Chester Clinic

 

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Efficient Cooling has been written to provide a basic understanding of thecomplex subject of milk cooling. It is from the physiological, biological andbehavioural platforms presented here that DeLaval products are developed.
Our philosophy is to work in harmony with biology, the environment, andthe natural processes of life, towards optimal harvesting and cooled storageof nature's most perfect food, milk.
We would especially like to thank the following for all their help (in alphabetical order): Friesland Dairy FoodsDirector, Transport Raw Materials Mr A. P. M. Rombouts Campina Melk UnieUnion, Transport and Quality Chief Viro FoodEnergy Engineer Wever ArchitectsEngineer DeLaval International ABBusiness Unit Cooling The NetherlandsOctober, 2000.
DeLaval International AB, Tumba, Sweden.
All rights reserved. No part of this book may be reproduced, in any form or byany means, without the permission of the publisher.
This publication has been printed on paper which follows the recommendationsmade by the Swedish Environment Federation.
Printed by Sandströms & Söner, September 2000 List of contents
– Where does milk come from?– What is in milk? Why cool milk? .9
– Milk hygiene and quality
– Bacteria
– Conclusion
Milk collection.28
– How to transport raw milk to the dairy plant
– Milk collection points and centres
– Summary
Cooling technology .32
– Heat – an energy difference
– Cooling with basic facilities
– Modern cooling systems
Demands on cooling equipment.43
– Material
– Cleaning
– Cooling medium (refrigerants)
– Norms for milk tanks
DeLaval cooling systems .49
VIII Future aspects .51
Milk is one of the most important products for human consumption. Its highquality is vital, and cooling is one of the most efficient and effective ways tomaintain milk's freshness. The demand on milk producers is to produce milkwith a composition that meets the needs of consumers.
All chemical processes depend on temperature. At lower temperatures, chemi-cal processes are slowed down and chemical spoilage is delayed. Milk containsseveral nutrients that are necessary for the life of all living beings. It is also theperfect growing medium for micro-organisms, although at 4 °C micro-organ-isms cannot duplicate and the microbiological spoilage of milk is avoided.
After having followed the right milking and hygienic procedures, quickly cool-ing milk to 4 – 3 °C is the best way to avoid microbiological growth andchemical changes.
In the first human societies, snow and water were used for cooling food. Later,the theory of cooling by evaporation was developed and practised for a longtime. Old Egyptian nations evaporated water in porous vases to cool their food.
Cooling media such as refrigerants (food cooling) and ammoniac (non-foodcooling) are used in present-day refrigeration equipment. In the future, thesecooling media will be replaced by others that have a less negative impact onthe environment.
The aim of this booklet is to introduce the reader to the complex, but also fas-cinating process of milk cooling. We will learn the cooling principle, the rea-sons for cooling and the different ways of cooling milk. This book may not an-swer all your questions, but it can be a beginning for further reading and learn-ing.
Where does milk
Where does milk come from?
For young mammals and human infants, milk is the first food ingested. In mostcases, it continues to be the sole constituent of the diet for a considerable peri-od of time. Milk is a complex biological fluid, the composition and physical characteris-tics of which vary from species to species, reflecting the dietary needs of theyoung mammal. The major constituent of milk is water, but depending on thespecies, milk contains varying quantities of lipids, proteins and carbohydratesthat are synthesised within the mammary gland. Also present are smaller quan-tities of minerals and other fat-soluble and water-soluble components deriveddirectly from blood plasma, specific blood proteins and intermediates of mam-mary synthesis. The domestication of animals such as the cow, and the avail-ability of milk surplus to the requirement of the young mammal, has meantthat animal milk has also become part of the adult human diet.
Lactating animals
Many animals are kept to produce milk for human consumption. The most im-
portant are cows, buffaloes, sheep (ewes), goats, horses (mares), donkey and
camels. These animals form the basis of commercial milk production in vari-
ous parts of the world.
Figure II.1 Lactating animals The various species produce significantly different quantities of milk. Evenwithin the same species there are wide variations in production, largely de-pending on: Figure II.2Factors affecting intra-species variation • Domestic purpose (adapted from van den • Breed and genetic quality Berg 1988, p.3) • Environmental conditions • Physiological conditions • Level of management In general, the dominant milk-producing animals in a region reflect the geo-graphic and climatic conditions. Goats, for example, can be successfullyfarmed in mountainous regions with poor grazing areas, which would be quiteunsuitable for other animals.
Milk production is not always the main reason for keeping these animals.
Mares, asses and camels are principally kept as draught, pack or riding ani-mals, while milk production is a secondary concern. In many parts of theworld the cow is of overwhelming importance in milk production, and in somecountries milk from species other than the cow is not legally defined as milk.
Because of different circumstances for the young mammals, milk can be of dif-ferent consistency. For example, reindeers that live in very cold areas need tohave a thick adipose tissue under the skin. The young must consume milk witha high fat content that allows them to quickly develop this protective tissue.
The pups of rats are born naked and therefore they need milk that contains theprotein necessary to develop a fur coat. (for more information about milk fromdifferent lactating animals, see Alfa Laval Agri AB 1995, Chapter II).
Figure II.3 Composition of milk (g/100g) of different species What is in milk?
What is in milk?
As J.C.T van den Berg explains: "Milk is the first food the newly born humanbeing or mammal receives. To serve its purpose, it is a food that contains allthe nutrients the newborn requires. Even beyond the suckling period, it is stillthe most complete food for human beings and mammals.
Some of the essential minerals and vitamins such as iron and vitamin D are,however, not present in sufficient amounts, or in optimum proportions, to fulfilthe requirements for complete nutrition. During the first period of its life, theyoung animal therefore makes up for the shortage of certain nutrients in milkby exploiting the reserves it receives from its mother at birth, which are nor-mally sufficient until its diet includes other foods. To make the nutrients easilyconsumable and digestible, they are available in a liquid state, partly as a solu-tion, partly as dispersion or suspension. There is a wide variation in the bal-ance of components in milk from various mammals, although the componentsthemselves are basically the same" (van den Berg 1988, p.5) Figure II.4 Composition of raw milk Vitamins and enzymes Quantities of the various main constituents of raw milk from cows can varyconsiderably; between cows of different breeds and between individual cowsof the same breed. The numbers in Figure II.4 give examples of the composi-tion of milk. Water is the principal constituent and it is the carrier of all othercomponents. Cows' milk consists of about 87 % water and 13 % dry substancethat is suspended or dissolved in the water. Beside total solids, the term solids-non-fat (SNF) is used in discussing milk composition.
Fat
Fat weighs less than water and exists as small globules or droplets dispersed in
the milk serum (see Figure II.5). The diameter of these globules ranges from
0.1 to 20 µm (1 µm = 0.001 mm), and their average size is 3 – 4 µm. There are
some 15 billion globules per ml milk. The emulsion is stabilised by a thin
membrane, only 5 – 10 nm thick ( 1 nm = 10 -9 m), which surrounds the glob-
ules and has a complicated composition.
Figure II.5Fat droplets in milk Because of its lower weight, fat rises up and floats on the surface of milk,causing a cream layer. The taste of this fat (butter) is creamy and somewhatsweet, and it has a light yellow colour.
Protein
Proteins are the most important nutrient in milk and an essential part of our
diet. They are present as a solution in milk, and the proteins we consume are
broken down into simpler compounds in the digestive system and the liver.
These compounds are then conveyed to the cells of the body, where they areused as construction material for building the body's own protein. The greatmajority of the chemical reactions that occur in an organism are controlled bycertain active proteins, the enzymes. Proteins are giant molecules built up ofsmaller units called amino acids, and a protein molecule consists of one ormore interlinked chain(s) of amino acids.
Casein
The proteins in milk consist to 80 % of casein, which in turn is made up of a
number of components that together form complex particles or micelles.
Whey protein
Proteins are built up completely differently and therefore also have totally dif-ferent characteristics. In general, whey proteins have very high nutritional val-ues and they are widely used in the food industry. Whey protein is also calledserum protein Non-protein nitrogenous compounds (NPN)
The presence of nitrogen is one of the main characteristics of proteins, buttraces of non-protein nitrogenous products are also found in milk.
Figure II.6 Percentage of different compounds in milk Minerals and salt
Milk contains a number of minerals, with a total concentration of < 1 %. The
most important salts are calcium, sodium, potassium and magnesium. These
occur as phosphates, chlorides, citrates and caseinates.
Vitamins
Vitamins are organic substances which occur in very small concentrations in
both plants and animals. Vitamins give milk its taste and are essential for nor-
mal life processes. Their chemical composition is usually extremely complex,
and the various vitamins are designated by capital letters, sometimes followed
by numerical subscripts, e.g. A, B1, B2. Milk contains many vitamins and
among
the best known are A, B1, B2, C and D. Vitamins A and D are soluble in fat, or
fat solvents, while the others are soluble in water.
Enzymes
Enzymes (catalysts) are a group of proteins produced by living organisms.
They have the ability to trigger chemical reactions and to affect the course and
speed of such reactions, and are able to do so without being consumed.
The action of enzymes is specific: each type of enzyme catalyses only one type
of reaction. Two factors that strongly influence enzymatic action, are tempera-
ture and pH. Several of the enzymes in milk are utilised for quality testing and
control.
LIPASE splits fat into glycerol and free fatty acids. When milk has been dam-aged, lipase causes differences in taste. For example, excess free acids in milkand milk products result in a rancid taste. Many micro-organisms produce li-pase.
PEROXIDASE is activated if the milk is heated to 80 °C for a few seconds.
This can be used to prove the presence or absence of peroxidase in milk andthereby check whether or not a pasteurisation temperature above 80 °C hasbeen reached.
CATALASE splits hydrogen peroxide into water and free oxygen. Milk fromdiseased udders has a high catalase content, while fresh milk from a healthyudder contains only an insignificant amount.
PHOSPHATASE is able to split certain phosphoric-acid esters into phosphoricacid and alcohol. Phosphatase is destroyed by ordinary pasteurisation (72 °Cfor 15 seconds). The phosphatase test can be used to determine whether thepasteurisation temperature has been attained.
III. Why cool milk?
In early times, people kept animals and cultivated vegetables to provide fortheir own needs. Animals were utilised not only for heavy work, but also as asource of food; cows were used for the production of milk and meat.
Families in these early times were almost completely self-sufficient. However,during industrialisation and profession specialisation, farmers became suppli-ers for consumers, and the process began whereby farms grew in size, scalingup all the time. Less farms with more animals is a trend that continues today.
The distance between the farm, the dairy and the consumer became greater, asdid the time lapse between milking and the drinking of milk. Milk storage onthe farm, and the time taken to bridge the gap between producer and consumergave bacteria the chance to acclimatise and grow in this nutritious liquid. It be-came a problem to keep milk quality at the same level as just after milking. If you lower the temperature of stored milk, chemical processes and microbio-logical growth will slow down, delaying the reduction in quality. This knowl-edge enabled farmers, transporters, and dairy organisations to provide milk toconsumers after a time delay, without an unacceptable impact on quality.
Cooling is a very good method to keep the quality of milk at a high level.
Refrigerating milk on the farm has two main aims: – to inhibit bacterial spoilage– to extend storage on the farm so as to decrease milk transport costs.
Full hygiene in all aspects of milk production is essential in the production ofquality milk. A critical aspect is to ensure that the growth of bacteria duringthe storage interval must also be curtailed. At body temperature, bacteria inmilk will multiply very quickly and even milk with a low initial bacteria countwill sour rapidly. Milk produced under hygienic conditions will retain good quality for a periodof up to 15 to 20 hours. However, it is not only the storage temperature that isimportant; the cooling time to reach storage temperature, normally 4 °C, isalso critical. Bulk milk coolers have been specially designed to cool the milkto 4 °C within a specified time period.
Milk hygiene and
Milk hygiene and quality
One general definition of quality could be: "the consumer gets what he or sheexpects". Quality is extremely important, and milk producers are increasinglybeing expected to show that everything has been done to meet quality stan-dards. If the producer succeeds in doing so, the consumer will have faith in thequality of the product, creating all-round benefits.
The quality of milk involves many different aspects. In this chapter we willdiscuss the main influences on the quality of raw milk: – physical hygiene– chemical hygiene– microbiological hygiene.
Figure III.1Hygiene influences on milkquality Physical hygiene
Density, freezing point, osmotic pressure and acidity are examples of physical
hygiene. The density of normal milk varies between 1.028 and 1.038 g/cm³ de-
pending on the milk composition. The freezing point of milk is the only reli-
able parameter to check milk for dilution with water. Between individual cows,
the freezing point has been found to vary from -0.54 to -0.59 °C. The acidity
of a solution depends on the concentration of hydronium ions [H+] in it. When
the concentrations of hydronium [H+] and hydroxyl [OH-] ions are equal, the
solution is neutral (pH = 7).
Chemical hygiene
The different components of milk, especially fat and protein, may undergo
chemical changes during storage. These changes are normally of two kinds,
oxidation and lipolysis. The products of these reactions can cause off-flavour-
ing in milk and butter.
OXIDATION. The oxidation of fat gives milk a metallic flavour, whilst itgives butter an oily, tallowy taste. The presence of iron and copper salts accel-erates the start of auto-oxidation and the development of metallic flavour,which is also caused by the presence of dissolved oxygen and exposure tolight, especially direct sunlight or light from fluorescent tubes.
When exposed to light, the amino acid methionine is degraded to methional.
This is the principal contributor to the sour ‘sunlight flavour'. Since methion-ine does not exist separately in milk, but is one of the components of milk pro-teins, fragmentation of the proteins must occur incidentally for the develop-ment of the sour flavour.
To avoid the oxidation of fat and protein in milk, the most important issue is tocontrol contact with oxygen and direct sunlight. When the milk is awaiting fortransport, it must be protected from direct sun light.
LIPOLYSIS. The break down of fat into glycerol and free fatty acids is calledlipolysis. Lipolysed fat has a rancid taste and smell. High storage temperaturesencourage lipolysis, but the responsible lipase cannot act unless the fat glob-ules have been damaged. In normal farming and dairying routines there aremany opportunities for fat globules to be damaged, for example by pumping,stirring and splashing the milk. In addition, sharp edges and curves in milktubes can damage the fat globules. These details must not be overlooked wheninstalling a milking system.
Microbiological hygiene
Food poisoning and food infections can be the result of poor microbiological
milk hygiene. These dangerous microbiological aspects can be reduced by
milk cooling and it is important to study them.
‘Micro-organisms' is the collective term for ‘all small living organisms whichare not visible to the eye and occupy an intermediate position between the veg-etable and animal kingdoms'. They are found everywhere; in the atmosphere,in the water and in the soil. Since they break down organic material, micro-or-ganisms play a very important role in the natural cycle.
There are thousands of micro-organic species which are important to the exis-tence and economic structure of human society. For example, during the break-down of dead organic material certain species form simple chemical elements that plants can then re-use. Micro-organisms increase soil fertility and cropproduction, which result in more food harvested. Certain species are present inanimal intestines and are essential for food digestion.
Figure III.2Micro-organisms play a very important role in nature (Tetra Pak 1995) Some micro-organisms are used in food processes, for example, cheese, yo-ghurt, pickles, beer and wine production, as well as in acid production for foodpreservation.
Other micro-organisms produce toxic substances that kill other organisms. Oneexample is the mould penicillum, which produces the substance penicillin.
Other micro-organisms cause diseases in animals and plants, reducing a na-tion's food supply, whereas others cause food deterioration such as mould, dis-colouring, etc.
Fig III.3Micro-organisms are used in food processing(Tetra Pak 1995) Bacteria are single-celled organisms that multiply mostly by binary fission, i.e.
by splitting into two. The simplest method of classifying bacteria is accordingto their appearance, yet to be able to see bacteria they must first be stained,then studied under the microscope at a magnification of about 1 000. The mostwidely used method of staining bacteria is called Gram dyeing, and bacteriaare divided into two main groups according to their Gram stain characteristics:(i) red gram negative, and (ii) blue gram positive.
Morphology of bacteria
In the word morphology, ‘morph' stands for form and ‘ology' for the study of.
Morphology of bacteria therefore means the study of the form of bacteria.
Morphological features include:
Figure III.4 Morphological features of bacteria • Cell structure • Mobility, i.e. the ability to move in a liquid spore and capsule formation SHAPE OF BACTERIA. Bacteria shapes can be divided into three categories:spherical, rod-shaped and spirals. The relative position of bacteria to each oth-er is another important distinguishing characteristic. Figure 3.4 shows howspherical bacteria (cocci) occur in different formations. Diplococci arrangethemselves in pairs; Staphylococci form clusters (Greek ‘staphylon' = ‘bunchof grapes'); while streptococci form chains (Greek ‘streptos' = ‘chain').
Figure III.5Spherical bacteria occur in different formations (adapted fromTetra Pak 1995) The figure below shows rod and spiral-shaped bacteria respectively. The rodbacteria (bacilli) vary in both length and thickness, and they also form chains.
Spiral bacteria (spirillum) are also of varying lengths and thickness, and havedifferent numbers of turns.
Figure III.6Rod and spiral shaped bac-teria (adapted from TetraPak 1995) SIZE OF BACTERIA. Cocci vary in size between 0.4 and 1.5 micrometres (1 micrometre = 0.001 mm). The length of bacilli can vary between 2 and 10 mi- crometres, although some species are larger and some are smaller.
CELL STRUCTURE OF BACTERIA. Like all other cells, bacteria contain asemi-liquid, proteinous substance called cytoplasm. Cytoplasm also containsstarch, fat and enzymes that are involved in the metabolism of the cell. Eachcell has nuclear material (DNA), the genetic information that controls the cell'slife and reproduction. In the cells of higher animals and botanical species, thenucleus, contrary to the basic substance of the cell, also contains the substanceprotoplasm.
Figure III.7 Schematic view of a bacte-rial cell The above figure shows a schematic view of the structure of a bacterium. Thenuclear material is suspended freely in the basic substance of the bacteria cell(cytoplasm). The cytoplasm is surrounded by a cytoplasmic membrane thatperforms many vital functions, including regulation of the exchange of salts,nutrients and metabolic products between the cell and its environment. The cy-toplasmic membrane is in turn enclosed in a further envelope, the actual wallof the cell. This serves as the ‘skeleton' of the bacterium, giving it a definiteshape. Some bacteria have the ability to form a protecting capsule (see FigureIII.10).
MOBILITY OF BACTERIA. Some cocci and many bacilli are capable ofmoving in a liquid nutrient medium. They propel themselves with the help offlagella, which are similar to long hairs growing out of the cytoplasmic mem-brane (see Figure III.8). The length and number of the flagella vary from onetype of bacteria to another. Bacteria generally move at speeds of between 1 and10 times their own length per second, with the cholera bacterium, as one of thefastest, is able to travel 30 times its length per second.
Figure III.8Rod- and spiral-shapedbacteria Bacterial spore and capsule formation
The spore is a form of protection against adverse conditions, including:
Figure III.9 • Heat and cold • Presence of disinfectants Bacteria spore protects • Lack of moisture • Lack of nutrients There are various types of endospore formation in bacteria Figure III.10 Spore and capsule formation (adapted from Tetra Pak 1995) Only a few types of genera of bacteria form spores. Of these, bacillus andclostridium are the best known. Under adverse conditions, these organismsgather nuclear material and some food reserves in one area of the cell. Duringspore formation, the vegetative part of the bacteria cell dies. The spore thengerminates back into a vegetative cell and, if conditions become favourableagain, starts reproduction.
The cell eventually dissolves and the spore is released. Spores have no metab-olism. They can survive for years in dry air, and they are more resistant thanbacteria to chemical sterilants, antibiotics, drying and ultraviolet light. Theyare also resistant to heat. For example, it takes 20 minutes at 120 °C to killthem with 100 % certainly. However, spore-forming bacteria in the vegetativestate, like all other bacteria, are killed in a few minutes by boiling them at 100 °C Figure III.11 Growing conditions forbacteria • Presence/Absence of free oxygen • Growth inhibiting agents Temperature
Temperature is the greatest single factor affecting bacteria growth, reproduc-
tion and food deterioration. Bacteria can only develop within certain tempera-
ture limits, and these limits vary from one species to another.
Figure III.12Temperature conditionsand classification of bacteria by temperature There are enormous differences between the various species of bacteria. Somespecies grow at temperatures close to freezing point, in exceptional cases evena few degrees Celsius below, whereas others need considerably higher temper-atures. In general, growth of bacteria in milk and milk products is considerably re-duced by cooling to below 10 °C , while temperatures as low as 4 or 3 °C arerequired to almost completely stop almost all activity. Storage of milk at lowtemperatures will, however, not destroy bacteria. Freezing may lead to a slowdestruction of the product as ice crystals rupture cell walls.
Maximum temperature is the temperature above which bacteria will cease todevelop, while optimum temperature is the temperature at which bacteria de-velop best. If the temperature is increased above the maximum, bacteria arequickly killed by heat. It takes much more heat to kill bacterial spores.
Bacteria are classified into the following temperature categories: Figure III.13 Bacteria classification by PSYCHROPHILIC are the cold-loving bacteria. They are frequently found inraw milk and usually originate from contaminated water. For this reason theyare sometimes called water bacteria. In many cases, the adulteration of milkwith water actually means an inoculation of the milk with this kind of bacteria.
PSYCHROTROPHIC are cold-tolerant bacteria are also found in dust frombarns, feed and from other sources. If unpasturized milk is stored for long peri-ods on the farm or at the milk plant, psychrotrophics may well spoil it. Themajority of psychrotrophic bacteria are actually mesophilic, having an opti-mum temperature in the same range as normal mesophilic bacteria (see below).
MESOPHILIC bacteria differ from psychotrophic bacteria by being able togrow at very low temperatures. Under normal conditions they are destroyed bypasteurisation, but may be found in pasteurised milk as a result of recontami-nation.
THERMOPHILIC. bacteria from soil, hay or other dry and dusty feeds maycontaminate raw milk on the farm. Milk solids that accumulate in improperlysanitised milking utensils are also a common source of contamination.
Enormous populations of thermophilic bacteria may build up in dairy plants ifmilk is kept at high temperatures over long periods, or in dairy equipment thatis used continuously for extended periods and not sanitised properly.
Light
Light is not essential to bacteria because they do not contain chlorophyll and
synthesise food in the same way as plants do. Instead, light tends to kill bacte-
ria as it contains ultraviolet light, a chemical activating ray that causes changes
in the cell protein. In nature, the bacteria-killing effect of sunlight plays an im-
portant role, especially concerning bacteria-filled dust in the air. It is the pri-
mary reason why sunny streets and light rooms are much poorer in bacteria
than dark and stuffy places.
Acidity
A suitable acidity level is very important for the proper development of micro-
organisms. In milk it is the pH that is decisive and not the titratable acidity. At
the normal pH of milk, many micro-organisms are able to develop, but some,
like mould and yeast, prefer a more acidic environment. Others, like most of
the protein-fermenting bacteria, stop reproduction at increased acidity.
The acid produced by lactic acid bacteria will prevent the development of cer-tain putrefying bacteria and actually preserve milk, although it becomes sour.
Lactic acid bacteria themselves can also only tolerate a certain acidity, al-though not all types are equally sensitive. This means that during the process of acidification of milk, various species of lactic acid bacteria may succeedeach other. Normally, acid production stops in milk at pH 4.2. Oxygen demand
While all higher level organisms require free oxygen (O²) to live, this is not al-
ways the case for micro-organisms. Moulds require oxygen because of theirway of reproduction, and the same goes for many types of yeast and bacteria.
However, other yeasts and bacteria, however, are not dependent on the pres-ence of free oxygen, and some do not tolerate oxygen at all. Micro-organisms can be classified into groups according to their oxygen re-quirements: AEROBIC. Most yeasts, all moulds and a large number of bacteria belonghere. These require free molecular oxygen for their development.
ANAEROBIC. Includes most of those bacteria that flourish in the absence ofoxygen.
FACULTATIVE AEROBIC/ANAEROBIC. These organisms can grow in aer-obic as well as anaerobic conditions, although often exhibit a preference forone or the other. A typical example of this group are the ordinary lactic acidbacteria, which develop more quickly at the bottom of a can or bottle than atthe top. As a result, the milk at the bottom of containers starts to acidify first.
Sometimes, the top layer of the milk seems sufficiently ‘fresh' whilst the milkat the bottom is already sour.
MICRO-AEROPHILIC. These only grow in areas with a low oxygen concen-tration.
Water and osmotic pressure
Water is the major component of the bacteria cells, and considerable quantities
are required for the production of new cells. Dried products, such as milk pow-
der, are protected from bacteriological deterioration because they lack water.
The drying process itself does not destroy all micro-organisms. and many sur-
vive long storage periods in dry products. Immediately after drying, the bacter-
ial count of milk powder decreases only slowly, and it may take years before
the product becomes more or less sterile. High storage temperatures will help
promote the destruction of the bacteria. In addition to the water content of the
product, the osmotic pressure of the water is important.
Nutrients
Nutrients are required for the development of micro-organisms because they
supply the ‘building materials' for new cells. Furthermore, the breakdown of
complex compounds into simpler compounds delivers the energy required for
the cells to function. The breaking down of compounds in combination with
the production of other compound is named fermentation.
Milk is rich in nutrients and is as such an excellent nutrient for many micro-or-ganisms. However, since the requirements of the various organisms vary, notall micro-organisms find all the nutrients they need in milk, and so not all areable to grow.
Reproduction of bacteria.
Bacteria normally reproduce asexually by fission. First, the size of the cell in-
creases. The clear material then gathers in one area of the cell and divides into
two identical parts. The parts that move away from each other result in two or-
ganisms that may break away or remain together, in turn resulting in different
but characteristic arrangements.
Figure III.14 Reproduction by fission Figure III.15 Development of bacteria with a generation time of 20 minutes Figure III.14 Figure III.15 The concept ‘generation time' was introduced to indicate the rate of growth ofmicro-organisms. It is the time a certain species or strain requires to double innumber during the exponential phase of the growth curve.
Figure III.16 shows the growth curve of bacteria transferred to a substrate byinoculation. Development phase (a) is called the ‘lag phase', and is the delaybefore the bacteria start to reproduce, as they must first acclimatise to the newenvironment. The lag phase may also be observed in a culture that has beendormant, for example, one that has been stored at a low temperature prior toinoculation. The length of this first phase varies according to how many of thebacteria were inhibited at the moment of inoculation. If viable, growing bacte-ria are used and there is no period of incubation; reproduction then begins atonce.
After the lag phase, the bacteria begin to reproduce quickly for the first fewhours. Development phase (b) is called the ‘log phase', because reproductionproceeds logarithmically.
Figure III.16 a Lag phase
Growth curve of bacteria (adapted from b Log phase
Tetra Pak 1995) During phase (b), toxic metabolic waste products accumulate in the culture.
The rate of reproduction therefore eventually slows down, and as bacteria areconstantly dying so a state of equilibrium is reached between the death of oldcells and the formation of new ones. This next phase (c) is called the ‘station-ary phase'. In the following phase (d), the formation of new cells ceases com-pletely and the existing cells gradually die off. At the end of phase (d) the cul-ture is extinct, hence the ‘mortality phase'.
The shape of the curve, i.e. the length of the various phases and the gradient ofthe curve in each phase, varies with temperature, food supply and other growthparameters Bacteria in milk
When milk is secreted in the udder it is virtually sterile. But before the milk
leaves the udder, bacteria manage to enter through the teat channel and infect
it. These bacteria are normally harmless and few in number, only a few tens or
hundreds per ml. However, in cases of bacterial udder inflammation (mastitis),
milk can be heavily contaminated with bacteria and may even be unfit for con-
sumption, not to mention the suffering it causes the cow. There are always con-
centrations of bacteria in the teat channel, but most of them are flushed out at
the beginning of milking. It is therefore advisable to collect the first bacteria-
rich jets of milk from diseased animals
Figure. III.17Bacteria enter through theteat channel Figure III.18During udder inflammationthe milk is heavily infectedby bacteria Figure III.17 Figure III.18 Infection on the farm
In the course of handling on the farm, milk is liable to be infected by various
micro-organisms, mainly bacteria. The degree of infection and composition of
bacterial population depend on the cleanliness of the Cows' environment and
those surfaces with which the milk comes into contact, for example, the
pail/milking machine, strainer, transport churn or tank and agitator. Milk-cov-
ered surfaces are usually much greater sources of infection than the udder.
When cows are milked by hand, bacteria can get into the milk via the milker,the cow, the litter and/or the ambient air. The magnitude of the influx dependslargely on the skill and the hygiene-consciousness of the milker. Certain dan-gers are eliminated in machine milking, but another one is added, namely themilking machine itself. A very large number of bacteria can enter the milk ifthe milking equipment is not cleaned properly Temperature and bacteria count in milk
Due to its very specific composition, milk is susceptible to contamination by a
wide variety of bacteria. Farm milk may contain anything from a few thousand
bacteria per ml, from a farm with good hygiene practices, to several million if
the standard of cleaning, disinfection and cooling is poor. Daily cleaning and
disinfection of all milking equipment is therefore the most decisive factor for
the bacteriological quality of milk. For milk to be classed as top quality, the
bacteria count (Colony Forming Units/CFU), should normally be less than 100
000 per ml. In some countries, 10 000 per ml can be reached easily
Rapid cooling to below 4 °C greatly contributes to the quality of the milk onthe farm. This treatment slows down the growth of the bacteria in the milk,thereby greatly improving its keeping qualities. The influence of temperatureon bacterial development in raw milk is shown in Figure 3.14 . Starting from300 000 CFU/ml, we can see the speed of development at higher temperaturesand the effect of cooling to 4 °C.
Figure III.19 Bacterial development inraw milk (adapted from Tetra Pak 1995) Million bacteria per ml Cooling to 4 °C, or even 2 °C, in conjunction with milking makes it possible todeliver milk at two- or three-day intervals, provided that the milkcontainer/tank is well insulated.
In situations of non-hygienic farming and infection, the initial bacteria countrises sharply and bacterial reproduction starts at an already high level.
Combined with am optimum temperature, bacterial growth is enormous. Toavoid development of bacteria it is important to keep the number of bacteria assmall as possible, partly by directly cooling the milk to around 4 °C. However, it is vital to recognise that cooling is a compliment, not a substitute,for hygienic working conditions. Avoiding infections through good hygienepractices, and cooling the milk as soon as possible after milking, combine toensure high milk quality. Cooling is a good expedient, and with efficient cool-ing you can help win the battle against micro-organisms.
Figure III.20Bacteria development by different start colony count and two different tempera- Storage time (hours) Principal bacteria in milk
Many of the bacteria in milk are casual visitors. They can live, and possibly re-
produce. Milk is, however, often an unsuitable growth medium for them. Some
of these bacteria die when competing with species which find the environment
more congenial. Groups of bacteria that occur in milk can be divided into:
Figure III.21Principal bacteria groups If you wish to find out more about the positive and negative aspects of bacte-ria, the dairy microbiological handbooks are good reference works (e.g. R. K.
Robinson 1983).
Natural protection of milk against bacterial growth
Among mammals, milk is the last nutritional link between mother and off-
spring among mammals. Besides being a complete, well-balanced diet for the
newborn, milk also contains anti-microbial agents that protect the suckling
young from various infectious diseases.
Figure. III.22 Calves must obtaincolostrum if they are tosurvive The knowledge that milk, and in particular colostrum (the first milk after par-turition), contains immune factors essential for the survival of offspring is veryold. Thousands of years ago, herdsmen recognised that newborn lambs, kidsand calves must obtain the first milk (colostrum) if they want to survive.
Today, it is well documented that milk contains several antibacterial factors.
The best known of these are the immunoglobulins, which can be found in highconcentrations in colostrum and which provide an immediate immunisation ofthe newborn.
Figure III.23Antibacterial factors in Milk also contains non-specific factors as lysozyme, lactorferrin and peroxi-dase. This type of peroxidase, which is called lactoperoxidase, is identical tothe peroxidase present in salvia and gastric juice.
Fungi
Fungi are a group of micro-organisms that are frequently found in nature
among plants, animals and human beings. Different species of fungi vary a
great deal in structure and method of reproduction. Fungi may be round, oval
or threadlike. The threads may form a network, visible to the naked eye. Fungi
are divided into yeasts and moulds.
YEASTSYeasts are single-cell organisms of spherical or cylindrical shape and the sizeof yeast cells varies considerably. For example, brewer's yeast, saccharomycescerevisiae, has a diameter in the order of 2 – 8 mm, and a length of 3 – 15 mm.
Yeast cells of certain other species may be as large as 100 mm.
Figure III.24 Structure of the yeast cell/adapted from Tetra Pak1995) Figure III.25Budding yeast cells (adapt-ed from Tetra Pak 1995) Figure III.24 Figure III.25 Yeast cells normally reproduce by budding, though there are other methods.
Budding is an asexual process. A small bud develops on the cell wall of theparent cell. The cytoplasm is shared for a while by parent and offspring, buteventually the bud is sealed off from the parent cell by a double wall. The newcell does not always separate from its parent, but may remain attached to itwhile the latter continues to form new buds. The offspring cell also form freshbuds of its own, which can result in large clusters of cells attached to each oth-er. Some types of yeast reproduce by forming spores (these are quite differentfrom bacterial spores).
Figure III.26 Yeast has the same need for nutrients as other Conditions for the growth living organisms, such as bacteria.
As for bacteria, although yeast needs less water; some can grow with very little water.
Yeast can grow in a pH value range of between 3 and 7 (optimum is between 4.5 and 5).
The optimum temperature is normally between20 and 30 ºC.
Yeast can grow both with and without the presence of atmospheric oxygen. Yeast cells are facultatively anaerobic, which means that in the presence of oxygen they grow better.
Yeasts are usually undesirable in dairy products because they often ruin them.
However, Russian ‘Kefir' and Finnish ‘Viile' are examples from a small prod-uct group where yeasts are necessary to give the correct quality. In the brew-ing, wine, baking and distilling industries, yeast organisms are valuable co-workers.
MOULDSMoulds belong to quite different groups of fungi. They consist of thread-likestrands of cells called mycelium.
The mould fungi has a many-branched body called the mycelium, which maybe microscopically small, or large enough to be seen with the naked eye. Themycelium consists of individual threads called hyphae. These hyphae consti-tute the vegetative part of the fungus. The part responsible reproduction con-sists of hyphae that often grow straight up and carry spores.
Figure. III.27Penicillum with conidiophores producing chains of coni-dia (adapted from TetraPak 1995) Figure III.28 Moulds can grow on materials with a very low Conditions for the growth water content and can extract water from air.
Moulds can grow in a pH value range of between 3 and 8.5 The optimum temperature is normally between20 and 30 ºC.
Moulds usually grow in aerobic conditions.
There are many different families of moulds. Groups that are of importance inthe dairy industry include penicillium and milk mould, geotrichum candidum.
Bacteriophages
Bacteriophages are viruses, i.e. bacterial parasites. By themselves they can sur-
vive, but they can only grow or replicate within bacterial cells. They have very
specific hosts, e.g. single species of strains of bacteria. Bacteriophages, or
phages, can only be seen by means of an electron microscope.
The micro-organisms used in the dairy industry are called ‘starter cultures'. A
starter culture is a mixture of organisms. The quality of the starter culture is
preserved until after arrival at the dairy by maintaining high standards of hy-
giene in all steps of the processing chain.
As milk is usually contaminated with bacteriophages, it is important that the
milk used for starter cultures, usually skimmed milk, is heated to inactivate the
phages. Fig 3.21 shows what happens if this is not done, or if the milk is re-
contaminated by phages at a later time.
Figure III.29 Growth of starter bacteria and phages andinfluence on infected starter culture single strain culture (adapted from Tetra Pak1995) Phage-infected single strain culture Structure of Bacteriophages
Figure III.30 Structure of bacterio-phages (adapted from Tetra Pak 1995) Reproduction of phages
Phages only attack bacteria, usually young actively growing ones, within
which they can reproduce. The bacteria subsequently disintegrate, releasing a
crowd of 10 to 200 phages per bacterium that then attack new victims.
1 The phage attaches to the surface of its host and the DNA is injected into the cell.
Figure III.31The reproduction stages of 2. The cellular machinery then produces new phage DNA and phage proteins.
3 The new phages are assembled inside the bacterial cell, which is then lysed.
4. The mature phages are released.
The great variety of bacteria, yeasts and moulds, and their widely varied activi-ties, are of the utmost importance for life on earth in general, and humanity inparticular. Micro-organisms in soil and water are responsible for degradingavailable sources of organic nourishment into forms that plants can assimilate.
By doing so they also perform an indirect service to the animal kingdom.
Human beings also benefit more directly from micro organisms. Lactic-acidforming micro-organisms, for example, can be used to preserve fodder (silagefor livestock). The same principle is applied to the preparation of certain foodssuch as sauerkraut, green olives and cucumbers.
Micro-organisms are of paramount importance in the manufacture of dairyproducts such as yoghurt, cheese and cultured butter. Choice of the right typesof micro-organisms is an essential factor for maximising the quality of suchproducts.
It should be mentioned here that milk may contain residues of antibiotics ema-nating from treatment of cows suffering from mastitis; the most commonly oc-curring being penicillin. This is in spite of regulations saying that milk fromcows treated with antibiotics must not be sent to the dairy.
It would be a false idealisation of micro-organisms not to mention that some ofthem, the pathogenic micro-organisms, are regarded as mankind's worst ene-mies. Although it is true that pathogens are far outnumbered by the harmless oruseful ones, their effects are much more obvious.
Almost all over the world, governments have passed laws requiring pasteurisa-tion of milk that is produced at a dairy and intended for consumption. A typicaltemperature/time combination for pasteurisation is 72 °C /15 - 20 seconds,which kills all pathogens. It is important is to know that cooling is a compliment and not a replacementfor hygienic working practices, and that prevention is better than cure.
Avoiding infections is the first priority.
Cooling is the weapon against growth, and with efficient cooling and goodcare the battle against micro-organisms can be won. Milk quality rises, as doesthe quality of all milk products. This leaves only one winner, human health.
(For even more detailed coverage of the micro-organisms present in milk, seeTetra Pak 1995) IV. Milk collection
How to transport
How to transport raw milk to the dairy plant
raw milk to the
After milking, milk should be cooled and stored in the milk room of the farm or dairy plant. Milk for industrial processing can be transported to the dairyplant by the farmers themselves, or it can be picked up at the dairy plant. Inboth cases, it is possible to contract out these collection activities to third par-ties, for example, professional transporters.
Due to organisational or economic difficulties, it may not be possible to coolthe milk on the farm. In areas far away from the dairy plant it may be trouble-some to collect milk and take it directly to the plant. In such cases, especiallyif there are many small suppliers, it is be preferable to take milk first to a col-lection point, and then transport it from there to the dairy plant or milk collec-tion centre. Figure IV.1Transport from farm to Non-cooled milk 35° C Cooled milk 3 – 4° C Can collection
Milk that is available in cans, whether on the farm or at the collection point,
can be picked up and transported by many convenient means of transportation
(bicycles, small barrows or trucks). The cans should be protected against the
sun, both while they are at the roadside awaiting collection and during trans-
portation
Figure IV.2Can collection (Tetra Pak1995) It is advisable to use insulated, or even refrigerated trucks to transport cooledmilk in cans over long distances and under high ambient temperatures. Whenthere are many individual suppliers, there are many different types of milkcans, providing logistic and cleaning problems. It is therefore advisable to usestandard shape milk cans with a smooth surface.
Bulk collection
Milk available from the farm in bulk, for instance from farm cooling tanks,
should also be picked up in bulk. It is not a good practice to use cans to trans-
port milk that is already available in bulk (storage) tanks, because there is an
extra risk of contamination. Furthermore, the temperature of milk in cans is
more difficult to control than milk in bulk, and filling, emptying and cleaning
of milk cans demands much labour and is costly.
Truck-mounted tanks or road tankers can be used for the transport of milk inbulk. The tanks should be insulated and may be covered by a shield to protectagainst strong sunshine. On the farm, or at the collection centre, the loadinghose from a milk transport truck is connected to the outlet valve on the storagetank, and the milk is pumped over. Pumping is stopped as soon as the coolingtank has been emptied, thereby preventing air from being mixed into the milk.
The tanker is fitted with a flow meter and pump so that the volume is automat-ically recorded. In other cases, the storage tank has to be calibrated to makedip-stick measurements reliable.
Figure IV.4 Bulk collection at the farm(Tetra Pak 1995) The tank of the bulk collection vehicle is divided into a number of compart-ments in order to prevent the milk from slushing around during transportation.
Figure IV.5Milk transport truck (TetraPak 1995) cooled milk 4 – 3 °C Milk collection points and centres
points and centres
In scarcely-populated areas, or areas where individual suppliers are far awayfrom the dairy plant and difficult to reach, milk has to be transported over longdistances. Transportation to the dairy plant will also take much time. In thesecases, it is advisable to collect and cool the milk in a milk collection centre(MCC) before transportation takes place.
Figure IV.6Uncooled milk will bebrought to the collectioncentre (Tetra Pak 1995) The difference between a collection point and a MCC is mainly based on cool-ing and size. A milk collection point can be a small, central place where smallsuppliers can deliver their milk. The reception capacity is likely to be between50 – 500 litres a day in cans or milk containers. There is no cooling equipmentpresent at the milk collection point, so the milk should be collected andbrought to the MCC within two hours after milking. At the MCC, there is al-ways cooling equipment and, in most cases, quality testing facilities. The milkmust be collected and cooled to < 4 °C not later than three hours after milkinghas been completed. The reception capacity of a collection centre is generallybetween 500 and 16 000 litres/day.
Figure IV.7Example of a MCC MCC ground plan
Farmer routeFarm supply Farmer with milk cans Heat recovery unit Storage/cooling tank 10. Milk transport to the dairy and entrance for goods 11. Milk can cleaner12. Drying frame for cans13. Farm supply shop14. Publication board15. Office MCC manager16. Storage goods Logistic advantages
Transporting cooled milk from a storage tank at a farm or collection centre has
many advantages. It enables the plant to organise an efficient system of collec-
tion and transportation. Since the milk has been cooled, it can be picked up at
the farm or the collection centre at any hour of the day, without the risk of
spoilage. In contrast, uncooled milk must be picked up as quickly as possible
after milking, which leads to peak hours in the operation.
Figure IV.8Quick and efficient coolinggives only winners Each type of collection has its own advantages. Can collection is for smallfarms; bulk collection is for larger suppliers; and there are combinations withtransport tanks for farms in between.
The most important issue is that milk must be cooled as quickly as possible.
Once this has been accomplished, all parties (farmer, dairy and consumer) willbenefit.
1. Milk cooling requires an adequate supply of electricity and water. These are not always available on the farm and sometimes can only be arrangedat relatively high costs.
2. Even though electricity and water may be available, the volume of daily milk production may be too small to justify a cooling system, and it wouldbe too expensive to cool a small amount of milk on the farm and too ex-pensive to collect it. Due to regulations, smaller amounts of milk aresometimes cooled on the farm, but this milk is then expensive to transport.
In such cases, it is possible to transport the cooled milk in an insulatedvessel to a collecting point, where a tanker collects milk from several sup-pliers.
3. Bulk collection of milk on farms not only requires a supply of water, elec- tricity and a certain daily production of milk, but also good road access formilk transport trucks.
4. If a dairy intends to introduce bulk collection of cooled milk in areas with many low producing farms (and where the milk is not cooled), substantialresources are required.
V. Cooling technology
Heat – an energy
Heat – an energy difference
Material can occur in three different forms: gas, solid or liquid. Each of theseforms is called a state of aggregation. For example, water (H²O) can exist as vapour, ice or water. The transformation from one state to another occurs at astationary point, and at this point the heat content changes, while the tempera-ture does not. The hidden amount of heat is called latent heat. The stationary point where ice becomes water is called the melting point, witha temperature of 0 °C, and the amount of heat needed to melt 1 kg of ice is 93Watt. The temperature at which water becomes vapour is called boiling point,or 100 °C at 1 Bar, while the latent heat is 268 Watt. It should be noted thatpressure only influences the boiling point, and not the melting point.
Figure V.1State of aggregation of ma- Evaporation heat
Many cooling processes involve the evaporation heat of a liquid. If a liquid
evaporates, it needs heat. This heat is taken from the surroundings of the evap-
orating liquid.
Figure V.2Cooling as it once was inEgypt An early example of cooling by evaporation can be found in ancient Egypt.
Stone bottles, called Gandis, were filled with water. Because of the porous ma-terial, some of the water seeped through to the outside of the bottle wall andevaporated. This evaporation took the heat out of the bottle and therefore outof the water inside.
Cooling with basic facilities
Cooling with basic
If the milk must be stored on the farm for long periods of time, any cooling method is better than no cooling. However, if cooling facilities are basic andthe time required to transport milk to the collection centre or dairy plant iscomparatively short, it is advisable to deliver milk as soon as possible to thenearest milk collection centre.
Several systems are available for cooling milk. The simplest systems use waterfrom a main or well. If abundant quantities of well water are available, themilk cans can be immersed in the well. This method, however, is not advisableif the well water is also used for drinking, because the immersion of cans easi-ly leads to contamination of the well. Simple systems of cooling that use waterwill bring the milk to a temperature only 3 – 5 °C above that of the water. Thismeans that water at a temperature of 11 – 12 °C is able to cool milk to about15 °C (at the lowest). Apart from the fact that this temperature is still high, wa-ter of 11 – 12 °C will generally not be available in warm tropical conditions.
Such conditions require artificial cooling with special equipment.
Cooling rings
Whenever running water is available, milk can be cooled by putting a perforat-
ed tubular ring around the neck of the can of warm milk. After the ring has
been connected to the mains, water will spray onto the can and flow over its
surface. If ice water from a cold water tank is used, the water should be col-
lected under the can and recirculated, for example, by standing the can on a
rack over the cold water tank.
Surface coolers
Surface coolers consist of a series of small-diameter horizontally arranged
tubes. Mounted on top of each other, these tubes terminate at each end in a
header. The headers connect the tubes, thus allowing the cooling agent to cir-
culate through them.
The warm milk is distributed over the surface of the cooler, i.e. over the set ofhorizontal tubes, by means of a spreader-pipe or a tray with small openings fit-ted on the top of the upper tube. Surface coolers may consist of two indepen-dent sections on top of each other. The upper section is cooled with water fromthe mains or from a well, whilst iced water or direct cooling is applied in thelower section. The surface cooling system, also called ‘open cooling system',is simple, but requires a proper sanitisation programme. Special care must betaken to prevent airborne contamination.
Ice-cones
If small amounts of milk have to be collected and transported over long dis-
tances, and it is not technically or economically feasible to cool the milk in ad-
vance, metal ice-cones may be used. These cones are inserted in the milk cans,
so that the rim of the cone rests on the collar of the can and fits sufficiently
tightly to prevent milk splashing out during handling and transport. The conetakes up about one-third of the volume of the can. If the cones are filled with crushed ice, the milk can be cooled from 30 °C to 5 – 10 °C during transport.
The cones and the ice can be taken to the farms or collection centres by themilk transport truck. The ice should be transported in an insulated box, and thecones must be properly sanitised after they have been used; preferably at thechilling centre or dairy plant.
Figure V.3Cooling with water and ice– simple and almost alwaysappropriate techniques Water tanks Water tanks
The simplest cooling system involves an open tank with cold water. Milk cans
have be inserted into the tank, where they are immersed in the water up to their
‘neck'. The water must be refreshed continuously or at regular intervals.
To allow de-aeration of the milk during cooling, the lids of the cans should beloosened. The tank may be covered with a lid to protect the milk from flies anddust. If well water or water from a main supply is used, this system only en-ables slow cooling to comparatively high temperatures. Better results are ob-tained by using iced water, and the cooling rate can be improved further byforced circulation of the iced water in the tank. To limit losses of cold by radia-tion, the tank and its cover must be insulated. Modern cooling systems
Cooling systems transfer the heat of the milk via a cooling agent to either airor water. This transfer goes via a separated wall, so there is never direct con-tact with the milk. The refrigerant, or cooling agent absorbs the heat of themilk inside the evaporator. Each refrigerant has, by a certain pressure, its ownboiling point. The cooling rate depends on the design of the equipment. The fi-nal temperature depends on the thermostat setting or milk flow through theplate coolers. Large differences in temperature increase the rate of cooling.
High speed and turbulent motion of liquids along the wall will improve theheat transfer rate.
If milk is cooled in a modern way, electricity is needed to generate the temper-ature required. The electricity runs the condensing unit, which condenses theevaporated liquid and makes the process a continuous cycle.
Cooling cycle
The cooling cycle can be divided into a low- and high-pressure side
Figure V.4 A simple cooling cycle Low pressure = low temperature High pressure = high temperature Low-pressure side
The evaporator is partially filled with refrigerant. When the compressor starts,
the gas above the liquid will be sucked away. Due to this, the pressure will de-
crease. The liquid starts to boil as soon as the pressure sinks below the pres-
sure of the present temperature. Parts belonging to the refrigerant will evapo-
rate and take the heat out of the remaining cooling medium. This makes the re-
maining part colder. If the temperature reduces below the milk temperature,
the heat will flow from the milk to the boiling refrigerant. This heat causes an
amount of refrigerant to evaporate. The temperature will remain constant once
the quantity of heat, which is transported by the compressor, is in balance with
the amount of heat from the milk.
High-pressure side
The high-pressure side of the compressor is connected to the condenser. The
purpose of the condenser is to remove the condensation heat to the surrounding
area. The compressor pumps gas into the condenser. As long as the pressure
remains below the pressure belonging to the condensing temperature, only the
pressure will rise. As soon as the pressure rises above the pressure belonging
to the condensing temperature, a heat transfer will start from the gas to the sur-
rounding area. First the ‘super heat' is taken away. The super heat is the tem-
perature difference between the heated gas above boiling point and the boiling
point. Condensation will start after this. To condensate with a certain capacity,
a particular temperature difference is needed. The pressure will be constant as
soon as the temperature difference is large enough to condensate all of the gas
pumped in by the compressor.
To make this process continuous, the liquid in the condenser must be fed backinto the evaporator. Since the pressure in the condenser is always higher thanin the evaporator, this can be easily done by establishing a pipe connectionfrom condenser to evaporator. If a valve is mounted in this pipe, the amount ofrefrigerant can be adjusted. Normally this valve is automatic, and is called thethermostatic expansion valve. This valve measures the pressure of the evapora-tor and the temperature of the suction pipe. The valve opens more or less ac-cording to the super heat.
Figure V.5The individual parts of acooling installation Figure V.6Key to the individual parts Key to figure V.5
of a cooling installation A gas pump creating low pressure in the evapo- rator (low temperature) and high pressure in the condenser (high temperature).
Mainly used for protection of the condensing side of the installation. If the pressure gets too high, the pressostat stops the compressor. Also used as protector against low pressure caused by refrigerant leakage and as a switch to stop the compressor at the end of a pump-down cycle.
The part where the refrigerant condenses.
The heat in the gas is released into the air and the gas turns into liquid.
4. Liquid receiver Meant to be a storage place for the refrigerant. If the installation is in operation, the receiver is al- most empty. If the installation stops and a pump- down system is installed, the refrigerant will be stored in the receiver.
5. Filter / Dryer The filter is used to take all solid parts out of the liquid. The dryer is used to remove moisture pre- sent in a very small amount in the refrigerant.
6. Solenoid valve In installations with a pump-down system, this valve stops the liquid flow to the evaporator.
Gives the possibility to check if there is suffi- cient refrigerant in the installation.
Gives the same amount of refrigerant, in a liquid form, back to the evaporator as the com- pressor takes out as a gas Part where the refrigerant evaporates and conse- quently takes the heat out of the milk.
Controls the temperature of the cooled milk, switching the compressor on or off depending on the temperature.
Direct expansion cooling
This is the most common milk cooling system. The bottom of the tank has
been designed as an evaporator, while the heat of the milk goes through the
stainless steel wall to the refrigerant. The refrigerant evaporates, which takes
the heat away from the milk. Since direct expansion tanks do not have a cold
buffer, energy must always be available. In this type of system, the milk is
cooled directly and agitated after arrival in the tank.
Figure V.7Direct expansion cooling system – the mostcommon choice Icebank cooling
In indirect cooling systems, the evaporator is situated in a reservoir filled with
the heat carrier, which is mostly water. The evaporator consists of a system of
coils or pipes in which the cooling medium evaporates and cools the heat carri-
er.
Figure V.8Milk cooling with an ice-bank system The biggest advantage of an icebank system is that it allows the cooling capac-ity to be stored in an isolated reservoir with a heat carrier and ‘cold buffer' or‘ice buffer'. In areas where there is not sufficient energy, an icebank systemprovides an efficient cooling solution. The formation of ice around the pipes inthe reservoir forms the cold buffer that can be used for cooling the milk. Thecold buffer makes it possible for cooling in areas where energy in peak times ismore expensive, or where the use of electricity is limited, and means that thecooling system can be turned off to avoid an energy rush during milking. Theproduction of cold can occur in periods when energy is inexpensive, and canbe extended over a longer period, enabling a small compressor to be used. The energy efficiency of the indirect system is lower than that of the directsystem, because cooling of the carrier demands extra energy. The energy con-sumption of an icebank cooler is 23 W/l. There are two types of chilled waterequipment. The first is the ice builder, which accumulates ice between milk-ings using a small condensing unit that runs up to 18 hours a day. The secondis the package chiller, which has a large condensing unit that runs only duringmilking.
Pre-coolers
Milk comes from the cows to an end unit, from where it is pumped at a con-
stant rate through a filter to the plate cooler. The plate cooler consists of corru-
gated stainless steel plates. The milk flows over one side of these plates, whilst
on the other side tap or well water flows in the opposite direction. When the
milk leaves the plate cooler its temperature has been reduced to 2 – 4 °C above
the water temperature, prior to final cooling and storage in the cooling tank.
From milking machine Milk temperature of raw milk, 35 °C Milk temperature 2 – 4 °C higher than well water temperature Milk temperature 4 °C Figure V.9 Figure V.10Milk/water flow in a pre- cooler heat exchanger Pre-cooling with cold tap water lowers the total and running costs for the plantby reducing the demand for chilled water. A prerequisite for this is, of course,a supply of inexpensive natural cold water. It is always possible to combinepre-cooling with other cooling systems to reduce the energy costs even more.
If tap water has been used for pre-cooling, it is advisable to recycle the cooledor cold water by using it as drinking water for cattle. If tap water is not re-used, the costs will annul the energy costs savings, whereas if well water hasbeen used for pre-cooling, this aspect is less important.
Instant cooling
Today, farms are becoming larger and larger, meaning more work, more cows
and more milk – and less time between milkings. This process provides farm-
ers with potential cooling problems, because all the milk has to be cooled and
stored. The sheer quantity of milk, combined with high milk flows and longer
milking periods, makes it more difficult for conventional bulk tanks to cope.
Quicker milking means greater milk amount per time. Overloaded cooling sys-tems mean slower cooling and higher bacteria counts, and long cooling timesinvolve prolonged agitation with the risk of buttering. Maintaining taste andquality is made more difficult, which puts the entire milk production at risk.
Instant cooling is an in-line system, which cools the milk in a matter of sec-onds before it reaches the storage tank.
Figure V.11 Instant cooling, from 35 – 3 °C silo (storage) tank The milk goes from the cows to the end unit and balance tank, from where it ispumped at a constant rate through a filter to the plate cooler. The plate cooler isthe heart of the cooling system and consists of corrugated stainless steel plateson one side of which the milk flows in one direction, while on the other side,chilled water flows in the opposite direction. When the milk leaves the platecooler, its temperature has been reduced to a temperature 2 – 4 °C above thewater temperature. The milk is pumped continuously to the insulated storagetank, where it can be kept, with occasional agitation, until collection.
Figure V.12The flow of milk/water in adeep cooler heat exchang-er/plate cooler Ecombies
Ecombies involve a two-step cooling process. It is very advantageous to com-
bine instant cooling with pre-cooling using chilled water. Pre-cooling with
cold tap or well water lowers the total costs, including running costs for the
plant by reducing the demand for chilled water.
Figure V.13Milk cooling with an ecombi-cooler In pre-cooling, the plate heat exchanger is divided into two sections. In thefirst section, the milk is cooled with cold tap or well water. In the second sec-tion, the milk is cooled down to the final storage temperature using chilled water.
Figure V.14Milk/water flow in anecombi-cooler heat exchanger VI. Demands on cooling equipment
It is important that the consistency and quality of milk does not change duringstorage. In order to store milk and maintain high milk quality, proper coolingequipment is essential. When determining the suitable type of cooling equip-ment, the following questions must be answered: – What is the daily milk volume?– What is the number of milkings for storage (total storage capacity)?– What cooling capacity is needed?– What is the ambient temperature?– Which are the suitable options to ensure efficient cooling? Cooling and agitation performances
Critical factors here are the number of milkings, ambient temperature and milk
cooling time.
Figure VI.1 The numeral (2) designates a tank for two milkings Number of milkings The numeral (4) designates a tank for four milkings The numeral (6) designates a tank for six milkings Figure VI.2Classification according to (PT) in °C
temp. (SOT) in °C
PT, Performance Temperature – ambient temperature to be used when measur-ing the milk cooling temperature. SOT, Safe Operating Temperature – highest limit of the range of ambient tem-peratures at which the equipment is required to function. Figure VI.3Milk cooling time Milk cooling time (from 35 °C to 4 °C)
0 – maximum acceptable cooling time of 2 hoursI – maximum acceptable cooling time of 2.5 hoursII – maximum acceptable cooling time of 3 hoursIII – maximum acceptable cooling time of 2.5 hours For example, cooling equipment with the code 2BII is designed for two milk-ings, with calculated cooling capacity at an ambient temperature of 32ºC. Thecooling time (35 ºC – 4 ºC) for each milking will take less than three hours.
In practice, the required cooling capacity becomes lower as the number ofmilkings becomes larger. This is because the relative added milk volume issmaller.
Milk cooling rate
If a tank for two milkings is empty, or contains 50 % of its rated volume ofmilk at 4 °C and then 50 % of volume is added in one batch at 35 °C, all of themilk should be cooled to 4 °C no longer than the specific cooling time. With four milkings, the respective tank stages are: empty, 25 %, 50 %, 75 %and 100 % respectively. With six milkings: empty, 16.7 %, 33.3 %, 50 %, 66.7%, 83.3 % and 100 % of its rate volume for the same temperatures.
Figure VI.4 Temperature course of milkin a storage tank (72-hourperiod). Different materials can be used to construct a milk cooling tank, each of whichhas its advantages and disadvantages.
Figure VI.5 Advantages and disadvantages of materialsused for the construction of cooling tanks easy to cleaneasy to buildscratch proofshock proofacid proof difficult to clean difficult to adapt difficult to repair General
Materials in contact with cleaning water and chemicals must be resistant to
cleaning and disinfecting agents, in normal conditions of dosage and tempera-
ture. This is to avoid tainting the milk.
Stainless steel
The chief alloying element in stainless steels is chromium (CR), which in con-
centrations above 12 – 13 % forms a passive layer on the metal. Increasing
chromium content leads to a stronger passivity and thus a higher corrosion re-
sistance. Although chromium makes the steel stainless, it cannot resist certain
more aggressive environments. Other elements are therefore added to modify
the structure, mechanical properties and corrosion resistance. These elements
are Nickel (NI), Molybdenum (Mo), Nitrogen (N) and Copper (Cu). Stainless
steel is available in many different quantities. Most milk tanks meet the quality
grade AISI 304, and in special cases AISI 316.
Cleaning
Cleaning can not be passed over. The careful cleaning of a milk cooling systemprovides the chance to avoid infections, while cooling delays micro bacterialgrowth and chemical processes. Avoiding bacterial growth by quick cooling,and good cleaning clearly pays good return on any extra cost that might be in-curred.
Because of the nature of the product, milk, it is necessary to clean the milkingequipment after every milk turn is complete. This means that the total installa-tion must be free from any remainders of milk, one reason being that the mostimportant life condition for bacteria, the presence of food, is taken away. Byusing high temperatures and thoroughly disinfecting the installation, most bac-teria will be killed. A holistic look at why and how to clean can be found inEfficient Cleaning from DeLaval.
External hygiene – Clean the tank with soapy water of a special cleansing agent– Pay attention to the lid and rubber seals.
– Clean valve with sweeper and check the condition of the rubber seals.
Condensing unit hygiene– Ensure sufficient fresh air supply – Remove dust, hay, cobwebs, etc. Areas that need to be checked when cleaning the cooling equipment– Inner surfaces must be smooth and clean– Dark places, and where water has been mixed with fat and stays in drops.
– The agitator wing.
– The tank interior. If necessary, climb in the tank and clean with a brush.
Cooling medium (refrigerants)
For milk cooling, mainly halogenic cooling agents are used. These are indicat-ed by the letter ‘R' (standing for refrigerant), followed by a code. This codegives the following proportions in R of– Carbon [C]– Hydrogen [H]– Fluorine [F]– Chlorine [Cl] Halogenic cooling agents are described by the following items– In the vapour phase they are odourless and non-irritating – They are not poisonous (except by open fire) – They cause no corrosion – They are neither inflammable nor explosive.
R for Refrigerants
R12The first widely used artificial refrigerant. Yet because of effects on the ozonelayer, and the negative influence of greenhouse gases, it is not longer allowed.
Production has therefore been stopped. Boiling point = [1 x 105 Pa] (°C) – 30.% R22Presently the most widely used artificial refrigerant. Disadvantage is that it stillhas some effect on the ozone layer (5 % of R12). Boiling point = [1 x 105 Pa] (°C) – 40 % R134aReplacement for R12, with no ozone and only a slight greenhouse effect.
Disadvantages are that it requires special oil and that it is rather difficult tochange an existing R12 installation to R134a.
Boiling point = [1 x 105 Pa] (°C) – 26.5 % R404aReplacement for R22, with no ozone and only a slight greenhouse effect.
Disadvantages are that it requires special oil and that it is rather difficult tochange an existing R22 installation to R404a.
Boiling point = [1 x 105 Pa] (°C) – 46.4 % R407cReplacement for R22, with no ozone and only a slight greenhouse effect. Disadvantages are that it requires special oil and that it is rather difficult tochange an existing R22 installation to R407c.
Boiling point = [1 x 105 Pa] (°C) – 44.% R507Replacement for R22, with no ozone and only a slight greenhouse effect. Disadvantages are that it requires special oil and that it is rather difficult tochange an existing R22 installation to R507.
Boiling point = [1 x 105 Pa] (°C) – 46.5 % Norms for milk tanks
Norms for milk
tanks
Requirements and norms for milk cooling tanks: LIDSThe opening, closing and locking operations require a positive action.
Accidental opening, closing and locking shall not be possible.
AGITATORSNo hazardous part of the agitator shall come into contact with the operator.
Non-protected parts shall be present on the agitator shaft, with the exception ofthe agitator blades and accessories for the cleaning system. STABILITYThe tank should be constructed in a way that, under normal operating condi-tions, it shall not tilt or move when subjected to an external force of 750N ap-plied at any accessible points.
THERMAL INSULATIONThe tank should be provided with thermal insulation so that milk at 4 °C shallnot exceed 7 °C within 12 hours when the rated volume is allowed to standundisturbed, without refrigeration.
FREEZING OF MILKWhen the tank is in use, ice shall not form under the milk surface during cool-ing or storage.
AGITATION OF MILKOperation of the agitator shall not cause milk to overflow when the tank con-tains any volume of milk up to 100 % of its rated volume. The agitator shall becapable of producing a uniform distribution of fat throughout the milk in anoperating time of not more than 2 minutes and, after that, when the milk is al-lowed to stand non-agitated for 1 hour.
VII. DeLaval cooling systems
DeLaval offers the world's largest range of milk cooling systems for on thefarm use. Able to meet virtually all raw milk cooling and storage demands,DeLaval supplies farmers with herds from one, up to more than one thou-sands cows.
DeLaval DX– direct expansion cooling
systems
Figure VII.1 Open DX tank Compared to other cooling systems, the direct expansion principlegives the highest efficiency in cooling technology, combined with thelowest possible energy consumption. The stainless-steel, dou-ble-plated evaporator has a large exchange surface to ensurerapid cooling of the milk. This range includes open cylindricaltanks (from 300 – 1 800 litres); open rectangle tanks (from 1000 – 3 000 litres), and horizontal closed oval tanks (from 1150 – 32 000 litres).
DX with cleaning and control unit
Current modern demands regarding milk Figure VII.2Closed DX tank with clean- cooling and tank cleaning for closed cool- ing and control unit ing tanks are embraced in the range ofDeLaval cleaning and control units. The unit is microprocessor controlled and pro- vides: several cleaning programmes, adjustable to the conditions on the farm; and control and alarm functions, including memory to store data regarding the development of the milk quality during the time the milk is stored at the farm. A higher level of comfort can be reached with automatic dosing. The cleaning and control unit range has been developed with unique cost and en-vironmental saving features built in, such as low water and electricity con-sumption.
DeLaval MC – mobile cooling
Figure VII.3 Mobile Cooling systems are specially developed DeLaval MC for high quality cooling of small quantities ofmilk, and are used to transport the cooled milk toa pick-up point for collection by a milk tanker.
Mobile cooling is especially practical when, dueto lack of infrastructure, small mountain roads orsmall quantities of milk, the milk collection truckcannot reach the farm.
DeLaval IB – icebank cooling systems
The condensing units of icebank tanks are used to produce ice in the periods
between milkings. This ice is frozen around the evaporator pipes in the lower
Figure VII.4 water basin. When warm milk DeLaval IB comes into the tank, the ice water isspayed against the outside of theinner tank. The cold water takes theheat out of the milk. These systemsare used in areas where peak elec-tricity consumption during milkingshould be avoided; where low nighttariffs offer economical benefits; orthe electricity supply in general istoo weak to support larger condens-ing units.
DeLaval IN – instant cooling systems
This system consists of a water-cooled plate cooler, P30 clip-on, and is able to
cool the milk very quickly. The chilled water used is from a water chiller or ice
builder. Instant cooling is often used with large herds and Figure VII.5 milking round the clock, such as for the Voluntary Milking Plate cooler System (VMS). In cases where the raw milk quality is poor(due to bad hygienic conditions and milking routines), in-stant cooling stops growth of the bacteria immediately.
The system consists of: plate coolers; heat exchangers; con-densing units with water-chillers; ice-builders; storage tankswith maintenance cooling, pumps and control unit.
units
This range is specifically designed for
milk cooling on the farm using environ-
Figure VII.6 mentally friendly freon gas. The con- DeLaval condensing unit densing units are equipped with a pistonor scroll compressor. It is available in anextensive range of capacities.
VIII. Future aspects
One of the most labour intensive and time-consuming jobs in dairy milk pro-duction is the milking itself, which takes place at least twice a day. Demandshave therefore been growing for automatic milking systems to solve this.
DeLaval offers the total automatic system, VMS, the Voluntary MilkingSystem.
Due to a continuous low quantity milk flow, 24 hours per day, the VMS systemrequires an efficient, specially-designed cooling concept, and DeLaval hastherefore developed a professional cooling system.
VMS cooling
Figure VIII.1VMS cooling system The new IN/VMS systems uses a 300 litre receiver, equipped with pump, platecooler and control unit. The milk flows from the VMS into the receiver tank, ispumped via a plate heat exchanger and cooled before it reaches the main cool-ing tank. When the main storage tank is emptied, the milk pump is blocked andthe milk will stay in the receiver tank during emptying and cleaning of the tank.
With this system, the milk is cooled with a P30 Clip-on Plate Cooler.
The VMS milk supply pipes to the cooling tank and instant cooling system areautomatically cleaned every eight hours. The storage tank is cleaned by the uniton the tank after the milk is collected.
Cooling media and
Cooling media and the environment
The ozone layer is a worldwide environmental concern, and the expulsion ofchloride-flour-hydrocarbons (CFK's) from cooling devices has been one of thecauses. DeLaval uses environmentally friendly tank insulation materials andrefrigerants, and is constantly researching for new ways to reduce any environ-mental impact of cooling materials. IX. The environment
DeLaval and the environment
DeLaval's business mission is: "To drive progress in milk production." Caring for the environment is an essential part of DeLaval's corporate strategy.
By continuously improving products, processes and work place impact forboth people and animals from an environmental standpoint, we will contributeto beneficial environmental conditions for customers as well as society. At thesame time, we will create sustainable business activity and enhance our lead-ing position in the market.
Guiding principles in our environmental activities:
Proactiveness
• By building up understanding and know-how together with target orienta-
tion, we will achieve a leading position in terms of the environment whichwill create competitive edge as well as freedom of action.
• We will exceed the minimum standards set by the authorities.
Comprehensive overview
• Environmental aspects will be considered in our decision making process.
We will balance the technically possible, economically reasonable and ecologically acceptable.
• We will strive to use natural resources sparingly and prefer to use recyclable materials where available.
• Products and activities that have negative environmental risks will be avoid- • We will strive to transfer environmentally sound technologies through our • We will put the same environmental demands on our suppliers as we do on our own company. Planning and follow-up
• Environmental endeavours will be pursued continuously and systematically.
These endeavours should be made an integrated part of normal operations. • Regular follow-up and evaluation of our environmental plans will be Development
• We will develop products and services that have less negative environmental
• The total life cycle of each product will be considered, from its raw material origin and production to usage and disposal/recycling.
Marketing
• We will promote our customers' choice of environmentally sound products
by focusing on and explaining the environmental benefits.
• Marketing with environmental arguments must be based on facts and Information and education
• By education and information, all employees should be stimulated to become
environmental-conscious and to participate in the activities.
• Individual initiative for environmental improvements will be encouraged and • Dialogue concerning environmental issues of the company will be charac- terised by openness and objectivity.
Alfa Laval Agri AB. (1995). Efficient Milking. Tumba.
For further
reading
Alfa Laval Agri AB. (1996). Efficient Cleaning. Tumba.
Boerekamp, J.A.M, & Slaghuis, B. A.(1993). Stromend diepkoelen en conven-tioneel koelen van rauwe melk. Lelystad European Standard (1996). Food Processing Machinery - Bulk milk coolers onfarms - Requirements for construction, performance and suitability for use,safety and hygiene. CEN, Brussels.
Harrigan, W.F., & McCane, M.E. (1976). Laboratory Methods in food andDairy Microbiology. Reading, Girvan.
Judkins, H.F., & Keener, H.A. (1960). Milk production and processing. NewYork.
Melkwinning (1986). Min. van landbouw en visserij, 4th Edition. Wageningen.
Melkwinning – van der Haven, M.C., De Koning, C.J.A.M., Wemmenhoven,H., & Westerbeek, R. (1996). Praktijk onderzoek Rundvee schapen en paarden(PR). Lelystad.
Netherlands Government Gazette (1995). Technical requirements for refrigera-tion equipment. Den Haag.
Robinson, R. K. (1983). Dairy Microbiology. Vol. 1.Reading.
Robinson, R.K. (1990). The Microbiology of Milk, 2nd Edition. Reading.
Slater. A, K. (1991). The Principles of Dairy Farming, 11th Edition.
Alexandria Bay.
Tetra Pak Processing Systems (1995). Dairy processing handbook. Lund.
van den Berg, J.C.T. (1988). Dairy technology in the tropics and subtropics.
Pudoc, Wageningen.
van Muijen, M.W.C. (1994). Melkkunde. Bolsward.
Varnam, A.H., & Sutherland, J.P. (1994) Milk and Milk Products. Reading.
Walstra, R, & Jenness, P. (1984). Dairy chemistry and physics. Wageningen,Minnesota.
Walstra, P., & Jellema, A. (1985). Zuiveltechnologie1. Fysische processen.
Wageningen. The manufacturer reserves the right to make design changes. 11603-en/0010

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