of triglyceride digestion. J. S. Patton, R. D. Vetter, M. Hamosh, B. Borgström, M. Lindström, M. C. Carey (Review paper) pp. 29-42.
  • Seed structure and composition of potential new crops. D. W. Irving, R. Beckerpp. 43-54.
  • The combined effects of the calcium activated factor and cathepsin D on skeletal muscle. E. A. Elgasim, M. Koohmaraie, A. F. Anglemier, W. H. Kennick, E. A. Elkhalifa pp. 55-62.
  • Microstructure of meat emulsions in relation to fat stabilization. C. M. Lee(Review paper) pp. 63-72.
  • Early research on the fibrous microstructure of meat. H. J. Swatland pp. 73-82.
  • Application of electron spin resonance techniques to model starch systems. L. E. Pearce, E. A. Davis, J. Gordon, W. G. Miller pp. 83-88.
  • Development of microstructure in a cream cheese based on Queso blanco cheese. M. Kaláb, H. W. Modler pp. 89-98.
  • A fluorescence microscopic study of cheese. S. H. Yiu pp. 99-106.
  • Small angle X-ray scattering by hydrated wheat starch granules. M. Yang, J. Grider, J. Gordon, E. A. Davis pp. 107-114.
  • Seed microstructure: Review of water imbibition in legumes. B. G. Swanson, J. S. Hughes, H. P. Rasmussen pp. 115-124.
  • The microstructure of wheat: Its development and conversion into bread. D. B. Bechtel pp. 125-134.
  • The application of light and scanning electron microscopy during flour milling and wheat processing. R. Moss pp. 135-142.
  • Ultrastructural studies on the cultivation processes and growth and development of the cultivated mushroom Agaricus bisporus. D. A. Wood, G. D. Craig, P. T. Atkey, R. J. Newsam, K. Gull (Review paper) pp. 1443-164.
  • Comparison of the microstructure of firm and stem-end softened cucumber pickles preserved by brine fermentation. W. M. Walter Jr., H. P. Flemming, R. N. Trigiano pp. 165-172.
  • Ultrastructural study of yam tuber as related to postharvest hardness. L. Sealy, S. Renaudin, D. J. Gallant, B. Bouchet, J. M. Brillouet pp. 173-181.

Food Microstructure Vol. 4, Number 2, 1985

  • Microstructural changes in maturing seeds of the common bean (Phaseolus vulgaris L.). J. S. Highes, B. G. Swanson pp. 183-190.
  • Changes in typical organelles in developing cotyledons of soybean. K. Saio, K. Kondo, T. Sugimoto pp. 191-198.
  • Rheological and ultrastructural studies of wheat kernel behaviour under compression as a function of water content. A. Al Saleh, D. J. Gallant pp. 199-212.
  • Structure of coarse and fine fractions of corn samples ground on the Stenvert hardness tester. Y. Pomeranz, Z. Czuchajowska pp. 213-220.
  • Relation between microstructure, destabilization phenomena and rheological properties of whippable emulsions. W. Buchheim, N. M. Barfod, N. Krog pp. 221-232.
  • Thermal analysis microscopy for the study of phase changes in fats. J. M. deMan, A. N. Mostafa, A. K. Smith pp. 233-240.
  • Combining microscopy and physical techniques in the study of cocoa butter polymorphs and vegetable fat blends. J. D. Hicklin, G. G. Jewell, J. F. Heathcockpp. 241-248.
  • Crystal morphology of cocoa butter. D. M. Manning, P. S. Dimick (Review paper) pp. 249-266.
  • Structure formation in acid milk gels. I. Heertje, J. Visser, P. Smits pp. 267-278.
  • Rheological and scanning electron microscopy examination of skim milk gels obtained by fermenting with ropy and non-ropy strains of lactic bacteria. S. M. Schellhaass, H. A. Morris pp. 279-288.
  • Observations on the air-serum interface of milk foams. B. E. Brooker pp. 289-296.
  • Effects of emulsifying agents on the microstructure and other characteristics of process cheese - A review. M. Caric, M. Gantar, M. Kaláb (Review paper) pp. 297-312.
  • Properties of calcium caseinates with disparate performance in imitation cheese. K. Fleming, R. Jenness, H. A. Morris, R. Schmidt pp. 313-322.
  • Ultrastructural and biochemical investigations of mature human milk. R. J. Carroll, J. J. Basch, J. G. Phillips, H. M. Farrell Jr. pp. 323-331.
  • Particle structure in spray-dried whole milk and in instant skim milk powder as related to lactose crystallization. Z. Saito (Review paper) pp. 333-340.
  • Microstructure of spray-dried and freeze-dried microalgal powders. L. P. Linpp. 341-348.
  • Effect of ionizing irradiation and storage on mushroom ultrastructure. I. The gills of Agaricus bisporus (Lge.) Imbach and Pleurotus ostreatus (Jacq. ex Fr.) Kummer. A. Keresztes, J. Kovács, E. Kovács pp. 349-356.
  • Ultrastructural utilization of plants by herbivores. L. H. Harbers (Tutorial paper) pp. 357-364.

Food Microstructure Vol. 5, Number 1, 1986

  • Features of food microscopy. D. F. Lewis (Review paper) pp. 1-18.
  • Fixing conditions in the freeze substitution technique for light microscopy observation of frozen beef tissue. M. N. Martino, N. E. Zaritzky pp. 19-24.
  • Changes in the microstructure of skipjack tuna during frozen storage and heat treatment. L. E. Lampila, W. D. Brown pp. 25-32.
  • Freeze texturization of proteins: Effect of the alkali, acid and freezing treatment on texture formation. F. I. Consolacion, P. Jelen pp. 33-40.
  • The fine structure of the endomysium, perimysium and intermyofibrillar connections in muscle. M. W. Orcutt, T. R. Dutson, F. Y. Wu, S. B. Smith pp. 41-52.
  • Action of polyphosphates in meat products. D. F. Lewis, K. H. M. Groves, J. H. Holgate pp. 53-62.
  • Microscopical observations on the structure of bacon. C. A. Voyle, P. D. Jolley, G. W. Offer (Review paper) pp. 63-70.
  • Accuracy and utility of sarcomere length assessment by laser diffraction. P. A. Koolmees, F. Korteknie, F. J. M. Smulders pp. 71-76.
  • Cryo-scanning electron microscopy of microorganisms in a liquid film on spoiled chicken skin. T. A. McMeekin, D. McCall, C. J. Thomas pp. 77-82.
  • Texture and microstructure of soybean curd (tofu) as affected by different coagulants. J. M. deMan, L. deMan, S. Gupta pp. 83-90.
  • Observations on the microstructure and rheology of ovalbumin gels. I. Heertje, F. S. M. van Kleef pp. 91-98.
  • The microstructure of the hen's egg shell - a short review. R. M. G. Hamilton(Review paper) pp. 99-110.
  • Ultrastructure of cooked spaghetti. M. A. Pagani, D. J. Gallant, B. Bouchet, P. Resmini pp. 111-130.
  • Microstructure of mealy and vitreous wheat endosperms (Triticum durum L.) with special emphasis on location and polymorphic behaviour of lipids. A. Al Saleh, D. Marion, D. J. Gallant pp. 131-140.
  • Ultrastructure of maize starch granules. A review. D. J. Gallant, B. Bouchet(Review paper) pp.141-156.
  • Mucilage in yellow mustard (Brassica hirta) seeds. I. R. Siddiqui, S. H. Yiu, J. D. Jones, M. Kaláb pp. 157-162.
  • Rheological and particle size changes in corn oil-in-water emulsions stabilized by 7S soybean proteins. M. Reeve, P. Sherman pp. 163-168.
  • Mechanical properties of cheese, cheese analogues and protein gels in relation to composition and microstructure. M. L. Green, K. R. Langley, R. J. Marshall, B. E. Brooker, A. Willis, J. F. V. Vincent pp. 169-180.
  • Ultrastructural aspects and physico-chemical properties of ultrahigh-temperature (UHT)-treated coffee cream. W. Buchheim, G. Falk, A. Hinz pp. 181-192.

Food Microstructure Vol. 5, Number 2, 1986

  • The skinned fiber technique as a potential method for study of muscle as a food. R. G. Cassens, T. J. Eddinger, R. L. Moss pp. 193-196.
  • Current concepts of muscle ultrastructure with emphasis on Z-line architecture. M. Yamaguchi, H. Kamisoyama, S. Nada, S. Yamano, M. Izumimoto, Y. Hirai, R. G. Cassens, H. Nasu, M. Muguruma, T. Fukazawa(Review paper) pp. 197-206.
  • Comparative microscopy and morphometry of skeletal muscle fibers in poultry. C. S. Williams, J. W. Williams, R. A. Chung pp. 207-218.
  • Effect of processing and cooking on the structural and microchemical composition of oats. S. H. Yiu pp. 219-226.
  • Determination of internal color of beef ribeye steaks using digital image analysis. K. Unklesbay, N. Unklesbay, J. Keller pp. 227-232.
  • The structure of gluten gels. A.-M. Hermansson, K. Larsson pp.233-240.
  • Microstructure of lentil seeds (Lens culinaris). J. S. Hughes, B. G. Swanson pp. 241-246.
  • Structural characteristics of Eleusine corocana (finger millet) using scanning electron and fluorescence microscopy. C. M. McDonough, L. W. Rooney, C. F. Earp pp. 247-256.
  • Fluorescence characterization of the mature caryopsis of Sorghum bicolor (L.) Mooench. C. F. Earp, L. W. Rooney pp. 257-264.
  • Lipolytic changes in the milk fat of raw milk and their effects on the quality of milk products. E. Kirst pp. 265-276.
  • The development of structure in whipped cream. B. E. Brooker, M. Anderson, A. T. Andrews pp. 277-286.
  • Relationship between microstructure and susceptibility to syneresis in yoghurt made from reconstituted nonfat dry milk. V. R. Harwalkar, M. Kaláb pp. 287-294.
  • Electron dense granules in yoghurt: Characterization by X-ray microanalysis.E. M. Parnell-Clunies, Y. Kakuda, R. Humprey pp. 295-302.

Food Microstructure Vol. 6, Number 1, 1987

  • Product morphology of fatty products. I. Heertje, M. Leunis, W. J. M. Van Zeyl, E. Berends pp. 1-8.
  • Scanning electron microscopy studies of cellular changes in raw, fermented and dried cocoa beans. A. S. Lopez, P. S. Dimick, R. M. Walsh pp. 9-16.
  • Microbial cell division and separation: Effect of citrate on the growth of group N streptococci. S. Ita, T. Kobayashi, K. Ozaki, T. Morichi, M. Saitoh pp. 17-24.
  • The crystallization of calcium phosphate at the surface of mould-ripened cheeses. B. E. Brooker pp. 25-24.
  • The size distribution and shape of curd granules in traditional Swiss hard and semi-hard cheeses. M. W. Rüegg, U. Moor pp. 35-46.
  • The morphology of black tea cream. R. D. Bee, M. J. Izzard, R. S. Harbron, J. M. Stubbs pp. 47-56.
  • Microstructural studies of texturized vegetable protein products: Effects of oil addition and transformation of raw materials in various sections of a twin screw extruder. S. Gwizda, A. Noguchi, K. Saio pp. 57-62.
  • The influence of ingredients and processing variables on the quality and microstructure of Hokkien, Cantonese and instant noodles. R. Moss, P. J. Gore, I. C. Murray pp. 63-74.
  • Effect of ionizing irradiation and storage on mushroom ultrastructure II. The stipe and the upper part of the cap of Agaricus bisporus (Lge. Imbach). A. Keresztes, E. Kovács pp. 75-80.
  • Textural properties and structure of starch-reinforced surimi gels as affected by heat-setting. J. M. Kim, C. M. Lee, L. A. Hufnagel pp. 81-90.
  • Seed microstructure and its relationship to water uptake in isogenic lines and a cultivar of dry beans (Phaseolus vulgaris L.). M. A. Uebersax, K. Klomparenspp. 91-102.
  • Influence of delignifying agents on tissue structure in bermudagrass stems. D. E. Akin, L. L. Rigsby, F. E. Barton II, P. Gelfand, D. S. Himmelsbach, W. R. Windham pp. 103-113.

Food Microstructure Vol. 6, Number 2, 1987

  • Confocal scanning laser microscopy in food research: Some observations. I. Heertje, P. van der Vlist, J. C. G. Blonk, H. A. C. Hendrickx, G. J. Brakenhoff pp. 115-120.
  • An electron spin resonance study of stearic acid interactions in model wheat starch and gluten systems. L. E. Pearce, E. A. Davis, J. Gordon, W. G. Miller pp. 121-126.
  • Cellular rupture and release of protoplasm and protein bodies from bean and pea cotyledons during imbibition. S. C. Spaeth, J. S. Hughes pp. 127-134.
  • The microstructure and rehydration properties of the Phoenix oyster musheroom (Pleurotus sajor-caju) dried by three alternative processes. B. Li-Shing-Tat, P. Jelen pp. 135-142.
  • Fluorescence and light microscopic analysis of digested oat bran. S. H. Yiu, R. Mongeau pp. 143-150.
  • Physical and molecular properties of lipid polymorphs - a review. K. Sato(Review paper) pp. 151-160.
  • Effect of clotting in stomachs of infants on protein digestibility of milk. S. Nakai, E. Li-Chan pp. 161-170.
  • Effects of drying techniques on milk powders quality and microstructure: A review. M. Caric, M. Kaláb pp. 171-180.
  • Textural properties and microstructure of process cheese food rework. M. Kaláb, J. Yun, S. H. Yiu pp. 181-192.
  • The effects of polysorbate 80 on the fat emulsion in ice cream mix: Evidence from transmission electron microscopy studies. H. D. Goff, M. Liboff, W. K. Jordan, J. E. Kinsella pp. 193-198.
  • Structural binding properties of silvercarp (Hypophtalmichthys molitrix) muscle affected by NaCl and CaCl2 treatments. I. Shomer, Z. G. Weinberg, R. Vasiliver pp. 199-208.
  • Internal porosity of corn extrudate air cell wall. S. H. Cohen, C. A. Voyle pp. 209-211.

Food Microstructure Vol. 7, Number 1, 1988

  • The effects of gamma irradiation and calcium treatment on the ultrastructure of apples and pears. E. Kovács, A. Keresztes, J. Kovács (Review paper) pp. 1-14.
  • The microstructure of spray-dried microcapsules. M. Rosenberg, Y. Talmon, I. J. Kopelman pp. 15-24.
  • Functional and microstructural effects of fillers in comminuted meat products.F. W. Comer, P. Allan-Wojtas (Review paper) pp.25-46.
  • The retention of contractility of rabbit myofibrils during storage at 25°C. M. Nakamura, M. Muguruma, T. Fukazawa, M. Yamaguchi pp. 47-52.
  • The use of transmission electron microscopy to study the composition ofPseudomonas fragi attachment material. P. J. Herald, E. A. Zottola pp. 53-58.
  • Effects of lignification, cellulose crystallinity and enzyme accessible space on the digestibility of plant cell wall carbohydrates by the ruminant. M. S. Kerley, G. C. Fahey Jr., J. M. Gould, E. L. Iannoti (Review paper) pp. 59-66.
  • Changes in the ultrastructure of emulsions as a result of electron microscopy preparation procedures. M. Liboff, H. D. Goff, Z. Haque, W. K. Jordan, J. E. Kinsella pp. 67-74.
  • Lactose crystallization in commercial whey powders and in spray-dried lactose. Z. Saito (Review paper) pp. 75-82.
  • Development of microstructure in raw, fried, and fried and cooked Paneer made from buffalo, cow, and mixed milks. M. Kaláb, S. K. Gupta, H. K. Desai, G. R. Patil pp. 83-92.
  • Amino acid composition and structure of cheese baked as a pizza ingredient in conventional and microwave ovens. A. Paquet, M. Kaláb pp. 93-104.
  • Changes in the microstructure of Saint Paulin cheese during manufacturre studied by scanning electron microscopy. M. Rousseau pp. 105-114.
  • Sample holders for solid and viscous foods compatible with the Hexland Cryotrans CT 1000 assembly. M. Kaláb, P. Allan-Wojtas, A. F. Yang pp. 115-120.

Food Microstructure Vol. 7, Number 2, 1988

  • The application of cold stage scanning electron microscopy to food research.J. A. Sargent (Review paper) pp. 123-136.
  • Ultrastructural and textural properties of restructured beef treated with a bacterial culture and splenic pulp. E. A. Elkhalifa, N. G. Marriott, R. L. Grrayson, P. P. Graham, S. K. Perkins pp. 137-146.
  • Characterization of hypercontracted fibers in skeletal muscle of domestic turkey (Meleagris gallopavo). A. Sosnicki, R. G. Cassens, D. R. McIntyre, R. J. Vimini, M. L. Greaser pp. 147-152.
  • Porosity, specific gravity and fat dispersion in Blue cheese. K. M. K. Kebary, H. A. Morris pp. 153-160.
  • Application of scanning electron microscopy and X-ray microanalysis to investigate corrosion problems in plain tinplate food cans and examine glass and glass-like particles found in canned food. J. E. Charbonneau pp. 161-172.
  • The role of beta-lactoglobulin in the development of the core-and-lining structure of casein particles in acid-heat-induced milk gels. V. R. Harwalkar, M. Kaláb pp. 173-180.
  • Microstructure of shortenings, margarine and butter - a review. A. C. Juriaanse, I. Heertje (Review paper) pp. 181-188.
  • The effect of processing on some microstructural characteristics of fat spreads. I. Heertje, J. van Eendenburg, J. M. Cornelissen, A. C. Juriaanse pp. 189-194.
  • The effect of storage, processing, and enzyme treatment on the microstructure of cloudy Spartan apple juice particulate. D.-L. McKenzie, T. Beveridge pp. 195-204.
  • Scanning electron microscopy of cellular structure of Granny Smith and Red Delicious apples. G. Trakoontivakorn, M. E. Patterson, B. G. Swanson pp. 205-212.
  • Encapsulation of viscous foods in agar gel tubes for electron microscopy. M. Kaláb (Technical note) pp. 213-214.

Food Microstructure Vol. 8, Number 1, 1989

  • Electron microscopic localization of cholesterol in bovine milk fat globules. R. W. Martin Jr. pp. 3-10.
  • Advanced techniques for preparation and characterization of small unilamellar vesicles. G. Mason pp. 11-14.
  • Soluble and insoluble dietary fiber in cooked common bean (Phaseolus vulgaris) seeds. J. S. Hughes, B. G. Swanson pp. 15-22.
  • Heat-set gels based on oil/water emulsions: An application of whey protein functionality. R. Jost, F. Dannenberg, J. Rosset pp. 23-28.
  • Microstructural changes in wheat starch dispersions during heating and cooling. M. Langton, A.-M. Hermansson pp. 29-40.
  • Microstructure and texture of meat emulsions supplemented with plant proteins. A. P. Paulson, M. A. Tung pp. 41-52.
  • Effects of protein concentration in ultrafiltration milk retentates and the type of protease used for coagulation on the microstructure of resulting gels. D. D. Gavaric, M. Caric, M. Kaláb pp. 53-66.
  • The effect of irradiation on starch content in Golden Delicious apples. E. Kovács, A. Keresztes pp. 67-74.
  • Ultrastructural effects of postharvest treatments on the vacuolar inclusions in pear (Pyrus communis L. cv. Hardenpont) peel. A. Keresztes, K. Boka, E. Bacsy, E. Kovács pp. 75-80.
  • Image analysis of the fat dispersion in a comminuted meat system. P. A. Koolmees, P. C. Moerman, M. H. G. Zijderveld pp. 81-90.
  • An electron microscopic study of the adherence of Lactobacillus acidophilus to human intestinal cells in vitro. S. K. Hood, E. A. Zottola pp. 91-98.
  • Cereal structure and its relationship to nutritional quality. S. H. Yiu (Review paper) pp. 99-114.
  • Changes in the ultrastructure of beef muscle as influenced by acidic conditions below the ultimate pH. M. V. Rao, N. F. S. Gault, S. Kennedy pp. 115-124.
  • Rheology and microstructure of strained yoghurt (labneh) made from cow's milk by three different methods. A. Y. Tamime, M. Kaláb, G. Davies pp. 125-136.
  • Structural characteristics of Pennisetum americanum (pearl millet) using scanning electron and fluorescence microscopy. C. M. McDonough, L. W. Rooney (Review paper) pp. 137-150.
  • The structural basis of the water-holding, appearance and toughness of meat and meat products. G. Offer, P. Knight, R. Jeacocke, R. Almond, T. Cousins, J. Elsey, N. Parsons, A. Sharp, R. Starr, P. Purslow (Review paper) pp. 151-170.

Food Microstructure Vol. 8, Number 2, 1989

  • Influence of the extrusion process on characteristics and structure of pasta. M. A. Pagani, P. Resmini, G. Dalbon (Review paper) pp. 173-182.
  • Vacuole formation in wheat starchy endosperm. D. B. Bechtel, A. Frend, L. A. Kaleikau, J. D. Wilson, P. R. Shewry pp. 183-190.
  • Use of image analysis to predict milling extraction rates of wheats. A. D. Evers, R. P. Withey pp. 191-200.
  • Grittiness in a pasteurized cheese spread: A microscopic study. H. W. Modler, S. H. Yiu, U. K. Bollinger, M. Kaláb pp. 201-210.
  • The size distribution of casein micelles in camel milk. Z. Farah, M. W. Rüeggpp. 211-216.
  • Effect of heating to 200°C on casein micelles in milk: A metal shadowing and negative staining electron microscope study. V. R. Harwalkar, P. Allan-Wojtas, M. Kaláb pp. 217-224.
  • Composition and some properties of spray-dried retentates obtained by the ultrafiltration of milk. M. Kaláb, M. Caric, M. Zaher, V. R. Harwalkar pp. 225-234.
  • Fluorescence microscopy study on (1,3)-beta-D-glucan in barley endosperm.A. W. MacGregor, G. M. Ballance, L. Dushnicky pp. 235-244.
  • Microstructure of extruded mixtures of cereals and oil seed processing residues. A. Salgó, S. Török, S. Sándor pp. 245-252.
  • A method for light and scanning electron microscopy of drought-induced damage of resting peanut seed tissue. C. T. Young, W. E. Schadel pp. 253-256.
  • Observation of seeding effects on fat bloom of dark chocolate. I. Hachiya, T. Koyano, K. Sato pp. 257-262.
  • Microstructure of nunas: Andean popping beans (Phaseolus vulgaris L.). S. C. Spaeth, D. G. Debouck, J. Tohme, J. van Beem pp. 263-270.
  • The effect of chloride salts on the texture, microstructure and stability of meat batters. A. Gordon, S. Barbut pp. 271-283.

Food Structure Vol. 9, Number 1, 1990

  • Effect of high-pressure homogenization on a sterilized infant formula: Microstructure and age gelation. Y. Pouliot, M. Britten, B. Latreille pp. 1-8.
  • Identification and characterization of cocoa solids and milk proteins in chocolate using X-ray microanalysis. B. E. Brooker pp. 9-22.
  • Microstructure and firmness of processed cheese manufactured from Cheddar cheese and skim milk powder cheese base. A. Y. Tamime, M. Kaláb, G. Davies, M. F. Younis pp. 23-38.
  • Polymorphism of triglycerides - a crystallographic review. L. Hernqvist(Tutorial paper) pp. 39-44.
  • Particle characteristics of flake cut meat. P. R. Sheard, A. Cousins, P. D. Jolley, C. A. Voyle (Review paper) pp. 45-56.
  • Porosity, specific gravity and air content in Blue cheeses. K. M. K. Kebary, H. A. Morris pp. 557-60.
  • An enzyme/surfactant treatment and filtration technique for the retrieval ofListeria monocytogenes from ice cream mix. K. A. Hale, S. Doores, R. A. Walshpp. 61-68.
  • Light and scanning electron microscopy of the peanut (Arachis hypogaea L. cv. Florunner) cotyledon after roasting. C. T. Young, W. E. Schadel pp. 69-74.
  • A simple carrier for freezing difficult food samples in preparation for scanning electron microscopy. P. Allan-Wojtas pp. 75-76.

Food Structure Vol. 9, Number 2, 1990

  • The role of the interfacial protein film in meat batter stabilization. A. Gordon, S. Barbut pp. 77-90.
  • The effect of tumbling, sodium chloride and polyphosphates on the microstructure and appearance of whole-muscle processed meats. P. D. Velinov, M. V. Zhikov, R. G. Cassens pp. 91-96.
  • Microscopic measurement of apple bruises. N.-K. Kim, Y.-C. Hung pp. 97-104.
  • A method for the examination of the microstructure of stabilized peanut butter. C. T. Young, W. E. Schadel pp. 105-108.
  • Transmission and scanning electron microscopy of peanut (Arachis hypogaeaL. cv. Florigiant) cotyledon after roasting. C. T. Young, W. E. Schadel pp. 109-112.
  • Scanning electron microscopy: Tissue characteristics and starch granule variations of potatoes after microwave and conductive heating. J. Huang, W. M. Hess, D. J. Weber, E. A. Purcell. C. S. Huber pp. 113-122.
  • Thermographic behavior of coconut oil during wheat dough mixing: Evidence for a solid-liquid phase separation according to mixing temperature. C. Le Roux, D. Marion, H. Bizot, D. J. Gallant pp. 123-138.
  • Changes in the rheology and microstructure of ropy yogurt during heating. J. A. Tegatz, H. A. Morris pp. 133-138.
  • The role of cell wall structure in the hard-to-cook phenomenon in bean (Phaseolus vulgaris L.). I. Shomer, N. Paster, P. Lindner, R. Vasiliver pp. 139-150.
  • Encapsulation of viscous high-fat foods in calcium alginate gel tubes at ambient temperature. I. A. Veliky, M. Kaláb pp. 151-154.
  • Food Microstructure - cumulative index. D. N. Holcomb pp. 155-173.

Food Structure Vol. 9, Number 3, 1990

  • The structure-function relationship of polymeric sorbents for colloid stabilization of beer. G. Basarová (Review paper) pp. 175-194.
  • The use of the scanning electron microscope in investigating container corrosion by canned foods and beverages. E. J. Helwig, H. E. Bibler (Tutorial paper) pp.195-202.
  • Application of microscopy in the paper industry: Case histories of the Mead Corporation. A. J. Leonardi, B. A. Blakistone, S. W. Kyryk (Tutorial paper) pp. 201-214.
  • Simulation of a penetrometric test on apples using Voronoi-Delaunay tesselation. A. C. Roudot, F. Duprat, E. Pietri pp. 215-222.
  • The adsorption of crystalline fat to the air-water interface of whipped cream.B. E. Brooker pp. 223-230.
  • Fat polymorphism and crystal seeding effects on fat bloom stability of dark chocolate. T. Koyano, I. Hachiya, K. Sato (Review paper) pp. 231-240.
  • Preparation of cereals and grain products for transmission electron microscopy. D. B. Bechtel (Tutorial paper) pp. 241-252.
  • Development of sorghum (Sorghum bicolor (L.) Moench) endosperm varieties of varying hardness. J. M. Shull, A. Chandrashekar, A. W. Kirleis, G. Ejeta pp. 253-267.

Food Structure Vol. 9, Number 4, 1990

  • Gelation of myosin filament under high hydrostatic pressure. K. Yamamoto, T. Miura, T. Yasui pp. 269-278.
  • The microstructure of raw meat batters prepared with monovalent and divalent chloride salts. A. Gordon, S. Barbut pp. 279-296.
  • Rheological and microstructural changes of oat and barley starches during heating and cooling. K. Autio pp. 297-304.
  • The observation of the displacement of emulsifiers by confocal scanning laser microscopy. I. Heertje, J. Nederlof, H. A. C. M. Hendricks, E. H. Lucassen-Reynder pp. 305-316.
  • Microstructure of peanut seed: A review. C. T. Young, W. E. Schadel (Review paper) pp. 317-328.
  • Crystal morphology of shortenings and margarines. P. Chawla, J. M. deMan, A. K. Smith (Tutorial paper) pp. 329-336.
  • Emulsifiers as additives in fats: Effect of polymorphic transformations and crystal properties of fatty acids and triglycerides. J. Aronhime, S. Sarig, N. Garti(Tutorial paper) pp. 337-352.

Food Structure Vol. 10, Number 1, 1991

  • Electron microscopic studies on the ultrastructure of curdlan and other polysaccharides in gels used in foods. T. Harada, Y. Kanzawa, K. Kanenaga, A. Koreeda, A. Harada (Review paper) pp. 1-18.
  • Relationship between microstructure and in vitro digestibility of starch in precooked leguminous seed flours. J. Tovar, A. de Francisco, I. Björck, N.-G. Asp pp. 19-26.
  • Effects of phosphate and citrate on the gelation properties of casein micelles in renneted ultra-high temperature (UHT) sterilized concentrated milk. D. J. McMahon, P. A. Savello, R. J. Brown, M. Kaláb pp. 27-36.
  • Microstructure and firmness of labneh (high solids yoghurt) made from cow's, goat's and sheep's milks by a traditional method or by ultrafiltration. A. Y. Tamime, M. Kaláb, G. Davies, H. A. Mahdi pp. 37-44.
  • Structure and rheology of dairy products: A compilation of references with subject and author indexes. D. N. Holcomb (Bibliography) pp. 45-108.

Food Structure Vol. 10, Number 2, 1991

  • Influence of emulsifier on the competitive adsorption of whey protein in emulsions. J.-L. Courthauson, E. Dickinson, Y. Matsumura, A. Williams pp. 109-116.
  • Distribution of amylose and amylopectin in potato starch pastes: Effects of heating and Shearing. K. Svegmark, A.-M. Hermansson pp. 117-130.
  • Microstructure of adzuki beans (Vigna angularis cv. Express). A. Chilukuri. B. G. Swanson pp. 131-136.
  • Practical methods for identification of rice endosperm protein bodies and fecal protein particles in light microscopy. D. L. Barber, J. N. A. Lott, D. A. Harris pp. 137-144.
  • Recent developments in the application of X-ray microanalysis to the study of food systems. B. E. Brooker pp. 145-152.
  • Effect of extruder die temperature on texture and microstructure of restructured mechanically deboned chicken and corn starch. V. B. Alvarez. D. M. Smith, S. Flegler pp. 153-160.
  • Structures and properties of stabilized vitamin and carotenoid dry powders. V. E. Colombo, F. Gerber pp. 161-170.
  • Application of scanning electron microscopy and X-ray microanalysis to investigate corrosion problems in plain and enamelled three piece welded food cans. J. E. Charbonneau pp. 171-184.

Food Structure Vol. 10, Number 3, 1991

  • Magnetic resonance imaging of cheese structure. M. Rosenberg, M. J. McCarthy, R. Kauten pp. 185-192.
  • Structure, meltability, and firmness of process cheese containing White cheese. M. Kaláb, H. W. Modler, M. Caric, S. Milanovic pp. 193-202.
  • Extrusion cooking of pea flour: Structural and immunocytochemical aspects.H. Ben-Hdech, D. J. Gallant, B. Bouchet, J. Gueguen, J.-P. Melcion pp. 203-212.
  • Comparison of the effects of three different grinding procedures on the microstructure of "old-fashioned" non-stabilized peanut butter. C. T. Young, W. E. Schadel pp. 213-216.
  • Effect of irradiation and dielectric heating on soybean ultrastructure, trypsin inhibitor and lipoxygenase activities. E. Kovács, D. N. Lam, J. Beczner, I. Kisspp. 217-228.
  • Physicochemical and macromolecular properties of starch-cellulose fiber extrudates. R. Chinnaswamy, M. A. Hanna pp. 229-240.
  • Effect of chemical modification on the microstructure of raw meat batters. A. Gordon, S. Barbut pp. 241-254.
  • Textural and microstructural properties of frozen fish mince as affected by the addition of nonfish proteins and sorbitol. K. S. Yoon, C. M. Lee, L. A. Hufnagelpp. 255-266.
  • Ultrastructural evidence for temperature-dependent Ca2+ release from fish sarcoplasmic reticulum during rigor mortis. H. Ushio, S. Watabe, M. Iwamoto, K. Hashimoto pp. 267-275.

Food Structure Vol. 10, Number 4, 1991

  • Microstructural studies of gluten and a hypothesis on dough formation. T. Amend, H.-D. Belitz, pp. 277-288.
  • Light microscopy observations on the mechanism of dough development in Chinese steamed bread production. S. Huang, R. Moss pp. 289-294.
  • Scanning electron microscope observations of growth and ochratoxin-A production of Aspergillus alutaceus variety alutaceus (formerly A. ochraneuson gamma-irradiated barley. J. G. Szekely, W. S. Chelack, S. Delaney, R. R. Marquardt, A. A. Frohlich pp. 295-302.
  • Immunohistochemical techniques applied to raw and mildly heat treated meat systems. B. Egelandsdal, S. Kidman, A.-M. Hermansson pp. 303-316.
  • Pathology of turkey skeletal muscle: Implications for the poultry industry. A. A. Sosnicki, B. W. Wilson (Review paper), pp. 317-326.
  • Composition and structure of demineralized spray-dried milk permeate powder. M. Kaláb, M. Caric, S. Milanovic pp. 327-332.
  • Fat-holding in hamburgers as influenced by the different constituents of beef adipose tissue. A. Olsson, E. Tornberg pp. 333-344.
  • The effect of processing temperatures on the microstructure and firmness of labneh made from cow's milk by the traditional method or by ultrafiltration. A. Y. Tamime, M. Kaláb, G. Davies pp. 345-352.
  • The contribution of milk serum proteins to the development of whipped cream structure. E. C. Needs, A. Huitson pp. 353-360.
  • Natural systems for preventing contamination and growth of microorganisms in foods. K.-T. Chung, C. A. Murdock (Tutorial paper) pp. 361-374.

Food Structure Vol. 11, Number 1, 1992

  • A low-temperature scanning electron microscopy study of ice cream. I. Techniques and general microstructure. K. B. Caldwell, H. D. Goff, D. W. Stanley pp. 1-10.
  • A low-temperature scanning electron microscopy study of ice cream. II. Influence of selected ingredients and processes. K. B. Caldwell, H. D. Goff, D. W. Stanley pp. 11-24.
  • Distribution of aromatic compounds in coastal bermudagrass cell walls using ultraviolet absorption scanning microspectrophotometry. N. O. Ames, R. D. Hartley, D. E. Akin pp. 25-32.
  • Molecular strategies to improve protein quality and reduce flatulence in legumes: A review. B. O. de Lumen pp. 33-46.
  • Effects of processing on the microstructure of oat (Avena sativa) bran concentrate and the physicochemical properties of isolated beta-glucans. K. Autio, Y. Mälkki, T. Virtanen pp. 47-54.
  • Image analysis to determine intramuscular fat in muscle. T. Ishii, R. G. Cassens, K. K. Scheller, S. C. Arp, D. M. Schaefer pp. 55-60.
  • Structure and rheology of string cheese. S. Taneya, T. Izutsu, T. Kimura, T. Shioya pp. 61-72.
  • Effect of lactose and protein on the microstructure of dried milk. V. V. Mistry, H. N. Hassan, D. J. Robison pp. 73-82.

Food Structure Vol. 11, Number 2, 1992

  • Computer modeling: The adjunct micro technique for lipids. J. W. Hagemann, J. A. Rothfus (Review paper) pp. 85-100.
  • A moving optical fibre technique for structure analysis of heterogeneous products: Application to the determination of the bubble-size distribution in liquid foams. C. G. J. Bisperink, J. C. Akkerman, A. Prins, A. D. Ronteltap pp. 101-108.
  • A moving optical fibre technique for structure analysis of heterogeneous products: Application to different foodstuffs. J. C. Akkerman, C. G.J. Bisperink, A. D. Ronteltap pp. 109-114.
  • Characterization of the pore structure of starch based food materials.Z.Hicsasmaz, J. T. Clayton pp. 115-132.
  • Effect of chemical modifications on the stability, texture and microstructure of cooked meat batters. A. Gordon, S. Barbut (Review paper) pp. 133-146.
  • Structure of margarines made with low erucic acid rapeseed oil. J. Hojerová, S. Schmidt, J. Krempaský pp. 147-154.
  • Microstructure and texture of khoa. G. R. Patil, A. A. Patel, P. Allan-Wojtas, M. Kaláb pp. 155-164.
  • Effect of ionizing gamma-radiation on thermoluminescence and electron spin resonance intensities in milk protein concentrate powders. J. Kispéter, L. I. Horváth, L. I. Kiss pp. 165-170.
  • Microstructural differences among adzuki bean (Vigna angularis) cultivars. E. Engquist, B. G. Swanson pp. 171-180.
  • Scanning electron microscopy studies of a typical Spanish confectionery product "Xixona turrón". M. A. lluch, M. J. Galotto, A. Chiralt pp. 181-186.

Food Structure Vol. 11, Number 3, 1992

  • Mechanical properties of starch, protein, and endosperm and their relationship to hardness in wheat. G. M. Glenn, R. K. Johnston pp. 187-200.
  • Importance of enzymes to value-added quality of foods. J. R. Whitaker (Review paper) pp. 201-208.
  • Cryo-scanning electron microscopy investigation of the texture of cooked potatoes. J. T. van Marie, A. C. M. Clerkx, A. Boekestein pp. 209-216.
  • Effect of draw pH on the development of curd structure during the manufacture of Mozzarella cheese. L. J. Kiley, P. S. Kindstedt, G. M. Hendricks, J. E. Levis, J. J. Yun, D. M. Barbano pp. 217-224.
  • Influence of addition of polyol and food emulsifiers on the retrogradation rate of starch. M. Miura, A. Nishimura, K. Katsuta pp. 225-236.
  • Light microscopy measurements of ice recrystallization in frozen corn starch pastes using isothermal freeze fixation. C. Ferrero, M. N. Martino, N. E. Zaritzky pp. 237-248.
  • Relating spectral observations of the agricultural landscape to crop yield. C. L. Wiegand, A. J. Richardson (Tutorial paper) pp. 249-258.
  • Scanning electron microscopy and ultraviolet absorption microspectrophotometry of plant cell types of different biodegradabilities. E. E. Akin, L. L. Rigsby pp. 259-272.
  • Functional properties and microstructure of chicken breast salt soluble protein gels as influenced by pH and temperature. S. F. Wang, D. M. Smith pp. 273-285.

Food Structure Vol. 11, Number 4, 1992

  • Identification of proteins and complex carbohydrates in some commercial low-fat dairy products by means of immunolocalization techniques. B. L. Armbruster, N. Desai pp. 289-300.
  • Characterization of food packaging materials by microscopic, spectrophotometric, thermal and dynamic mechanical analysis. I. R. Urzendowski, D. G. Pechak (Review paper) pp. 301-314.
  • Heat-induced structural changes in acid-modified barley starch dispersions.K. Autio, K. Poutanen, T. Suortti, E. Pessa pp. 315-322.
  • Ultrastructural changes in cherimoya fruit injured by chilling. M. Gutiérrez, M. del Mar Sola, L. Pascual, M. I. Rodríguez-García, A. M. Vargas pp. 323-332.
  • Scanning electron microscopy structure and firmness of papain treated apple slices. Y. Luo, M. E. Patterson, B. G. Swanson pp. 333-338.
  • The cellular structure of selected apple varieties. K. G. Lapsley, F. E. Escher, E. Hoehn pp. 339-350.
  • Objective measurement of the visual aspect of dry sausage slices by image analysis. A.-C. Roudot, F. Duprat, M.-G. Grotte, G. O'Lidha pp. 351-360.
  • Physicochemical properties of irradiation modified starch extrudates. A. S. Sokhey, R. Chinnaswamy pp. 361-371.
  • Thermal processing effects on rice characteristics. S.-M. Chang, H.-C. Yangpp. 373-382.

Food Structure Vol. 12, Number 1, 1993

  • Interfacial viscoelasticity in emulsions and foams. E. H. Lucassen-Reynders(Review paper) pp. 1-12.
  • Age related changes in the microstructure of Mozzarella cheese. L. J. Kiely, P. S. Kinstedt, G. M. Hendricks, J. E. Levis, J. J. Yun, D. M. Barbano pp. 13-20.
  • An apparatus for a new microcube encapsulation of fluid milk in preparation for transmission electron microscopy. M. C. Alleyne, D. J. McMahon, N. N. Youssef, S. Hekmat pp. 21-30.
  • Whey proteins as microencapsulating agents. Microencapsulation of anhydrous milkfat - structure evaluation. M. Rosenberg, S. L. Young pp. 31-42.
  • Lactose crystallization in spray-dried milk powders exposed to isobutanol. C. A. Aguilar, G. R. Ziegler pp. 43-50.
  • Mechanism of hard-to-cook defect in cowpeas: Verification via microstructure examination. K. Liu, Y.-C. hung, R. D. Phillips pp. 51-58.
  • A comparison of the effects of oven roasting and oil cooking on the microstructure of peanut (Arachis hypogaea L. cv. Florigiant) cotyledon. C. T. Young, W. E. Schadel pp. 59-66.
  • Distribution and intracellular localization of titanium in plants after titanium treatment. G.Kelemen, A. Keresztes, E. Bácsy, M. Fehér, P. Fodor, I. Pais pp. 67-72.
  • Critical point drier as a source of contamination in food samples prepared for scanning electron microscopy. G. Larocque, A. F. Yang (Technical tip) pp. 73-74.
  • Microstructural studies in fat research. I. Heertje (Tutorial paper) pp. 77-94.
  • Practical aspects of electron microscopy in dairy research. M. Kaláb (Tutorial paper) pp. 95-114.
  • The stabilisation of air in foods containing fat - a review. B. E. Brooker(Tutorial paper) pp. 115-122.
  • Food microscopy and the nutritional quality of cereal foods. S. H. Yiu (Tutorial paper) pp. 123-133.

Food Structure Vol. 12, Number 2, 1993

  • Interfacial interactions, competitive adsorption and emulsion stability. J. Chen, E. Dickinson, G. Iveson pp. 135-146.
  • A comparative study of normal and hard-to-cook Brazilian common bean (Phaseolus vulgaris): Ultrastructural and histochemical aspects. E. Garcia, F. M. Lajolo, B. G. Swanson pp. 147-154.
  • Microstructure of black, green and red gram. E. Joseph, S. G. Crites, B. G. Swanson pp. 155-162.
  • Liquid holding capacity and structural changes during heating of fish muscle: Cod (Gadus morhua L.) and salmon (Salmo salar). R. Ofstad, S. Kidman, R. Myklebust, A.-M. Hermansson pp. 163-174.
  • Physico-chemical changes occurring in gamma irradiated flours studied by small-angle X-ray scattering. K Ciesla, T. Zóltowski, R. Diduszko pp. 175-180.
  • Microstructure and rheological properties of composites of potato starch granules and amylose: A comparison of observed and predicted structures. K. Svegmark, A.-M. Hermansson pp. 181-194.
  • Structure and composition of apple juice haze. T. Beveridge, V. Tait pp. 195-198.
  • Fluidization and its applications to food processing. N. C. Shilton, K. Niranjan(Tutorial paper) pp. 199-216.
  • Heat-induced structural changes in faba bean starch paste: The effect of steaming faba bean seeds. B. Kaczynska, K. Autio, J. Fornal pp. 217-224.
  • The surface coverage of fat on food powders analyzed by ESCA (electron spectroscopy for chemical analysis). P. Fäldt, B. Bergenståhl, G. Carlsson pp. 225-234.
  • Three-dimensional molecular modeling of bovine caseins. H. M. Farrell Jr., E. M. Brown, T. F. Kumosinski pp. 235-250.
  • Microstructure of Mozzarella cheese during manufacture. C. J. Oberg, W. R. McManus, D. J. McMahon pp. 251-258.
  • Composition and microstructure of commercial full-fat and low-fat cheeses.V. V. Mistry, D. L. Anderson pp. 259-266.
  • Microstructure of whey protein/anhydrous milkfat emulsions. M. Rosenberg, S. L. Lee pp. 267-274.

Food Structure Vol. 12, Number 3, 1993

  • Development of the food microscopist. D. F. Lewis (Key note paper) pp. 275-284.
  • The stabilisation of air in cake batters - the role of fat. B. E. Brooker pp. 285-296.
  • Water-holding properties of milk protein products - a review. W. Kneifel, A. Seiler (Review paper) pp. 297-308.
  • Relationship between the starch granule structure and the textural properties of heat-induced surimi gels. V. Verrez-Bagnis, B. Bouchet, D. J. Gallant pp. 309-320.
  • Biodegradation of cell types in normal and brown midrib mutant pearl millet (Pennisetum glaucum (L) R Br): Microspectrophotometric and electron microscopic studies of walls and wall layers. D. E. Akin, L. L. Rigsby. W. W. Hanna pp. 321-332.
  • Microstructural approach to legume seeds for food uses. K. Saio, M. Monma(Tutorial paper) pp. 333-342.
  • Structure and function of food products: A review. I. Heertje (Key note paper) pp. 343-364.
  • A tutorial and comprehensive bibliography on the identification of foreign bodies found in food. D. F. Lewis (Tutorial paper) pp. 365-378.
  • Identification of radiation treatment of mineral-enriched milk protein concentrate by complex test protocols. A comparison of thermoluminescence, electron spin resonance, and rheological investigations. J. Kispéter, L. I. Horváth, K. Bajúsz-Kabók, L. I. Kiss, P. Záhonyi-Racs pp. 379-384.

Food Structure Vol. 12, Number 4, 1993

  • Luminescencce techniques to identify the treatment of foods by ionizing radiation. G. A. Schreiber, B. Ziegelmann, G. Quitzsch, N. Helle, K. W. Bögl(Review paper) pp. 385-396.
  • Properties of irradiated starches. A. S. Sokhey, M. A. Hanna (Tutorial paper) pp. 397-410.
  • Hydrocolloids as food emulsifiers and stabilizers.N. Garti, D. Reichman (Review paper) pp. 411-426.
  • Changes in the microstructure of a comminuted meat system during heating.P. A. Koolmees, G. Wijngaards, M. H. G.. Tersteeg, J. G. van Logtestijn pp. 427-442.
  • An overview of the role of fat in nutrition and formulation and its measurement in the live animal, meat carcass and processed meat products. P. B. Newman (Tutorial paper) pp. 443-456.
  • Microstructure and fat extractability in microcapsules based on whey proteins or mixtures of whey proteins and lactose. D. L. Moreau, M. Rosenberg pp. 457-468.
  • Structure-compressive stress relationships in mixed dairy gels. J. M. Aguilera, J. E. Kinsella, M. Liboff pp. 469-474.
  • High-resolution scanning electron microscopy of milk products: A new sample preparation procedure. W. R. McManus, D. J. McMahon, C. J. Oberg pp. 475-482.
  • Microstructure of idli. E. Joseph, S. G. Crites, B. G. Swanson pp. 483-488.
  • Baking properties and microstructure of yeast-raised breads containing wheat bran:carrageenan blends or laminates. P. R. Belisle, B. A. Rasco, K. Siffring, B. Bruinsma pp. 489-496.

Studies of Food Microstructure

Published in 1981 by Scanning Electron Microscopy, Inc., AMF O'Hare, IL 60666, USA
Editors: D. N. Holcomb, M. Kaláb
ISBN: 0-931288-22-3 Library of Congress Catalog Card. No.: 81-84080 1981

Table of contents:

SFMS numbers refer to pages in Studies of Food Microstructure, SEM numbers refer to the original location of the papers in Scanning Electron Microscopy.

Part I - General papers

  • Some examples of scanning electron microscopy in food science. R. J. Carroll, S. B. Jones (Review paper) SFMS pp. 1-8, SEM/1979/III:253-259.
  • Preparation of food science samples for SEM. J. F. Chabot (Review paper) SFMS pp. 9-16, SEM/1979/III:279-286, 298.
  • Application of scanning electron microscopy for the development of materials for food. C.-H. Lee, C. K. Rha (Review paper) SFMS pp. 17-24, SEM/1979/III:465-471.
  • The use of microscopy to explain the behaviour of foodstuffs - a review of work carried out at the Leatherhead Food Research Association. D. F. Lewis(Review paper) SFMS pp. 25-38, SEM/1981/III:391-404.
  • Identification of foreign matter in foods. J. T. Stasny, F. R. Albright, R. GrahamSFMS pp. 39-50, SEM/1981/III:599-610, 560.

Part II - Meat foods

  • Scanning electron microscopy in meat science. C. A. Voyle (Tutorial paper) SFMS pp. 51-60, SEM/1981/III:405-413.
  • Preparation of muscle samples for electron microscopy. H. D. Geissinger, D. W. Stanley (Tutorial paper) SFMS pp. 61-72, SEM/1981/III:415-426, 414.
  • The effect of catheptic enzymes on chilled bovine muscle. S. H. Cohen, L. R. Trusal SFMS pp. 73-78, SEM/1980/III:595-600.
  • Microscopical observation on electrically stimulated bovine muscle. C. A. VoyleSFMS pp. 79-86, SEM/1981/III:427-434.
  • Scanning and transmission electron microscopy of normal and PSE porcine muscle. J. D. Cloke, E. A. Davis, J. Gordon, S.-I. Hsieh, J. Grider, P. B. Addis, C. J. McGrath SFMS pp. 87-98, SEM/1981/III:435-446.
  • Identification of fat and protein components in meat emulsions using SEM and light microscopy. F. K. Ray, B. G. Miller, D. C. Van Sickle, E. D. Aberle, J. C. Forrest, M. D. Judge SFMS pp. 99-104, SEM/1979/III:473-478.
  • Meat emulsions - fine structure relationships and stability. R. J. Carroll, C. M. Lee SFMS pp. 105-110, SEM/1981/III:447-452.

Part III - Milk products, gels, and mayonnaise

  • Scanning electron microscopy of dairy products: An overview. M. Kaláb (Review paper) SFMS pp. 111-122, SEM/1979/III:261-272.
  • Electron microscopy of milk products: A review of techniques. M. Kaláb(Review paper) SFMS pp.123-142, SEM/1981/III:453-472.
  • Microstructure and rheology of process cheese. A. A. Rayan, M. Kaláb, C. A. Ernstrom SFMS pp. 143-152, SEM/1980/III:635-643.
  • Electron microscopy and sensory evaluation of commercial cream cheese. M. Kaláb, A. G. Sargant, D. A. Froehlich SFMS pp. 153-162, SEM/1981/III:473-482, 514.
  • Morphological, ultrastructural and rheological characterization of Cheddar and Mozzarella cheese. M. V. Taranto, P. J. Wan, S. L. Chen, K. C. Rha SFMS pp. 163-168, SEM/1979/III:273-278.
  • Morphological and textural characterization of soybean Mozzarella cheese analogs. M. V. Taranto, C. S. T. Yang SFMS pp.19-178, SEM/1981/III:483-492.
  • Possibilities of an electron-microscopic detection of buttermilk made from sweet cream in adulterated skim milk. M. Kaláb SFMS pp.179-186, SEM/1980/III:645-651.
  • A scanning electron microscopical investigation of the whipping of cream. D. G. Schmidt, A. C. M. van Hooydonk SFMS pp.187-192, 210, SEM/1980/III:653-658, 644.
  • A comparison of the microstructure of dried milk products by freeze-fracturing powder suspensions in non-aqueous media. W. Buchheim SFMS pp. 193-202, SEM/1981/III:493-502.
  • SEM investigation of the effect of lactose crystallization on the storage properties of spray-dried whey. M. Saltmarch, T. P. Labuza SFMS pp. 203-209, SEM/1980/III:659-665.
  • Effect of acidulants and temperature on microstructure, firmness and susceptibility to syneresis of skim milk gels. V. R. Harwalkar, M. Kaláb SFMS pp. 211-221, SEM/1981/III:503-513.
  • Structure of various types of gels as revealed by scanning electron microscopy (SEM). V. E. Colombo, P. J. Spath SFMS pp. 223-230, SEM/1981/III:515-522.
  • Microstructure of mayonnaise and salad dressing. M. A. Tung, L. J. Jones SFMS pp. 231-238, SEM/1981/III:523-530.

Part IV - Foods of plant origin

  • Scanning electron microscopy of soybeans and soybean protein products. W. J. Wolf, F. L. Baker (Review paper) SFMS pp. 239-252, SEM/1980/III:621-634.
  • Soybean seed-coat structural features: pits, deposits and cracks. W. J. Wolf, F. L. Baker, R. L. Bernard SFMS pp. 253-266, SEM/1981/III:531-544.
  • An SEM study of the effects of avian digestion on the seed coats of three common angiosperms. L. B. Smith SFMS pp. 267-274, SEM/1981/III:545-552.
  • Microstructure of traditional Japanese soybean foods. K. Saio (Review paper) SFMS pp. 275-282, SEM/1981/III:553-559.
  • Effects of exogenous enzymes on oilseed protein bodies. R. D. Allen, H. J. Arnott SFMS pp. 283-292, SEM/1981/III:561-570.
  • Tannin development and location in bird-resistant sorghum grain. P. Morrall, N. v. d. W. Liebenberg, C. W. Glennie SFMS pp. 293-298, SEM/1981/III:571-576.
  • Light microscopy of plant constituents in animal feeds. J. G. Vaughan (Tutorial paper) SFMS pp. 299-304, SEM/1981/III:577-582.
  • The relationship between wheat microstructure and flourmilling. R. Moss, N. L. Stenvert, K. Kingswood, G. Pointing SFMS pp. 305-312, SEM/1980/III:613-620.
  • Scanning electron microscopy of flour-water doughs treated with oxidizing and reducing agents. L. G. Evans, A. M. Pearson, G. R. Hooper SFMS pp. 313-322, SEM/1981/III:583-592.
  • Structural studies of carrots by SEM. E. A. Davis, J. Gordon (Tutorial paper) SFMS pp. 323-332, SEM/1980/III:601-611.
  • The microstructure of orange juice. G. G. Jewell SFMS pp. 333-338, SEM/1981/III:593-598.

Note: This page was compiled by M. Kaláb on December 28, 1998 and corrected, with apologies to the authors concerned, on February 13, 2014. Corrections: Page numbers in Food Microstructure 1(1) and in Food Microstructure 4(2) for authors R.J. Carroll et al. 

This page was compiled by M. Kaláb on December 28, 1998 and updated on March 17, 2011.

Glossary of Terms

Boiling Point Elevation: One of the colligative properties. The boiling point of a solution is increased over that of water by the presence of solutes, and the extent of the increase is a function of both concentration and molecular weight.

Colligative Properties: Properties which depend on the number of molecules in solution, a function of concentration and molecular weight, rather than just on the total percent concentration. Such properties include boiling point elevation, freezing point depression, and osmotic concentration.

Emulsion: liquid droplets dispersed in another immiscible liquid. The dispersed phase droplet size ranges from 0.1 - 10 µ m. Important oil-in-water food emulsions, ones in which oil or fat is the dispersed phase and water is the continuous phase, include milk, cream, ice cream, salad dressings, cake batters, flavour emulsions, meat emulsions, and cream liquers. Examples of food water-in-oil emulsions are butter or margarine. Emulsions are inherently unstable because free energy is associated with the interface between the two phases. As the interfacial area increases, either through a decrease in particle size or the addition of more dispersed phase material, i.e. higher fat, more energy is needed to keep the emulsion from coalescing. Some molecules act as surface active agents (called surfactants or emulsifiers) and can reduce this energy needed to keep these phases apart.

Foam: a gas dispersed in a liquid where the gas bubbles are the discrete phase. There are many food foams including whipped creams, ice cream, carbonated soft drinks, mousses, meringues, and the head of a beer. A foam is likewise unstable and needs a stabilizing agent to form the gas bubble membrane.

Freezing point depression: of a solution is a colligative property associated with the number of dissolved molecules. The lower the molecular weight, the greater the ability of a molecule to depress the freezing point for any given concentration. For example, in ice cream manufacturing, monosaccharides such as fructose or glucose produce a much softer ice cream than disaccharides such as sucrose, if the concentration of both is the same.

Osmotic pressure: A chemical force caused by a concentration gradient. It is a colligative property and the principle behind membrane processing.

pH: is a measure of the activity of the hydronium ion (H3O+) which, according to the Debye-Huckel expression, is a function of the concentration of the hydronium ion [H3O+], the effective diameter of the hydrated ion and the ionic strength (µ m) of the solvent. For solutions of low ionic strength (µ m < 0.1) hydronium ion activity is nearly equivalent to [H3O+] which is normally abbreviated to [H+]. Then, for a weak acid (HA) dissociating to H+ and A- with a dissociation constant, Ka and pKa equal to -log10 Ka, the most important relationships are defined the following two equations:

Ka = [H+] [A-] / [HA]

pH = log 1 / [H+] = pKa + log [A-] / [HA]

Reynold's Number: a dimensionless expression used in predicting flow patterns:

Reynold's Number

Stoke's Equation:The velocity at which a sphere will rise or fall in a liquid varies as the square of its diameter:

Stokes Law

For example, a fat globule with a diameter of 2 microns will rise 4 times faster than a fat globule with a diameter of 1 micron.

Titratable acidity: Ameasure of titaratable hydrogen ions. Includes H+ ions free in solution and those associated with acids and proteins.

Dairy Science and Technology General References

alfa Laval/Tetra Pak 1995. Dairy Processing Handbook, Tetra Pak Processing Systems, S-221 86, Lund, Sweden.

Clark, S., M. Costello, M.A. Drake and F. Bodyfelt. 2009. The Sensory Evaluation of Dairy Products, 2nd ed. Springer.

Fox, P.F. and P.L.H. McSweeney. 1998. Dairy Chemistry and Biochemistry. Blackie Academic and Professional.

Fox, P.F. and P.L.H. McSweeney. Advanced Dairy Chemistry. 3rd ed. Vol. 1. Proteins (2003). Vol. 2. Lipids (2006). Vol. 3. Lactose, Water, Salts and Minor Constituents (2009). Springer. 

Fox, P.F., T. P. Guinee, T. M. Cogan and P.L.H. McSweeney. 2000. Fundamentals of Cheese Science. Aspen Publication.

Hui, Y. H. 1993. Dairy Science and Technology Handbook. Vol. 1. Principles and Properties. Vol. 2. Product Manufacturing. Vol.3. Technology and Engineering. VCH. 

Marshall, R. T., Goff, H. D. and Hartel, R. W. 2003. Ice Cream. 6th Edition. Kluwer Academic.

Marth, E. H. and J. L. Steele. 2001. Applied Dairy Microbiology, 2nd ed. Marcel Dekker Inc.

Robinson, R. K., ed. 1994. Modern Dairy Technology. Vol. 1. Advances in Milk Products. Vol. 2. Advances in Milk Processing . Elsevier, NY.

Walstra, P.,J.T.M. Wouters and T.J. Geurts. 2006. Dairy Technology, 2nd Ed.. CRC/Taylor & Francis.

Regulations

National and international regulations for milk processing and products are a complicated business! In order to provide easier access to relevant laws, please see the following websites as helpful starters: