dk3080ch1

Part I

Milk

© 2006 by Taylor & Francis Group, LLC

3

1 Milk: Main Characteristics

Milk is defined as the secretion of the mammary glands of mammals, its primarynatural function being nutrition of the young. Milk of some animals, especiallycows, buffaloes, goats and sheep, is also used for human consumption, either assuch or in the form of a range of dairy products. In this book, the word milk willbe used for the ‘normal’ milk of healthy cows, unless stated otherwise. Occasion-ally, a com-parison will be made with human milk.

This chapter is meant as a general introduction. Nearly all that is mentioned —with the exception of parts of Section 1.2 — is discussed in greater detail in otherchapters. However, for readers new to the field it is useful to have some idea of theformation, composition, structure, and properties of milk, as well as the variation —including natural variation and changes due to processing — that can occur inthese characteristics, before starting on the main text.

1.1 COMPOSITION AND STRUCTURE

1.1.1 PRINCIPAL COMPONENTS

A classification of the principal constituents of milk is given in Table 1.1. Theprincipal chemical components or groups of chemical components are those presentin the largest quantities. Of course, the quantity (in grams) is not paramount in allrespects. For example, vitamins are important with respect to nutritive value; en-zymes are catalysts of reactions; and some minor components contribute markedlyto the taste of milk. More information on milk composition is given in Table 1.3.

Lactose or milk sugar is the distinctive carbohydrate of milk. It is a disac-charide composed of glucose and galactose. Lactose is a reducing sugar.

The fat is largely made up of triglycerides, constituting a very complicatedmixture. The component fatty acids vary widely in chain length (2 to 20 carbonatoms) and in saturation (0 to 4 double bonds). Other lipids that are present includephospholipids, cholesterol, free fatty acids, monoglycerides, and diglycerides.

About four fifths of the protein consists of casein, actually a mixture of fourproteins: αS1-, αS2-, β-, and κ-casein. The caseins are typical for milk and have somerather specific properties: They are to some extent phosphorylated and have littleor no secondary structure. The remainder consists, for the most part, of the milkserum proteins, the main one being β-lactoglobulin. Moreover, milk contains nume-rous minor proteins, including a wide range of enzymes.

The mineral substances — primarily K, Na, Ca, Mg, Cl, and phosphate —are not equi-valent to the salts. Milk contains numerous other elements in trace



quantities. The salts are only partly ionized. The organic acids occur largely asions or as salts; citrate is the principle one. Furthermore, milk has many miscel-laneous components, often in trace amounts.

The total content of all substances except water is called the content of drymatter. Furthermore, one distinguishes solids-not-fat and the content of fat in thedry matter.

The chemical composition of milk largely determines its nutritional value; theextent to which microorganisms can grow in it; its flavor; and the chemicalreactions that can occur in milk. The latter include reactions that cause off-flavours.

1.1.2 STRUCTURAL ELEMENTS

Structure can be defined as the geometrical distribution of the (chemical) compo-nents in a system. It may imply, as it does in milk, that the liquid contains particles.This can have important consequences for the properties of the system. Forinstance, (1) chemical components are present in separate compartments, whichcan greatly affect their reactivity; (2) the presence of particles greatly affects somephysical properties, like viscosity and optical appearance; (3) interaction forcesbetween particles generally determine the physical stability of the system; and (4)the separation of some components (fat and casein) is relatively easy.

Figure 1.1 shows the main structural elements of milk. Of course, the figureis schematic and incomplete. Some properties of the structural elements are given

TABLE 1.1Approximate Composition of Milk

ComponentAverage Contentin Milk (% w/w)

Rangea

(% w/w)Average Content in

Dry Matter (% w/w)

Water 87.1 85.3−88.7 —Solids-not-fat 8.9 7.9−10.0 —Fat in dry matter 31 22−38 —Lactose 4.6 3.8−5.3 36Fat 4.0 2.5−5.5 31Proteinb 3.3 2.3−4.4 25

casein 2.6 1.7−3.5 20Mineral substances 0.7 0.57−0.83 5.4Organic acids 0.17 0.12−0.21 1.3Miscellaneous 0.15 — 1.2

Note: Typical for milks of lowland breeds.

a These values will rarely be exceeded, e.g., in 1 to 2% of samples of separatemilkings of healthy individual cows, excluding colostrum and milk drawn shortlybefore parturition.b Nonprotein nitrogen compounds not included.


1.1 Composition and Structure 5

in Table 1.2, again in a simplified form; the numerical data mentioned are meantonly to define orders of magnitude. The table clearly shows that aspects of colloidchemistry are essential for understanding the properties of milk and the manychanges that can occur in it. All particles exhibit Brownian motion; they have anelectrostatic charge, which is negative at the pH of milk. Their total surface areais large.

Fat globules. To a certain extent, milk is an oil-in-water emulsion. But thefat globules are more complicated than emulsion droplets. In particular, thesurface layer or membrane of the fat globule is not an adsorption layer of one

FIGURE 1.1 Milk viewed at different magnifications, showing the relative size of struc-tural elements (A) Uniform liquid. However, the liquid is turbid and thus cannot behomogeneous. (B) Spherical droplets, consisting of fat. These globules float in a liquid(plasma), which is still turbid. (C) The plasma contains proteinaceous particles, which arecasein micelles. The remaining liquid (serum) is still opalescent, so it must contain otherparticles. The fat globules have a thin outer layer (membrane) of different constitution.(From H. Mulder and P. Walstra, The Milk Fat Globule, Pudoc, Wageningen, 1974.)

C× 50000

B× 500

A× 5

Milk

Plasma

Fatglobules

Fatglobule

SerumMembrane

Caseinmicelles



single substance but consists of many components; its structure is complicated.The dry mass of the membrane is about 2.5% of that of the fat. A small part ofthe lipids of milk is found outside the fat globules. At temperatures below 35°C,part of the fat in the globules can crystallize. Milk minus fat globules is calledmilk plasma, i.e., the liquid in which the fat globules float.

Casein micelles consist of water, protein, and salts. The protein is casein.Casein is present as a caseinate, which means that it binds cations, primarilycalcium and magnesium. The other salts in the micelles occur as a calciumphosphate, varying somewhat in composition and also containing a small amountof citrate. This is often called colloidal phosphate. The whole may be calledcalcium-caseinate/calcium-phosphate complex. The casein micelles are notmicelles in the colloid-chemical sense but just ‘small particles.’ The micelles have

TABLE 1.2Properties of the Main Structural Elements of Milk

Milk

Plasma

Serum

Fat Globules Casein MicellesGlobular Proteins

LipoproteinParticles

Main components Fat Casein, water,salts

Serum protein Lipids, proteins

To be considered as Emulsion Fine dispersion Colloidal solution

Colloidal dispersion

Content(% dry matter)

4 2.8 0.6 0.01

Volume fraction 0.05 0.1 0.006 10−4

Particle diametera 0.1−10 µm 20−400 nm 3−6 nm 10 nmNumber per ml 1010 1014 1017 1014

Surface area (cm2/ml milk)

700 40000 50000 100

Density(20°C; kg ⋅ m−3)

920 1100 1300 1100

Visible with Microscope Ultramicroscope Electron microscope

Separable with Milk separator High-speedcentrifuge

Ultrafiltration Ultrafiltration

Diffusion rate(mm in 1h)a

0.0 0.1–0.3 0.6 0.4

Isoelectric pH ∼3.8 ∼4.6 4−5 ∼4

Note: Numerical values are approximate averages.

a For comparison, most molecules in solution are 0.4 to 1 nm diameter, and diffuse, say, 5 mmin 1 h. 1 mm = 103 µm = 106 nm = 107 Å.


1.2 Milk Formation 7

an open structure and, accordingly, contain much water, a few grams per gramof casein. Milk serum, i.e., the liquid in which the micelles are dispersed, is milkminus fat globules and casein micelles.

Serum proteins are largely present in milk in molecular form or as very smallaggregates.

Lipoprotein particles, sometimes called milk microsomes, vary in quantityand shape. Presumably, they consist of remnants of mammary secretory cellmembranes. Few definitive data on lipoprotein particles have been published.

Cells, i.e., leukocytes, are always present in milk. They account for about0.01% of the volume of milk of healthy cows. Of course, the cells contain allcytoplasmic components such as enzymes. They are rich in catalase.

Table 1.3 gives a survey of the average composition and structure of milk.

1.2 MILK FORMATION

Milk components are for the most part formed in the mammary gland (the udder)of a cow, from precursors that are the results of digestion.

Digestion. Mammals digest their food by the use of enzymes to obtain simple,soluble, low-molar-mass components, especially monosaccharides; small pep-tides and amino acids; and fatty acids and monoglycerides. These are taken upin the blood, together with other nutrients, such as various salts, glycerol, organicacids, etc. The substances are transported to all the organs in the body, includingthe mammary gland, to provide energy and building blocks (precursors) formetabolism, including the synthesis of proteins, lipids, etc.

In ruminants like the cow, considerable predigestion occurs by means ofmicrobial fermentation, which occurs for the most part in the first stomach orrumen. The latter may be considered as a large and very complex bio-fermenter.It contains numerous bacteria that can digest cellulose, thereby breaking downplant cell walls, providing energy and liberating the cell contents. From celluloseand other carbohydrates, acetic, propionic, butyric and lactic acid are formed, whichare taken up in the blood. The composition of the organic acid mixture dependson the composition of the feed. Proteins are broken down into amino acids. Therumen flora uses these to make proteins but can also synthesize amino acids fromlow-molar-mass nitrogenous components. Further on in the digestive tract themicrobes are digested, liberating amino acids. Also, food lipids are hydrolyzed inthe rumen and partly metabolized by the microorganisms. All these precursors canreach the mammary gland.

Milk Synthesis. The synthesis of milk components occurs for the greaterpart in the secretory cells of the mammary gland. Figure 1.2 illustrates such acell. At the basal end precursors of milk components are taken up from the blood,and at the apical end milk components are secreted into the lumen. Proteins areformed in the endoplasmic reticulum and transported to the Golgi vesicles, inwhich most of the soluble milk components are collected. The vesicles grow insize while being transported through the cell and then open up to release theircontents in the lumen. Triglycerides are synthesized in the cytoplasm, forming


8M

ilk: Main C

haracteristicsTABLE 1.3Composition and Structure of Milka

SERUM

Water 790 g Organic acids Proteins

citrate 1600 mg casein +

Carbohydrates formate 40 mg β-lactoglobulin 3.2 g

lactose 46 g acetate 30 mg α-lactalbumin 1.2 g

glucose 70 mg lactate 20 mg serum albumin 0.4 g

others oxalate 20 mg immunoglobulins 0.8 g

others 10 mg proteose peptone +

Minerals others

Ca, bound 300 mg Gases Nonprotein nitrogenous

Ca, ions 90 mg oxygen 6 mg compounds

Mg 70 mg nitrogen 16 mg peptides +

K 1500 mg Lipids amino acids 50 mg

Na 450 mg glycerides + urea 250 mg

Cl 1100 mg fatty acids 20 mg ammonia 10 mg

phosphate 1100 mg phospholipids 100 mg others 300 mg

sulfate 100 mg cerebrosides 10 mg Enzymes

bicarbonate 100 mg sterols 15 mg acid phosphatase

others peroxidase

Trace elements many others

Zn 3 mg Vitamins, e.g. Phosphoric esters ~300 mg

Fe 120 µg riboflavin 2 mg Others

Cu 20 µg ascorbic acid 20 mg

many others

a Approximate average quantities in 1 kg milk. Note: The water in the casein micelles contains some small-molecule solutes.

FAT GLOBULE

triglycerides diglycerides monoglycerides

Glycerides40 g0.1 g10 mg

60 mg

60 mg100 mg0.3 mg

Fatty acidsSterolsCarotenoidsVitamins A, D, E, KWaterOthers

CASEIN MICELLE

caseinproteose peptone

lipaseplasmin

CaphosphatecitrateK, Mg, Na

Protein

Salts

WaterEnzymes

850 mg1000 mg150 mg

26 g+

~80 g

MEMBRANE

LIPOPROTEINPARTICLE

lipidsprotein

enzymeswater

LEUKOCYTE

Many enzymese.g., catalase

Nucleic acidsWater

water +protein 700 mgphospholipids 250 mgcerebrosides 30 mgglycerides +fatty acids 15 mgstreols 15 mgother lipidsenzymes alkaline phosphatase xanthine oxidase many othersCu 4 µgFe 100 µg

2 g



small globules, which grow while they are transported to the apical end of thecell. They become enrobed by the outer cell membrane (or plasmalemma) whilebeing pinched off into the lumen. This type of secretion is called merocrine,which means that the cell remains intact.

Table 1.4 gives some information about the synthesis of specific components.Most are synthesized in the cell. Others are taken up from the blood but, generally,not in the same proportion as in the blood; see, especially, the salts. This meansthat the cell membranes have mechanisms to reject, or allow passage of, specificcomponents. Some substances, notably water and small lipophilic molecules, can

FIGURE 1.2 Stylized diagram of a mammary secretory cell. Below is the basal part, ontop the apical part of the cell. The cell is bounded by other secretory cells to form theglandular epithelium. See text for further details. (From P. Walstra and R. Jenness, DairyChemistry and Physics, Wiley, New York, 1984. With permission.)

Ribosomes

Basementmembrane

Golgi apparatus

Cytosol

Lysosome

Outer cellmembrane(plasmalemma)

Golgi vesiclewith caseinmicelles

Endoplasmicreticulum

Mitochondrion

Nascentfat globule

Junctionalcomplex

Microvillus

5 µm

Nucleus

LUMEN


10 M

ilk: Main C

haracteristicsTABLE 1.4Synthesis of Important Milk Components

Milk Component Precursor in Blood Plasma Synthesis of Component

NameConcentration

(% w/w) NameConcentration

(% w/w)In the

Secretory Cell?Specificfor Milk?

Specific for the Species?

Water 86 Identical 91 No No No Lactose 4.7 Glucosea 0.05 Yes Yes NoProtein

Caseins 2.60.320.120.01

Yes Yes Yesb

β-lactoglobulin Amino acids 0.04 Yes Yes Yesα-lactalbumin Yes Yes YesLactoferrin Yes No YesSerum albumin 0.04 Identical 3.2 No No YesImmunoglobulins 0.07 Most are identical 1.5 No No Yes

Enzymes Trace Various — Yesc Noc YesLipids

Triglycerides 4 Partly Partly

Phospholipids 0.03 Some lipids 0.3Citric Acid 0.17 Glucosea 0.05 Yes No NoMinerals Identical No No No

Ca 0.13 0.01Pd 0.09 0.01Na 0.04 0.34K 0.15 0.03Cl 0.11 0.35

a Glucose can also be formed in the secretory cells from some amino acids.b All proteins are species specific, but comparable proteins occur in the milk of all ruminants.c Is not true for all enzymes.d In various phosphates.

Acetic acid

-Hydroxy butyric acid

A

0 01

0 006

.

.βccylglycerols



pass the cell more or less unhindered. Some other components, such as serumalbumin and chlorides, can ‘leak’ from the blood into the milk by passing throughthe spaces between secretory cells. Also, some leukocytes somehow reach thelumen. Finally, cell remnants, such as part of the microvilli depicted in Figure 1.2and tiny fragments of cytoplasm that occasionally adhere to a fat globule, aresecreted and form the lipoprotein particles of Table 1.2.

Excretion. The glandular epithelium, consisting of layers of secretory cells,form spherical bodies called alveoli. Each of these has a central lumen into whichthe freshly formed milk is secreted. From there, the milk can flow through smallducts into larger and still larger ones until it reaches a cavity called the cistern.From the cistern, the milk can be released via the teat. A cow has four teats andhence four separate mammary glands, commonly called (udder) quarters.

Excretion of the milk does not happen spontaneously. The alveoli have tocontract, which can be achieved by the contraction of muscle tissue around thealveoli. Contraction is induced by the hormone oxytocin. This is released intothe blood by stimulation of the teats of the animal, be it by the suckling youngor by the milker. The udder is not fully emptied.

Lactation. When a calf is born, lactation — i.e., the formation and secretionof milk — starts. The first secretion greatly differs in composition from milk (seeSubsection 2.7.1.5). Within a few days the milk has become normal and milk yieldincreases for some months, after which it declines. The yield greatly varies amongcows and with the amount and the quality of the feed taken by the cow. For milchcows, milking is generally stopped after about 10 months, when yield has becomequite low. The duration from parturition to leaving the cow dry is called thelactation period, and the time elapsed after parturition is the stage of lactation.

1.3 SOME PROPERTIES OF MILK

Milk as a Solution. Milk is a dilute aqueous solution and behaves accord-ingly. Because the dielectric constant is almost as high as that of pure water, polarsubstances dissolve well in milk and salts tend to dissociate (although this dis-sociation is not complete). The ionic strength of the solution is about 0.073 M. ThepH of milk is about 6.7 at room temperature. The viscosity is low, about twice thatof water, which means that milk can readily be mixed, even by convection currentsresulting from small temperature fluctuations. The dissolved substances give milkan osmotic pressure of about 700 kPa (7 bar) and a freezing-point depressionclose to 0.53 K. The water activity is high, about 0.995. Milk density (ρ20) equalsabout 1029 kg⋅m−3 at 20°C; it varies especially with fat content.

Milk as a Dispersion. Milk is also a dispersion; the particles involved aresummarized in Table 1.2. This has several consequences, such as milk beingwhite. The fat globules have a membrane, which acts as a kind of barrier betweenthe plasma and the core lipids. The membrane also protects the globules againstcoalescence. The various particles can be separated from the rest.

The fat globules can be concentrated in a simple way by creaming, whicheither occurs due to gravity or — more efficiently — is induced by centrifugation.



In this way cream and skim milk are obtained. Skim milk is not identical to milkplasma, though quite similar, because it still contains some small fat globules.Cream can be churned, leading to butter and buttermilk; the latter is rather similarin composition to skim milk.

Likewise, casein micelles can be concentrated and separated from milk, forinstance, by membrane filtration. The solution passing through the membrane isthen quite similar to milk serum. If the pores in the membrane are very small, alsothe serum proteins are retained. When adding rennet enzyme to milk, as is donein cheese making, the casein micelles start to aggregate, forming a gel; whencutting the gel into pieces, these contract, expelling whey. Whey is also similarto milk serum but not quite, because it contains some of the fat globules and partof the κ-casein split off by the enzyme. Casein also aggregates and forms a gelwhen the pH of the milk is lowered to about 4.6.

Moreover, water can be removed from milk by evaporation. Altogether, arange of liquid milk products of various compositions can be made. Some exam-ples are given in Table 1.5.

Flavor. The flavor of fresh milk is fairly bland. The lactose produces somesweetness and the salts some saltiness. Several small molecules present in verysmall quantities also contribute to flavor. The fat globules are responsible for thecreaminess of whole milk.

Nutritional value. Milk is a complete food for the young calf, and it canalso provide good nutrition to humans. It contains virtually all nutrients, most ofthese in significant quantities. However, it is poor in iron and the vitamin Ccontent is not high. It contains no antinutritional factors, but it lacks dietary fibre.

Milk as a Substrate for Bacteria. Because it is rich in nutrients, manymicroorganisms, especially bacteria, can grow in milk. Not all bacteria that needsugar can grow in milk, some being unable to metabolize lactose. Milk is poorin iron, which is an essential nutrient for several bacteria, and contains someantibacterial factors, such as immunoglobulins and some enzyme systems. More-over, milk contains too much oxygen for strictly anaerobic bacteria. Altogether,the growth of several bacteria is more or less restricted in raw milk, but severalothers can proliferate, especially at high ambient temperatures.

1.4 VARIABILITY

Freshly drawn milk varies in composition, structure, and properties. Even within themilk from a single milking of one cow, variation can occur. The fat globules varyin size and, to some extent, in composition, and the same applies to casein micelles.

Natural Variation. The main factors responsible for natural variation in milkare the following:

• Genetic factors: Breed and individual.• The stage of lactation: This can have a significant effect. Especially

the milk obtained within 2 or 3 d after parturition tends to have a verydifferent composition; it is called colostrum or beestings.


1.4 Variability 13

• Illness of the cow: Especially severe mastitis (inflammation of theudder) can have a relatively large effect. The milk tends to have anincreased content of somatic cells.

• Feed: The amount and the quality of the feed given strongly affectmilk yield. However, the effect of the cow’s diet on milk compositionis fairly small, except for milk fat content and composition.

In a qualitative sense, cows’ milk is remarkably constant in composition.Nevertheless, individual milkings show significant differences in composition.The variation is small in milk processed at the dairy, because this consists ofmixtures of the milk of a large number of cows from many farms.

Other Causes. As soon as the milk leaves the udder, it becomes contaminated,for instance, with oxygen and bacteria (milk within the udder of a healthy cowtends to be sterile). Contamination with other substances can occur. The temper-ature of the milk generally decreases. These factors can lead to changes in milkproperties. Far greater changes occur during long storage and in milk processing(see the next section).

1.5 CHANGES

Milk is not a system in equilibrium. It changes even while in the udder. This ispartly because different components are formed at various sites in the mammarysecretory cell and come into contact with one another after their formation.Furthermore, several changes can occur due to the milking, the subsequent low-ering of the temperature, and so on. Changes may be classified as follows:

1. Physical changes occurring, for instance, when air is incorporatedduring milking: Because of this, additional dissolution of oxygen andnitrogen occurs in milk. Moreover, a new structural element is formed:air bubbles. Milk contains many surface-active substances, predomi-nantly proteins, which can become attached to the air–water interfaceformed. Furthermore, by contact with the air bubbles, fat globules maybecome damaged, i.e., lose part of their membrane. Fat globules maycream. Creaming is most rapid at low temperature because the globulesaggregate to large flocs during the so-called cold agglutination (Sub-section 3.2.4). On cooling, part of the milk fat starts to crystallize, themore so at a lower temperature. But even at 0°C part of the fat remainsliquid. The presence of fat crystals can strongly diminish the stabilityof fat globules against clumping.

2. Chemical changes may be caused by the presence of oxygen: Severalsubstances may be oxidized. In particular, light may induce reactions,often leading to off-flavors. Composition of salts can vary, for example,with temperature.

3. Biochemical changes can occur because milk contains active enzymes:Examples are lipase, which causes lipolysis; proteinases, which cause



proteolysis; and phosphatases, which cause hydrolysis of phosphoricacid esters.

4. Microbial changes are often the most conspicuous: The best-knowneffect is production of lactic acid from lactose, causing an obviousdecrease in pH. Numerous other changes, such as lipolysis and pro-teolysis, may result from microbial growth.

Cooling of the milk to about 4°C is generally applied to inhibit many of thechanges mentioned, especially growth of microorganisms and enzyme action. Inmany regions, the milk is already cooled at the farm, directly after milking, in aso-called bulk tank. The milk should be kept cold during transport to the dairyand subsequent storage.

Processing. At the dairy milk is always processed. Of course, this causes changesin composition and properties of the milk, as it is intended to do. These changes canbe drastic, as the following examples will show, and it can be questioned whether theresulting product can still be called milk; however, it is standard practice to do so.The most common processes applied are briefly described in the following paragraphs.

Heat treatment is virtually always applied, primarily to kill harmful bacteria.It also causes numerous chemical and other changes, the extent of which dependson temperature and duration of heating. Low pasteurization (e.g., 15 s at 74°C)is a fairly mild treatment that kills most microorganisms and inactivates someenzymes but does not cause too many other changes. High pasteurization (e.g.,15 s at 90°C, but varying widely) is more intense; all vegetative microorganismsare killed, most enzymes are inactivated, and part of the serum proteins becomeinsoluble. Sterilization (e.g., 20 min at 118°C) is meant to kill all microorganisms,including spores; all enzymes are inactivated; numerous chemical changes, such asbrowning reactions, occur; and formic acid is formed. UHT (ultrahigh-temperature)heating (e.g., at 145°C for a few seconds) is meant to sterilize milk while mini-mizing chemical changes; even some enzymes are not inactivated fully.

Separation, usually by means of a flow-through centrifuge called a cream sep-arator, yields skim milk and cream. The skim milk has a very low fat content, 0.05to 0.08%. Milk skimmed after gravity creaming has a much higher fat content.Unless stated otherwise, the term skim milk will refer to centrifugally separated milk.By mixing skim milk and cream, milk may be standardized to a desired fat content.

Homogenization (i.e., treatment in a high-pressure homogenizer) of milk leadsto a considerable reduction in fat globule size. Such milk creams very slowly butis also altered in other respects. All types of sterilized milk or, more generally,all long-life liquid milk products are homogenized in practice.

Evaporation removes water, producing milk that is more concentrated. Manyproperties are altered; the pH decreases, for example.

Membrane processes may be applied to remove water; this is called reverseosmosis. Ultrafiltration separates milk into a concentrate and a permeate that israther similar to milk serum. Electrodialysis removes some inorganic salts.

Fermentation or culturing of milk, usually by lactic acid bacteria, causesconsiderable alteration. Part of the lactose is converted to lactic acid, causing a


1.5 Changes

15

TABLE 1.5Gross Composition and Some Properties of Milk and Liquid Milk Products

Milk ProductFreshMilk

BeverageMilk

BeverageMilk

SkimMilkb

WhippingCream

EvaporatedMilk

Whey(Sweet)b

Buttermilk (Sour)c

Yogurt(Plain)d

Heat Treatmenta No LP ST LP HP ST LP HP HP

Contents (% w/w)Fat 4.0 3.5 3.5 0.07 35 8.0 0.05 0.4 3.5Casein 2.6 2.6 2.6 2.7 1.7 5.7 trace 2.6 2.5Other proteins 0.70 0.70 0.70 0.66 1.15 1.5 0.81 0.9 0.8Lactose 4.6 4.6 4.6 4.8 3.1 9.5 5.0 3.9 3.7Salts 0.85 0.85 0.85 0.88 0.57 1.85 0.66 0.88 0.85Water 87.1 87.5 87.5 90.6 58.4 73.1 93.3 90.2 87.5

Properties pH 6.7 6.7 6.6 6.7 6.7 6.2 6.6 4.6 4.4Viscosity (mPa⋅s) 1.9 1.8 1.9 1.65 8.7 17 1.2 100e 400e

Density (kg⋅m−3) 1029 1030 1030 1035 990 1070 1025 1034 1031Fat globule size (µmf) 3.4 0.5 0.3 0.4 4.0 0.3 — — 0.4

Note: Approximate examples; values at room temperature.

a LP = low-pasteurized, HP = high-pasteurized, ST = sterilized.b Centrifuged.c Undiluted.d Stirred yogurt.e Highly variable.f d32 (see Subsection 3.1.4).



decrease in pH to such an extent that the casein becomes insoluble. This makesthe milk much more viscous. The bacteria also produce other metabolites, thekind and concentrations of which depend on the bacterial species.

Cheese making. As mentioned, milk can be clotted by adding rennet, whichcontains a specific proteolytic enzyme. The enzyme transforms the casein micellesin such a way that they start to coagulate. The resulting gel can be broken intopieces; stirring the material then results in the formation of curd particles andwhey. The curd contains the micellar casein and most of the fat, the liquid wheycontains most of the water-soluble components of the milk and some protein splitoff casein by the rennet. The curd is further processed to form cheese.

Table 1.5 gives some idea of properties and composition of milk treated invarious ways and of some liquid milk products, including whey. More extensiveinformation is given in the Appendix.

Suggested Literature

Most aspects are further discussed in later chapters, and literature will be mentioned there.A general reference for many aspects treated throughout the book: H. Roginski,J.W. Fuquay, and P.F. Fox, Eds., Encyclopedia of Dairy Sciences, Vols. 1–4,Academic Press, London, 2003.

A good monograph on milk synthesis, secretion, and collection, although slightly outdatedin a few aspects: B.L. Larson, Ed., Lactation, Iowa State University Press, Ames,Iowa, 1985.


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