Raw milk quality

 4.1 The Principal Milk Components

See also Dairy Chemistry and Physics in the Dairy Science and Technology Education website.

Cheese can be made from the milk of many mammals including goats, sheep, buffalo, reindeer, camel, llama, zebra and yak. The milk of ruminants is the best milk for cheese making because it contains high levels of the milk protein casein which is required to provide an adequate coagulum. Our consideration of milk composition will include only a summary of the proximate analyses of the most common dairy species and a few, relevant with respect to cheese making, comments about each component.

Proximate Analysis

Gross composition of food (also referred to as proximate analysis) means distribution of the total amounts of fats, proteins, carbohydrates, ash (mainly minerals such as calcium) and moisture or total solids. Typical proximate analysis profiles for cows', sheeps' and goats' milk are listed in Table 4.1. Further discussion refers only to cows' milk unless otherwise stated.

Milk fat

Fat content ranges from 2.0 to 7.0 kg/hl. An approximate average for regions where Holstein Fresian cattle predominate is about 3.9 kg/hl. With respect to cheese manufacture and quality the following properties are important.

  • Most diverse of all natural fats. We are routinely quantifying over 100 fatty acids ranging from four carbons to 22 carbons in length. Many more have been identified.
  • Traditional, but not entirely justified, nutritional concerns are: (1) About 70% of milk fatty acids are saturated; and (2) It contains cholesterol.
  • Positive nutritional factors are butyric acid (anticarcinogenic), saturated but mid to short length fatty acids (antihypertensive), and rumenic acid (anticarcinogenic).
  • Unique flavour of dairy fat is due to short chain fatty acids, especially butyric acid
  • The two major spoilage reactions in milk fat are (1) break down of the triglyceride fat structure releasing fatty acids such as butyric to create a rancid flavor; and (2) Oxidation of unsaturated fats creating an oxidized flavour (flat, cardboard flavour)
  • The melting properties of butter fat are significant to cheese texture.

Milk Proteins

Total milk protein ranges from about 2.5 to 5.5 kg/hl. The average for regions in which Holstein Fresians predominate is about 3.3 kg/hl. There are two major groups of proteins, the caseins (about 2.6 kg/hl) which I refer to as the 'cheese proteins' and the whey proteins (about 0.7 kg/hl) which as the name suggests are usually lost in the whey during cheese making. Caseins are not water soluble and so the cow packages them in water dispersable particles called micelles which along with caseins include most of the milk calcium, magnesium, phosphate and citrate (more about casein micelles in Coagulation. Unlike whey proteins which are very sensitive to heat, caseins are little affected by heating except that they react with heat denatured whey proteins. Table 4.2 lists the principal caseins and some properties which are most relevant to cheese making. Similarly, Table 4.3 lists some properties of the principal whey proteins.

Factors affecting gross milk composition

Species 

Cheese making principles are similar for milk of all species with some modifications required to account for high solids of some species such as buffalo and sheep. Cows' milk and goat's milk have similar cheese making properties except that:

  • Goats' milk cheese tends to ripen by lipolysis (fat breakdown) more than cows' milk cheese.
  • Goats' milk has smaller fat globules which allows higher fat recovery and possibly a smoother texture..
  • Cheese making quality of goats' milk is also improved due to higher levels of alpha-S1 casein in goats' milk which permit better coagulation.

Genetics

Through out the modern history of dairying, farmers have selectively bred dairy cattle to increase production or fat content or both. Recently, genetic selection has focussed on other milk properties such as increasing the proportion of milk protein to fat. Three genetic effects are most relevant to cheese making.

(1) Relative proportions of fat and protein (P/F ratio)

Fat content and protein content generally increase or decrease in parallel, but fat varies more with feed and season then protein. The same is true for breed (genetic) effects, such that genetic selection has produced the the following practical effects in modern dairying.

  • Higher fat breeds have lower protein/fat ratio. For example, a typical protein/fat ratio in Jersey milk is 0.7 relative to 0.84 in Holstein milk (Table 4.4).
  • Genetic selection over the last 100 years has produced considerable within breed improvement with respect to both milk production and increased protein and fat content. The result is greatly increased per cow production of milk protein and fat. However, once again, because fat responds more to genetic selection than protein, the result in many areas has been a gradual decrease in the average P/F ratio. For example in Ontario, the P/F ratio decreased from about 0.88 in 1970 to 0.85 in 1995 (estimated from data provided by Laboratory Services Division, University of Guelph.

(2) Relative proportions of fat and protein to other solids

With respect to other solids, mineral content (mainly Ca, Mg, and P) generally varies in proportion to protein content and lactose content is relatively stable. Because lactose is largely a wasted component, increasing protein and fat by feed or genetic selection has economic advantages in terms of feed conversion, milk transportation costs, and waste handling.

(3) Stage of lactation

Fat content tends to increase during lactation as milk production decreases. The result is that the relative proportion of protein to fat (protein/fat ratio or P/F) is highest at the peak of lactation (about 60 days of lactation) and lowest at the end of lactation. Protein distribution also changes during lactation with resulting effects on cheese ripening and flavour. In particular, the proportion of alpha-caseins decrease during lactation while the proportion of beta-casein increases.

Feed

Depending on the relative demand for butter fat versus milk non-fat solids, there may be incentive to change the relative proportions of milk protein and fat. The only short term means to do this is by changing the diet. Generally less roughage and more high energy feeds will encourage lower fat content with little decrease in protein content to provide a higher P/F ratio.

Season

Seasonal variation in milk composition is most important to cheese yield efficiency and composition control. Some important seasonal effects are listed below and illustrated to the right. These observations are based on Ontario data.

  • Fat content reaches a minimum in August and a maximum in October.
  • Protein content changes roughly in parallel with fat content, but the seasonal variations are smaller, causing high protein fat ratios (P/F) during the summer and low P/F ratios in the winter.
  • Casein content also varies with season which is most important because cheese making is dependent on casein NOT on total protein (more on this in Treatment of milk for cheese makingStandardization of milk for cheese making and Yield efficiency).

Milk as a growth medium

Cheese making depends on the growth of bacteria to produce acidity, flavour compounds, and ripening enzymes. It is, therefore, important to understand the characteristics of milk as a growth medium.

General Nutrients

Milk is a good source of all principal nutrients, including carbon, nitrogen and macro-minerals. Many micronutrients such as vitamins and micro-minerals are also available. However, milk is unique with respect to its sugar.

Milk Sugar

Carbohydrates, especially simple sugars such as sucrose (table sugar), can be utilized as sources of energy more quickly than fats and proteins. However, the energy currency of the cell is glucose (also called dextrose) so to use available carbohydrates, microorganisms must be able to convert them to glucose.

The only sugar naturally present is milk is lactose. Most microorganisms lack the enzyme lactase which is required to break lactose into its two component sugars, namely, glucose and galactose. Lactic acid bacteria which do have lactase readily break down lactose and use glucose as an energy source. Lactic acid bacteria, therefore, have a competitive advantage in milk; that is, they are able to out grow other bacteria which are unable to obtain glucose from lactose. Further, some lactic acid bacteria are able to convert galactose to glucose.

Acidity (pH)

Acidity as measured by pH is one of the most critical parameters with respect to both food safety and both process and quality control of fermented foods such as cheese. The concepts of acidity and pH are explained in Sections 3.5. The titratable acidity of milk typically varies from 0.12 to 0.19% lactic acid depending on composition, especially protein content. The pH of milk is near the physiological pH of 6.8 which, considering the following points, means that milk is a good growth medium with respect to acidity (pH).

  • Most organisms grow best at pH near physiological pH of 6.8. As explained in Section 3.5, titratable acidity (TA) is not a good predictor of acid effects on microbial growth and chemical properties such as protein functionality.
  • The major groups of microorganisms important to food preservation are in order of increasing acid tolerance: bacteria, yeasts and moulds.
  • Natural fermentation of warm raw milk by lactic acid bacteria reduces milk pH to less than 4.0 which prevents the growth of pathogenic bacteria and most spoilage bacteria

Moisture

Milk has a high moisture content (typically 87% for cows' milk) and with respect to available moisture, is an excellent growth medium. But, it must be understood that with respect to microbial growth, the critical parameter is water activity not moisture content. Water activity (aw) is an index of the availability of water for microbial growth. It is the availability of water in the food reported as a fraction of the availability of water from pure water. In other words, the aw of water is 1 and the aw of other substances is reported as decimal fractions of 1. Water activity is reduced by dissolved substances, varying directly with number of dissolved molecules rather than the weight. For this reason, relative to large molecules such as proteins, small molecules such as sugar and salt have a large effect on water activity. For example, jams are preserved by their high sugar content.

Microorganisms vary greatly in their ability to survive and/or grow at reduced water activity. However, acknowledging that exceptions exist, the minimum water activity for the principal groups of microorganisms are as follows:

  • Most bacteria: 0.90 - 0.91
  • Most yeast: 0.87 - 0.94
  • Most moulds: 0.70 - 0.80

Compare these values with typical aw values for milk, cheese and a few other foods.

  • Milk, fresh fruits and vegetables, fresh meats 60 - 98% moisture, aw 0.97 - 1.00
  • Most baked products, some cheese, some cured meats, 20-60% moisture, aw 0.88 - 0.96.
  • Dehydrated foods such as breakfast cereals. Less than 5% moisture, aw 0.20 - 0.30

Typical aw values for some cheese at the marketing stage are given below (Eck and Gillis, 2000). See also typical aw values for cheese families in Table 1.1.

Availability of oxygen

  • Cottage 0.988
  • Brie 0.980
  • Munster 0.977
  • Saint-Paulin 0.968
  • Edam 0.960
  • Cheddar 0.950
  • Parmesan 0.917

Availability of oxygen

With respect to oxygen requirements, microorganisms may be:

  • Aerobic: must have oxygen to grow
  • Anaerobic: can only grow in the absence of oxygen
  • Microaerophilic: require small amounts of oxygen
  • Able to grow with or with out oxygen.

Moulds require oxygen, so they can be eliminated by vacuum or gas flush packaging. Most yeast are aerobic (require oxygen) but some can grow anaerobically (in the absence of oxygen). Bacteria may fall into any of these categories, but lactic acid bacteria are micoaerophilic or anaerobic.

Milk will acquire some dissolved oxygen during milking, storage and handling, but it is used up quickly during bacterial growth.

Types of microorganisms and their activity in milk

The numbered list below identifies seven types of bacteria according to how they change the properties of milk. Often these changes are negative (spoilage) but as we will see in later sections, many of these bacteria are important to the development of cheese flavour. Before proceeding to the list, please note the following definitions:

  • Psychrotrophic refers to microorganisms which are able to grow at temperatures less than 7C. Cold milk storage and transport selects for psychrotrophic bacteria which are often proteolytic and lipolytic. Common psychrotrophic bacteria in milk are species of Micrococci, Bacilli, Staphyloccoci, Lactobacilli, Pseudomonas, and coliformsPseudomonas species are the most common and typically have the most impact on quality. At temperatures of 2 - 4C, bacterial growth in milk is mainly due to strains of Pseudomonas flourescens. Little growth occurs at temperature less than 2C.
  • Spore forming bacteria are able to exist in a highly stable form called 'spores'. In the spore state, these bacteria are able to withstand greater extremes of acidity, temperature and desiccation.
  • Enzymes are biological catalysts that accelerate the rates of biochemical reactions. Bacterial enzymes are most significant to milk spoilage and cheese ripening but it is important to distinguish between the enzyme and the bacterial source. For example, many psychrotrophic bacteria produce heat stable enzymes which remain active in milk and cheese even after the bacteria are killed by pasteurization.

Keeping the above definitions in mind, note the following types of microorganisms, grouped according to their impact on milk quality.

(1) Lactic acid bacteria which ferment lactose to lactic acid and other end products. Lactic acid bacteria (LAB) important to cheese making will be described further in Cultures. For now note the following:

  • As noted earlier, LAB are able to readily metabolize lactose so they have some competitive advantage over other microorganisms.
  • Notwithstanding, their ability to metabolize lactose, LAB prefer temperatures greater than 30C, so, depending on initial relative counts, psychrotrophic bacteria including some coliform and pseudomonas bacteria are able to outgrow LAB at room temperature.

(2) Proteolytic bacteria which degrade protein and cause bitterness and putrefaction. Most important in cheese milk are species of:

  • Pseudomonas which are psychrotrophic and produce heat stable lipases.
  • Bacillus which form heat stable spores and survive pasteurization

(3) Lipolytic bacteria which degrade fats and produce lipolytic rancidity. Again, the most common example in milk is the genus Pseudomonas. Several psychrotrophic species of Pseudomonas produce heat stable lipases as well as proteases.

(4) Gas producing microorganisms which cause cheese openness, floating curd in cottage cheese, and gassy milk.

  • Yeasts are always present in milk and are common contaminants during the cheese making process. They may cause 'yeast slits' in cheese and contribute to ripening of surface ripened cheese.
  • Coliform bacteria are always present in milk but their numbers can be minimized by good sanitation. Also, coliform bacteria compete poorly with lactic acid bacteria, so their numbers rapidly decrease in the presence of a rapidly growing lactic acid culture.
  • Clostridium tyrobutyricum is a thermoduric (survives pasteurization) spore forming organism of legendary fame among cheese makers. C. tyrobutyricum causes gas formation (carbon dioxide) during the later stages of ripening of Swiss and Dutch type cheeses. The resulting craters and cracks in the cheese are called 'late gas defect'. European cheese makers frequently check raw milk for thermoduric and/or spore forming bacteria to estimate potential for late gas defects. Five hundred spores per litre of milk are sufficient to cause late gas defect.
  • Propioni bacterium produces the desirable gas formation in Swiss type cheese.
  • Some lactic cultures, called heterofermentative, also produce carbon dioxide. See Cultures

(5) Ropy bacteria cause stringy milk due to excretion of gummy polysaccharides. Usually ropy bacteria such as Alcaligenes viscolactis are undesirable. However, in some fermented dairy products, ropy lactic acid bacteria such as certain subspecies of Lactococcus lactis are used to develop texture.

(6) Sweet curdling bacteria produce rennet-like enzymes which may coagulate milk. Common examples are the psychrotrophic spore formers Bacillus subtilis and Bacillus cereus.

(7) Numerous off flavours have been associated with specific milk contaminates. Some examples are:

  • Malty: S. lactis var maltigenes
  • Bitter: see (2) Proteolytic bacteria
  • Rancid: see (3) Lipolytic bacteria
  • Unclean: coliform bacteria
  • Fishy: Pseudomonas
  • Fruity: Pseudomonas 

Pathogenic Bacteria

This short course makes no attempt to provide comprehensive training on food safety with respect to cheese manufacture. However, some food safety principles will be discussed in the context of other topics, for example, acid control and food plant sanitation. Here, we mention only some characteristics of a few pathogens which are particularly significant to cheese making. We begin with definitions to distinguish between food infection and food intoxication.

Food infections are caused by organisms which grow in the gastro intestinal track. Illness occurs after ingestion of an infectious dose which depends on many factors including the health status of the person.

Food intoxication results from toxins produced by bacteria. Toxins may be present within the bacteria (endotoxin) or excreted outside the bacteria (exotoxin). The organism need not be alive or even present to cause illness. A good example, is Staphylococcus aureus. Like all the other pathogenic bacteria listed below, Staphylococcus aureus is destroyed by pasteurization but its enterotoxin survives pasteurization.

Pathogens: common before 1940

  • Corynebacterium diptheriae: causes diphtheria
  • Brucella abortus: causes brucellosis in cows and undulant fever in people.
  • Mycobacterium tuberculosis: causes TB in people.
  • Coxiella burneti: causes Q fever in people.

Pathogens which emerged during 1940 - 1970

  • Staphylococcus aureus: food intoxication caused by heat stable enterotoxins of which enterotoxin A is the most common.
  • Salmonella species: Salmonellosis is an infection caused by many species and strains of species of Salmonella. Salmonellosis is of great concern to the dairy industry, especially the cheese and milk powder sectors. Infectious doses can be extremely low, perhaps as low as a single organism.
  • Enteropathogenic E. Coli produce enterotoxins, some of which are heat stable. Of numerous species and strains, the most famous is E. Coli 0157 H7, which occurs frequently in raw milk. E. Coli0157 H7 is of particular concern because it is quite acid tolerant and is able to grow at refrigeration temperatures.

Recent Pathogens

  • Yersina enterocolitica is a psychrotrophic infectious agent.
  • Campylobacter jejuni, is an infectious agent which has passed Salmonella as the leading cause of diarrhoea all over the world.
  • Listeria monocytogenes is a psychrotrophic infectious agent, which requires special caution because it is acid tolerant and more heat stable than most pathogens, although it does not survive proper pasteurisation.
  • Bacillus cereus is mainly important as spoilage agent. However, some strains are mildly pathogenic which is problematic because Bacillus cereus forms heat stable spores which survive pasteurization and are able to grow at refrigeration temperatures.

Antibiotics

Lactic cultures are very sensitive to antibiotics. In most jurisdictions increasing penalties have greatly reduced antibiotic residues in milk. Nevertheless, antibiotic testing of all cheese milk is still recommended. See rapid screening tests in Section 3.10.

Mastitic Milk

Mastitis is an infection of the udder which negatively impacts milk quality. Pooling milk dilutes the effect of single infected cows and herds but in most jurisdictions the cumulative effect of mastitis, especially subclinical mastitis, is significant. Olson as cited in Eck and Gillis (2000) estimates a cheese yield loss of 1% if 10% of the milk is from cows with subclinical mastitis. Further, as noted below, the quality effects of mastitic milk are probably of more economic importance than the yield effects.

Causative organisms include human pathogens such as E. coli and Staphylococcus aureus. Nonbacterial infections such as prototheca infection also cause high SCC. Based on new automated procedures for bacteria counting, Ontario producer milk data suggests that prototheca is a common mastitic agent and frequently contributes to high SCC and bacterial counts.

Typical Ranges of Somatic Cells

Somatic cells include any type of 'body' cell in the milk, such as skin cells (epithelial) from the cows' udders and leucocytes of several types. Leucocytes are white blood cells which are part of the cow's immune response to infection in the udder, so they are used as an index of mastitis or udder infection. Several observations are relevant:

  • The milk of healthy cows should contain less than 100,000 somatic cells per ml of milk. Higher counts indicate subclinical mastitis (infection in the udder).
  • Clinical mastitis is associated with counts greater than 1,000,0000/ml
  • Producer milks in Ontario average about 200,000 cells/ml but counts less than 100,000 can readily be achieved with good herd management

Critical Ranges With Respect to Milk Quality

There is evidence that counts as low as 100,000 cells/ml affect cheese yield (Barbano et al, 1991, J. Dairy Sci. 74:369) and the quality of other dairy products such as ultra-high temperature milk. SCC in the range of 250,000 - 500,000 are associated with altered milk composition and decreased cheese yield. When counts exceed 1,000,000 cells/ml, altered milk composition and reduced cheese yield, are obvious.

Composition Effects

Gross composition effects of udder infection are not significant for SCC less than about 250,000/ml. Above that level the following trends are observed:

  • Little change in fat content
  • Increased mineral content, particularly more cloride.
  • Decreased lactose content which balances the osmotic effects of increased mineral content.
  • Protein effects:
    • Less casein
    • More whey proteins, especially immunoglobulins
    • More nonprotein nitrogen

Increased pH (up to 7.5 whereas 6.7 is normal)

Bacteriological Properties:

High SCC are normally associated with shedding of pathogenic (to humans) bacteria in the milk including E. coli, S. aureus and others. Basically, whatever organism is causing the udder infection, including the algae, prototheca will be present in the milk. Further, growth factors present in high SCC milk encourage growth of both E. coli and S. aureus (Amer. J. Vet. Res. 45:2504). The growth rates of some lactic cultures are also affected; Streptococcus thermophilis grows faster and Lactobacillus acidophilus is inhibited.

Significance To Cheese Milk

Cheese yield is affected in two ways:

  • Mastitic milk contains more plasmin, a heat stable milk protease which degrades protein and causes more protein to be lost in the whey.
  • Reduced casein directly affects cheese yield.
  • Poor curd formation (longer flocculation time, slower rate of curd firming, and reduced maximum firmness) contributes to yield loss as fines.

Perhaps more important than yield are the effects of subclinical mastitis on cheese quality (J. Dairy Res. 53:645). Modest levels of SCC cause several quality problems:

  • Decreased curd strength due to high whey proteins, low caseins, high pH and altered calcium-phosphate-caseinate balance. As noted above, these changes affect cheese yield, but they also impact quality.
  • Higher moisture cheese due to impaired curd syneresis.
  • Soft, less elastic, sticky and grainy cheese texture.
  • Increased flavour intensity, usually with off flavours

Significance to Fluid Milk

Very high counts (>2 million) will cause milk to taste salty and result in many quality problems. Lower counts, even as low as 300,000 can increase development of bitter flavour due to increased levels of plasmin. This is a particular problem with ultra-high-temperature processed milk because the enzyme is heat stable and the storage time is long enough to permit significant protein degradation.

Raw Milk quality tests

The following list is a summary of the most important raw milk quality tests. Procedures for some milk quality tests are described in Process and quality control procedures.

  1. Organoleptic
  2. Total plate counts: good < 3,000/ml; maximum raw milk 100,000
  3. Coliforms: good < 10/ml; concern > 25; max 100
  4. Psychrotrophes (grow at T < 7C): good < 1,000
  5. Somatic cell counts: good <100,000; concern >300,000;
  6. Rapid test for inhibitors
  7. Disk assay (official test for inhibitors)
  8. Added water: maximum freezing point -0.505C (-525H)
  9. Composition: fat, protein, lactose, total solids, and casein if possible.