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.