Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

Подождите немного. Документ загружается.

0020 Some processors use microfiltration in combin-

ation with pasteurization to extend milk shelf-life.

The skim milk fraction is filtered to remove bacteria,

sterile cream is added to adjust the fat content, and

then the product is pasteurized. Microfiltration re-

duces levels of spoilage bacteria while avoiding flavor

defects caused by high heat treatment of skim milk.

Cream is not as susceptible to this problem. Canadian

experience has demonstrated that the shelf-life of

pasteurized milk can be extended from 12–18 to 32

days by using microfiltration.

UHT

0021 The UHT treatment was developed to minimize

damage to milk components caused by in-container

sterilization while killing or inactivating all microor-

ganisms, so there is little likelihood of microorganisms

spoiling the product during storage and transport.

The product is said to be ‘commercially sterile.’ The

term ‘UHT’ may stand for ultraheat treatment or

ultrahigh temperature.

0022 UHT processing is being used increasingly for heat-

treating milk, as the shelf-life is extended from days to

months, and UHT milk can be stored and transported

without refrigeration. The use of UHT treatment varies

around the world. In Germany, France, and Spain,

more than 50% of liquid milk is consumed in UHT

form, whereas the process is rarely used in the USA.

0023 The taste and appearance of UHT-treated milk are

different from those of pasteurized milk. The milk

may taste cooked or burnt to some consumers, be-

cause of the development of a sulfurous flavor during

the heat treatment. The form of heat treatment used

influences the intensity of the sulfurous flavor. How-

ever, the flavor has generally dissipated by the time of

consumption, and the milk has a slightly brown color.

0024In some countries, the temperature and time condi-

tions for UHT treatment are specified by regulation.

Typical temperature–time conditions for UHT treat-

ment of milk are 130–150



C for 1–3 s. Products with

increased solids will require a higher temperature or

longer time to ensure effective heat treatment.

0025UHT treatment can be carried out using a number

of different systems. These include direct means of

heat treatment such as injection of steam into milk

and infusion of milk into a steam chamber or indirect

means using heat exchangers where the milk is separ-

ated from the heating medium. Scraped-surface heat

exchangers may be necessary for UHT treatment of

viscous products or products containing particulate

matter. When steam is used for direct heat treatment,

care must be taken to ensure that there is no dilution

of the milk. The steam must also be suitable for

contact with food products. All UHT equipment

must be sterile to prevent recontamination of the

milk with microorganisms. Products that have been

UHT-treated must be packed aseptically to ensure

optimum shelf-life.

0026The raw milk to be used for UHT treatment must

be selected carefully to ensure that there is minimal

Flow diversion to

raw milk line

if not heated to

pasteurization temperature

THERMOMETER

Heating

Cooling

Raw milk

Hot liquid

HOMOGENIZER

Regeneration

Pasteurized milk

Cold liquid

HOLDING TUBE

fig0001 Figure 1 Schematic diagram of a plate heat exhanger pasteurizer showing milk flow.

3954 MILK/Processing of Liquid Milk

chance of heat-stable indigenous or microbial enzymes

present causing gelation in the stored products.

0027 Some dairies use equipment designed for UHT

treatment to heat the product to 120–130



C without

a holding time. This is claimed to double the shelf-life

of product packed under nonaseptic conditions com-

pared with normal pasteurized milk.

In-container

sterilization

0028 The consumption of in-container sterilized milk has

declined with the introduction of UHT processing and

improved storage and distribution systems for pas-

teurized milk. The term ‘sterilized’ is used to describe

milk that has been packed in airtight containers and

subjected to temperatures in excess of 100



C. The

severe heat treatment of the milk results in a caramel-

ized, burnt flavor and a brown color. The product is

usually packed before heat treatment. If it is packed

after heat treatment, the containers must be sterilized

before use and then filled under aseptic conditions.

Typical heating parameters for packaged sterlized

milk are 115–120



C for 10–20 min. The tempera-

ture–time conditions applied depend on the compos-

ition of the product, the type and dimensions of the

package, and the desired degree of caramelization.

Homogenization

0029 Many liquid milks need to be homogenized to prevent

the separation of fat during storage and distribution.

The large fat globules are broken up to form small

globules that do not aggregate and rise to the surface.

Homogenization also improves the mouth-feel of

many products.

0030 The homogenization step normally takes place after

the preheat treatment in the pasteurizer. The normal

temperature of milk required for effective homogeniza-

tion is 60–70



C. Heating the milk prior to homogeniza-

tion helps to break up the fat globules and inactivates

milk lipase that may act on the disrupted fat. The hom-

ogenization process is normally in two stages with

14 000–20 000 kPa pressure for the first stage to reduce

the size of the fat globules and 3500 kPa for the second

stage to disrupt and prevent the reformation of fat glob-

ules into large clumps. UHT milk may be homogenized

before or after heating, although it is said that

homogenization after UHT treatment gives a more

stable product. (See Homogenization.)

Packaging

0031 After heat treatment, liquid milk products are packed

with minimal time delay. This helps reduce the

likelihood of postpasteurization contamination that

may affect the shelf-life.

0032Historically, returnable glass bottles were the con-

tainers commonly used, but technological advances

have seen the introduction and regular use of card-

board cartons, plastic bottles and sachets, and lamin-

ates of foil, plastic, and cardboard. Many of these

containers are single-trip and thus do not require spe-

cialized cleaning and sterilizing equipment in the dairy.

0033Selection of the type of package and packaging

method requires consideration of many factors. The

principal objective of packaging a product is to main-

tain the nutritional properties and quality of the

product during storage, transport, and distribution

to the consumer. Other factors to be considered

when selecting packaging material include the safety

of the packaging materials, compatibility of the prod-

uct and package, hygiene, accuracy and mechanical

efficiency of the packaging process, hygienic design

of packaging equipment, marketing and promotion

functions, consumer acceptance, energy consump-

tion, environmental impact, and economics.

0034The sterilization of the packaging material and

filling equipment for pasteurized products extends

the shelf-life of the pasteurized milk product com-

pared with the shelf-life of pasteurized milk packed

under normal conditions.

0035When packaging UHT products, the equipment

and packaging material must be sterilized to ensure

that no microorganisms enter the product. This re-

quires stringent cleaning and sanitation procedures

and quality control checks. (See Packaging: Pack-

aging of Liquids; Plant Design: Designing for Hy-

gienic Operation.)

Storage and Distribution

0036This step is very important in the chain of events from

raw materials to consumers, but its importance is

often overlooked. In many countries, regulations

may prescribe conditions for the temperature of the

product during storage and distribution and for

hygienic conditions in the storerooms and transport

vehicles.

0037A major factor influencing the shelf-life of pasteur-

ized milk is the temperature of the product after pas-

teurization. Many milk containers act as insulators;

therefore, the temperature of the milk as it leaves the

pasteurizer is critical to ensure that the pasteurized milk

reaches the maximum shelf-life. The effective tempera-

ture of cool storage rooms is dependent upon factors

such as the temperature of the external environment,

the temperature of incoming product, the refrigeration

capacity, and the isolation or separation of the cool

room from the external environment. It is generally

agreed that the temperature of milk should remain

below 5



C to ensure the maximum shelf-life. This

MILK/Processing of Liquid Milk 3955

requires an efficient cold chain during the transport of

milk from the pasteurizer to the customer.

0038 In many countries, storage of milk at temperatures

below 0



C experienced during winter may result in

freezing and subsequent damage to the milk during

thawing. Steps must be taken to minimize the possible

uncontrolled freezing and thawing of the product.

0039 Care must be taken during storage and distribution

to prevent possible contamination of milk with off-

flavors caused by close contact with chemicals or

other foods. Mechanical damage to the container

may permit contamination by microbes and other

material such as dust.

Factors that Influence Shelf-life

0040 The shelf-life of milk products is defined as the period

between manufacture or processing and when the

consumer considers the product unsuitable for use.

The product may be considered to be unsuitable for

consumption, because of the presence of flavor

defects and/or changes in physical appearance. (See

Storage Stability: Shelf-life Testing.)

0041 Factors determining the shelf-life of pasteurized

milk include:

0042 The presence of thermoduric bacteria in raw milk

that are able to grow under refrigeration, e.g.,

Bacillus cereus and B. circulans. Processors using

the extended shelf-life packaging technique often

encounter problems with shelf-life owing to the

presence of low numbers of Bacillus spp. in the

raw milk. (See Bacillus: Occurrence.)

0043 The presence of psychrotrophic bacteria in the

plant after the heat treatment step. These organ-

isms grow under refrigeration and eventually cause

spoilage as a result of their biochemical activity.

0044 The temperature of the product after heat treat-

ment. Ideally, milk should be stored between 1 and



C to minimize the growth of psychrotrophic

bacteria present in the milk from postpasteuriza-

tion contamination. However, in many production

situations, 1–2



C is not practical, and the recom-

mended temperature is 4–5



C, with some coun-

tries permitting higher storage temperatures.

Elevated temperatures allow bacteria to grow

more quickly, thus reducing the time required for

spoilage. (See Spoilage: Bacterial Spoilage.)

0045 The presence of heat-stable enzymes produced

by some species of psychrotrophic bacteria.

Pasteurization and UHT treatments easily kill the

bacteria, but the enzymes survive and are able to

break down protein or fat. The problem is more

pronounced in UHT products. Generally, a large

number of bacteria must be present to produce the

enzyme, and so low numbers of bacteria in raw

milk will minimize this risk. (See Spoilage: Chem-

ical and Enzymatic Spoilage.)

0046The action of lipase on damaged fat globules in

raw milk results in rancid flavors in the finished

products. Fat globules may be damaged during

pumping and agitation. Maintaining the tempera-

ture of raw milk below 4



C minimizes the lipo-

lytic activity. The lipase is inactivated during

pasteurization.

0047The general microbial growth in raw milk may

result in damage to the milk solids as a result of

metabolic activity. Subsequent handling of the

milk may result in precipitation of the milk pro-

teins or off-flavors. Numbers in excess of 5 million

per milliliter are generally required for this to be a

major concern.

0048Excessive temperatures (> 75



C) during pasteuriza-

tion are likely to induce the germination of micro-

bial spores present in the raw milk; these microbes

are then able to grow under low temperatures.

0049The action of heat-stable plasmin on proteins in

milk. This causes flavor defects and age gelation in

UHT products. Plasmin concentration increases

with time during the lactation period, the degree

of mastitic infection and the age of the cow.

Many consumer complaints relate to the presence of

off-flavors in the milk. Frequently, these off-flavors

may be caused by exposure of milk to sunlight or

artificial light. One form of off-flavor, called acti-

vated flavor, occurs quickly on exposure to sunlight.

The intensity of the off-flavor decreases over time.

The light-induced oxidized flavor is very strong and

objectionable, and does not decrease. Many dairies

now use opaque packaging material to reduce the

frequency of complaints from this problem.

Quality Assurance and Quality Control

0050Effective programs for the manufacture of safe, high-

quality milk products are based on preventative

systems for the control of microbial and other con-

tamination. Many dairies now implement Hazard

Analysis Critical Control Point-based programs to

identify and manage points in the process where

hazards can occur, including where pathogenic organ-

isms can survive and proliferate. Key areas of hygienic

control for the safe production of milk include raw

material quality, adequate heat treatment, avoidance

of postpasteurization contamination, and limiting the

growth of microorganisms during storage prior to

consumption. These practices will not only ensure

the microbial safety of milk products but also assist

in the prevention of milk spoilage.

3956 MILK/Processing of Liquid Milk

0051 Programs to ensure that the consumer receives milk

with acceptable properties, including safety and shelf-

life, include the following key elements:

0052 Inspection of incoming raw materials for organo-

leptic and compositional properties, plus the

presence of contaminants such as added water,

extraneous matter, residues, and bacteria.

0053 Monitoring of milk temperature during storage

and heat treatment using chart recorders.

0054 Monitoring of effectiveness of pasteurization heat

treatment using the alkaline phosphatase test and

operation of the diversion flow valve.

0055 Monitoring of effectiveness of the homogenization

process.

0056 Testing of product to ensure compliance to speci-

fications.

0057 Monitoring the effectiveness of cleaning and sani-

tation programs. This may involve the application

of tests to ensure that detergents and sanitizers are

used at the correct strength, temperature, and time

period, visual checks or swabs for the presence of

milk residues, and tests for bacterial indicators

such as Gram-negative psychrotrophs to deter-

mine the presence of postpasteurization contamin-

ation. (See Sanitization.)

0058 Monitoring the volume of packed containers.

0059 Monitoring the effectiveness of formation and

sealing of containers, including the presentation

of the pack.

0060 Monitoring the control of temperature and house-

keeping practices during the storage and distribu-

tion of the finished product.

A complete quality-management system will depend

upon the processing requirements of individual dairies.

The selection of process control tests should recognize

the need for quick results to enable an operative to take

corrective action, rather than for analytical precision.

(See Hazard Analysis Critical Control Point; Quality

Assurance and Quality Control.)

See also: Hazard Analysis Critical Control Point; Heat

Transfer Methods; Heat Treatment: Ultra-high

Temperature (UHT) Treatments; Homogenization; Milk:

Liquid Milk for the Consumer; Packaging: Packaging of

Liquids; Pasteurization: Pasteurization of Liquid

Products; Plant Design: Designing for Hygienic

Operation; Quality Assurance and Quality Control;

Recombined and Filled Milks; Sanitization;

Sterilization of Foods; Storage Stability: Shelf-life

Testing

Further Reading

Burton H (1988) Ultra-high-temperature Processing of

Milk and Milk Products. London: Elsevier.

Champagne CP, Laing RR, Roy D, Mafu AA and Griffiths

MW (1994) Psychrotrophs in dairy products: their

effects and their control. Critical Reviews in Food

Science and Nutrition 34: 1–30.

Fox PF (ed.) (1995) Heat-induced Changes in Milk, 2nd

edn. Brussels: International Dairy Federation.

International Dairy Federation (1986) Monograph on

Pasteurized Milk, Document 200. Brussels: Inter-

national Dairy Federation.

International Dairy Federation (1987) Factors Affecting the

Keeping Quality of Heat Treated Milk, Document 130.

Brussels: International Dairy Federation.

International Dairy Federation (1988) Recombination of

Milk and Milk Products. Brussels: International Dairy

Federation.

International Dairy Federation (1994) Recommendations

for the Hygienic Manufacture of Milk and Milk-based

Products, Document 292. Brussels: International Dairy

Federation.

International Dairy Federation (1995) Technical Guide for

the Packaging of Milk and Milk Products, Document

300. Brussels: International Dairy Federation.

International Dairy Federation (1996) Heat Treatment

and Alternative Methods. Brussels: International Dairy

Federation.

Lewis MJ (1994) Heat Treatment of Milk. In: Robinson RK

(ed.) Modern Dairy Technology, vol. 1 Advances in Milk

Processing, 2nd edn., pp. 1–60. London: Chapman &

Hall.

Marth EH and Steele JL (eds.) (1998) Applied Dairy Micro-

biology, pp. 405–460. New York: Marcel Dekker.

Society of Dairy Technology (1983) Pasteurizing Plant

Manual. Huntingdon, UK: Society of Dairy Technology.

von Bockelmann B (undated) Long Life Products from Raw

Materials to Finished Products. Lund: Tetra Pak Inter-

national.

Walstra P, Geurts TJ, Noomen A, Jellema A and van Boekel

MASS (1999) Dairy Technology, Principles of Milk

Properties and Processes. New York: Marcel Dekker.

Physical and Chemical

Properties

D Otter, New Zealand Dairy Research Institute,

Palmerston North, New Zealand

Introduction

0001Milk comes from a wide variety of animal sources,

e.g., human, cow, buffalo, goat, and sheep, and is an

important source of nutritional and physiological

components. The gross composition of milk from all

these sources has some similarities and this article

will discuss only bovine milk. It has a very complex

MILK/Physical and Chemical Properties 3957

composition, comprised principally of water (88%),

lactose (4.8%), fat (3.9%), protein (3.4%), and min-

erals (0.8%). These values however, can vary consid-

erably depending on cow breed, and time of lactation,

and between individual cows (Table 1).

Composition of Bovine Milk

Milk Fat

0002 The fat (or lipid) in milk appears as an emulsion of

globules dispersed in the milk serum that is held

together by a very thin membrane derived from the

plasma membrane of the mammary gland. The main

functions of the fat are as an energy source for the

neonate and a source of essential fatty acids (especially

linoleic acid, C

18:2

) and fat-soluble vitamins (A, D, E,

K). Triglycerides typically make up approximately

98% of the total milk fat; the remainder consists

of di- and monoglycerides, fatty acids, sterols, caro-

tenoids, vitamins, and phospholipids. The membrane,

in contrast, contains phospholipids, lipoproteins,

cerebrosides, proteins, nucleic acids, enzymes, and

trace elements (metals).

0003More than 400 distinct fatty acids have been

detected in milk, with C

14:0

(myristic acid), C

16:0

(palmitic acid), C

18:0

(stearic acid), and C

18:1

(oleic

acid) being the most abundant. The first three satur-

ated fatty acids are solid at room temperature while

the unsaturated oleic acid is liquid. The ratios of these

fatty acids can therefore affect the hardness of the fat

and any fat-based products. The fatty acids either

come from the plasma lipids or are synthesized in

the mammary gland. They therefore display marked

patterns based on seasonal and dietary variations.

0004Esterification of fatty acids is not random, with the

sn-1 and sn-2 positions on the glycerol comprising

mainly myristic, palmitic, stearic, or oleic acid and

the sn-3 principally butanoic (C

4:0

), hexanoic (C

6:0

)

or oleic. The relative concentrations of C

and C

fatty acids are important to keep the fat liquid at body

temperature.

Proteins

0005Milk contains hundreds of different proteins, al-

though most occur only in very small amounts. They

are generally classified according to their chemical

properties into caseins which is defined as those pro-

teins, which precipitate from solution when the pH of

milk is adjusted to 4.6, and whey proteins, which

remain soluble under these conditions. In addition,

milk contains the milk fat globule membrane pro-

teins, which are found on the surface of the fat

globules and are only released by mechanical action

such as churning cream into butter; and a group of

minor proteins/enzymes or protein-like material, i.e.,

the proteose-peptone and the nonprotein nitrogen

(NPN) fraction. This last group is generally included

in the whey proteins and includes casein fragments

produced by posttranslational proteolysis or during

casein processing.

0006Casein The caseins represent approximately 80% of

the milk protein and consist of four subgroups: a

-, b-, and k-casein. Each is heterogeneous and

includes between two and eight genetic variants that

differ from each other by one to 14 amino acids. The

frequency of genetic polymorphism is breed-specific

and can affect the processing properties of the milk

proteins, e.g., cheese-making properties and the con-

centration of protein in milk. All the caseins are phos-

phorylated to varying degrees (between one and 13

phosphoserines) and k-casein has up to nine glycosy-

lated forms. a

- and k-casein each contains two

cysteines per molecule which form intermolecular

disulfide bonds such that a

-casein exists as a dimer

and k-casein can exist as dimers through to decamers.

tbl0001 Table 1 Chemical composition of bovine milk

Component Concentration (gl

1

)

Lactose 36–55

Fat

Triacylglycerols 36–38

Diacylglycerols 0.1–0.23

Monoacylglycerols 0.006–0.015

Sterols 0.09–0.16

Sterol esters Trace

Unesterified fatty acids 0.04–0.17

Hydrocarbons Trace

Phospholipids 0.08–0.39

Protein 30–35

Caseins 24–28

-Casein 12–15

-Casein 3–4

b-Casein 9–11

k-Casein 2–4

Whey 5–7

b-Lactoglobulin 2–4

a-Lactalbumin 0.6–1.7

Bovine serum albumin 0.2–0.4

Immunoglobulins 0.5–1.8

Casein fragments

g-Casein 1–2

Proteose-peptones 0.6–1.8

Milk fat globule membrane 0.4

Salt 0.7–0.8

Calcium 1.1–1.3

Chloride 0.9–1.1

Iron 0.3–0.6

Magnesium 0.09–0.14

Phosphorus 0.9–1.0

Sodium 0.35–0.9

Potassium 1.1–1.7

3958 MILK/Physical and Chemical Properties

0007 The a

-, a

-, b-, and k-caseins contain 199, 207,

209, and 169 amino acids (mol. wt *23.6 kDa,

25.4 kDa, 24 kDa, and 19 kDa, respectively), the

composition of which is unusual in the following

ways:

0008 They contain high amounts of the apolar amino

acids valine, leucine, isoleucine, phenylalanine,

tyrosine, and proline (35–45%). The hydrophobic

nature of these apolar amino acids is counteracted

by the high content of phosphate and low content

of sulfur-containing amino acids (methionine and

cysteine) such that the caseins are reasonably sol-

uble in water.

0009 They all contain a high proline level that contrib-

utes to the low amounts of a-helix and b-sheet in

their secondary structure and this makes them

more vulnerable to proteolysis.

0010 They have a very low sulfur amino acid content

that lowers their nutritional and biological value

to approximately 80% of the value for egg albu-

men.

0011 They are rich in the essential amino acid lysine,

and can therefore be used to supplement lysine-

deficient food sources such as the cereal proteins

and other plants. The high cationic lysine levels

also make the caseins susceptible to nonenzymatic

Maillard browning when exposed to heat and re-

ducing sugars.

The high levels of polar and apolar amino acids

in caseins manifest themselves as clusters in the pri-

mary structure, resulting in both hydrophobic and

hydrophilic regions. This detergent-like arrangement,

together with a high surface hydrophobicity and

amphipathicity, gives the caseins good emulsification

properties and helps with micelle stabilization.

0012 The low amounts of a-helix and b-sheet give the

caseins very little secondary or tertiary structure,

resulting in ‘an open, flexible, mobile conformation’.

This lack of structure may give them stability against

heat and other denaturing agents. It also renders them

vulnerable to proteolytic attack and gives them high

digestibility and hence nutritional value. They readily

adsorb at air–water and oil–water interfaces, giving

them good foaming and emulsifying properties that

are used in the food industry.

0013 There are also a number of minor caseins, the g-

caseins, which are produced from the C-terminal end

of b-casein by proteolysis by plasmin, an indigenous

proteinase. They occur at approximately 3% of the

casein level but can be up to 10% depending on

the stage of lactation and health of the cow. The

remaining N-terminal segments of the b-casein com-

prise a large proportion of the proteose-peptone frac-

tion. a

-Casein in milk and isolated a

-casein can

also be hydrolyzed to a lesser degree by plasmin,

although the degradation products have not been

positively identified.

0014Micelle In milk the caseins interact with each other

and with Ca

2þ

to form large colloidal complexes

known as casein micelles. The micelle contains 63%

water with a dry-matter composition of approxi-

mately 94% protein and 6% colloidal calcium phos-

phate (calcium, magnesium, phosphate, and citrate).

Electron micrographs show that the micelles are

roughly spherical, with a diameter between 50 and

500 nm, and evidence suggests that the micelles are

made up of a large number of submicelles with diam-

eters of less than 20 nm. The colloidal nature of the

micelles results in light scattering that gives milk its

whitish-blue tinge.

0015Whilst the definitive structure for the casein micelle

has yet to be elucidated, there are a number of phe-

nomena which help explain the stability of the mi-

celle. Overall, calcium phosphate and hydrophobic

interactions between the submicelles help form the

micelles. The various caseins are distributed hetero-

geneously throughout the micelles, with k-casein lo-

calized predominantly on the surface of the micelles.

The solubility of the calcium salt of k-casein thus

helps to stabilize the calcium-sensitive a

-, a

-, and

b-caseins. Micelle stability is further enhanced by the

hydrophilic C-terminal of k-casein that is situated

outside the micelle complex, giving them a ‘hairy’

look under electron microscopy, and which contains

the protein’s carbohydrate moiety. The calcium ion

concentration dictates the micelle size, with removal

of colloidal calcium phosphate resulting in disinte-

gration of the micelles.

0016The micelles are physically stable and can with-

stand all the processes to which milk is routinely sub-

jected, such as heating (up to 140



C for 15–20 min),

compaction, homogenization, and high calcium ion

concentrations (200 mmol l

1

for temperatures up to



C). They are destabilized by alcohol (40% ethanol

at pH 6.7) and freezing and aggregate and precipitate

when the pH is reduced to 4.6 (the isoelectric point of

the caseins), when the colloidal calcium phosphate

concentration is reduced or when proteinases hydro-

lyze the C-terminal end of k-casein.

0017Precipitation/Coagulation The precipitation of milk

principally involves the precipitation of the casein

proteins. It can be caused by a number of different

mechanisms and is dependent on whether the casein is

present in either the micellar or nonmicellar form.

0018The coagulation of micellar casein can be induced

by lowering the natural pH of the milk using either an

acid or by allowing the growth of an acid-producing

MILK/Physical and Chemical Properties 3959

bacteria. As the pH is lowered to 4.9, the colloidal

calcium phosphate in the casein micelle becomes sol-

uble and forms calcium ions that can leave the mi-

celle. As the pH is lowered further and reaches the

isoelectric point of the casein proteins (approximately

4.6), the micelles aggregate and enlarge, creating a

dense coagulum. This aggregation and precipitation

is temperature-dependent.

0019 Coagulation can also be caused by the hydrolysis of

k-casein by the acid proteinase, chymosin. This is the

first step in the manufacture of most cheese. Chymo-

sin is traditionally found in rennet that has been

prepared from calf stomachs, although the calf chy-

mosin gene has also been cloned into a number of

microbial vectors for use in vegetarian cheeses. Chy-

mosin specifically cleaves the peptide bond between

the amino acids 105 (phenylalanine) and 106 (me-

thionine), producing para-k-casein (k-CN f1-105)

and glycomacropeptides (f106–169). Glycomacro-

peptides, which are soluble, hydrophilic, and contain

the different carbohydrate moieties, diffuse into the

whey, creating imbalances in the intermolecular

forces in the micelles that then aggregate, forming a

coagulum. Hydrophobic interactions then occur that

lead, in the presence of calcium ions, to water expul-

sion from, and shrinkage of, the coagulum (syneresis).

0020 The initial peptide bond cleavage is known as the

primary phase of rennet action, coagulation is the

secondary phase, and subsequent rennet action on

the casein components during cheese-ripening is

called the tertiary phase. The extent of the first two

phases is governed by the pH and temperature and the

secondary phase is calcium ion-dependent.

0021 Micelle destabilization and coagulation also occur

in the presence of approximately 40% ethanol at pH

6.7 and at lower ethanol concentrations when the

pH is lowered.

0022 Whey Proteins Whilst the caseins can be considered

as a related family of proteins, the whey proteins are

more diverse and encompass any protein or protein

fragment, e.g., from casein degradation, not removed

from skim milk by precipitation at pH 4.6 (i.e., not a

casein). These soluble proteins (or milk serum pro-

teins) have a well-ordered, globular structure and are

generally not precipitated at their isoelectric points

but are more heat-labile than the caseins.

0023 b-Lactoglobulin, the major bovine whey protein,

contains 162 amino acids with an approximate mol.

wt of 18 kDa and is only found in ungulates, although

a closely related protein occurs in other mammals.

More than four genetic variants have been observed

with the A and B variants being the most prevalent.

b-Lactoglobulin is rich in sulfur amino acids, contain-

ing five cysteine residues, and has a high biological

value of 110. At physiological pH it generally exists

as a dimer of mol. wt approximately 36 kDa linked by

up to three disulfide bonds.

0024At temperatures above 60



C denaturation of b-

lactoglobulin can lead to the exposure of the reactive

sulfur amino acids that can then form disulfide

bridges with k-casein and/or a-lactalbumin. As the

temperature is increased, other sulfurous compounds

such as hydrogen sulfide are produced, leading to the

‘cooked’ flavor of heat-treated milk. b-Lactoglobulin

also has a large number of lysine amino acids that can

undergo lactosylation, which alters the physicochem-

ical properties of b-lactoglobulin.

0025The principal biological function of b-lactoglobu-

lin is presently unknown but it can act as a carrier for

vitamin A (retinol). It can also stimulate lipolysis by

binding the free fatty acids which inhibit lipases.

0026a-Lactalbumin is found in the milk of most

mammals; it is the principal protein in human milk,

and is thus considered to be the ‘typical’ whey pro-

tein. It is a compact globular protein of 123 residues

(mol. wt *14 kDa) and has three genetic variants,

although the milk of western cattle contain only a-

lactalbumin B. It is relatively rich in tryptophan and

the sulfur amino acids cysteine and methionine. It

contains four intramolecular disulfides, exists as a

monomer and has 54 of its amino acids identical to

the corresponding residues in lysozyme. Up to 15% of

a-lactalbumin has been shown to be glycosylated to

varying degrees. It is also a metalloprotein, binding

one Ca

2þ

per mole (this increases its heat-stability).

0027a-Lactalbumin has an essential physiological role

in lactose synthesis in the udder. Although it is not an

enzyme, it acts as a ‘specifier protein’ with uridine

diphosphate-galactosyl transferase to form lactose

synthetase which catalyzes the transfer of galactose

to glucose to form lactose.

0028The immunoglobulins are a very complex group of

proteins that provide various types of immunity to the

cow and its offspring. Immunoglobulin levels in col-

ostrum (milk from the first 2–3 days postpartum) can

be up to 100 g l

1

as the colostrum provides passive

immunity to the neonate prior to its own immune

system functioning fully. This level falls rapidly

over the first week postpartum to approximately

0.6–1gl

1

during normal lactation.

0029Bovine serum albumin is a large molecule (mol.

wt*kDa) which is presumed to occur in milk as a

result of leakage from the blood in the mammary

gland during milk production. Although it can bind

metals and fatty acids, it probably has little functional

significance in bovine milk.

0030Lactotransferrin (or lactoferrin) occurs in trace

amounts in bovine milk, although in human milk

it represents up to 20% of the total nitrogen. It is

3960 MILK/Physical and Chemical Properties

similar to serum transferrin, is involved in iron trans-

port, and has an antibacterial role in the neonate gut.

0031 Membrane Proteins The membrane proteins are a

diverse group of proteins that, together with lipids,

form the milk fat globule membrane. Some are lipo-

proteins and all tend to have varying degrees of hydro-

phobicity and hence align themselves in a gradient

from the fat surface to the water. This attracts other

hydrophobic molecules such as phospholipids and

lipolytic enzymes that are adsorbed within the mem-

brane structure.

0032 Biologically Active Proteins and Peptides Milk con-

tains a wide variety of biologically active substances.

These include: (1) proteins such as lactoperoxidase,

lactotransferrin, immunoglobulins, and vitamin-

binding proteins (e.g., folate-binding protein); (2)

growth factors such as insulin-like growth factors

(IGF-1 and IGF-2), transforming growth factors

(TGF

, TGF

and TGF

), mammary-derived

growth factors (MDGF I and MDGF II), fibroblast

growth factors, platelet-derived growth factors

(PDGF), bombesin and bifidus factor; and (3) pep-

tides from milk protein hydrolysates, e.g., glyco-

macropeptides from k-casein, phosphopeptides from

caseins, caseinomorphins, immunomodulating pep-

tides, platelet-modifying peptide, angiotensin-con-

verting enzyme (ACE) inhibitor, calmodulin-binding

peptides, and bactericidal peptides from lactotrans-

ferrin.

0033 Enzymes Milk contains both indigenous and en-

dogenous enzymes. As many as 60 indigenous

enzymes have been reported, with the majority de-

rived from the milk fat globule membrane. Peroxid-

ase and alkaline phosphatase are indicators of

efficient pasteurization; catalase occurs naturally

and also results from mastitic infection; lipase re-

leases free fatty acids from milk fat, resulting in ran-

cidity; plasmin degrades caseins; and lysozyme and

lactoperoxidase possess antimicrobial activity.

0034 The endogenous enzymes derive from microorgan-

isms present in the milk and can cause both undesir-

able, e.g. rancidity, bitterness, or off-flavors, and

desirable, e.g., ripe cheese flavors, properties.

Lactose

0035 Lactose is the principal carbohydrate in milk and

varies with the breed of cow, individuality factors,

udder infection, and stage of lactation. It plays an

important role in osmotic pressure maintenance in

the mammary system and has an inverse relationship

with chloride in milk. Lactose is a disaccharide con-

sisting of galactose and glucose linked by a b1–4

glycosidic bond, with the glucose moiety being poten-

tially free (i.e., a reducing sugar) and existing as either

an a-orb-anomer.

Vitamins

0036Milk is a good source of the daily vitamin require-

ment of an adult person. It contains the fat-soluble

vitamins A, D, E, and K, and the water soluble

B-group vitamins B

, niacin, biotin, panthothenic

acid, B

, folate and B

, and ascorbic acid (vitamin

C). The amount of each vitamin varies with stage of

lactation and the animal’s diet and health.

Minerals and Salts

0037The soluble and colloidal phases of milk contain

varying amounts of different salts with the compos-

ition influenced by breed, individual cow, stage of

lactation, feed, mastitic infection, and season. The

most important cations are calcium, sodium, potas-

sium, and magnesium that occur as phosphates,

chlorides, citrates, and caseinates. Individual salt con-

centrations are dependent on the requirements to:

(1) maintain electric neutrality; (2) maintain milk

isotonic with blood, through lactose, sodium, potas-

sium, and chloride ion concentrations; and (3) form

casein micelles which constrain pH and calcium salt

levels and require calcium phosphate to complex with

casein. Variation in total calcium is due to changes in

citrate and casein during lactation.

0038Approximately 20 trace elements are found in

milk, including copper, iron, silicon, zinc, and iodine.

The mineral (ash) content of milk is approximately

0.7–0.8%. Some trace minerals are essential for

health, e.g., iron, zinc copper, and manganese, whilst

others can be toxic, e.g., aluminum, arsenic, cad-

mium, and lead. Deficiencies or excesses in these

minerals can result in a variety of physiological

symptoms.

Other Constituents

0039Somatic cells (white blood corpuscles or leukocytes)

are always present at low levels (200 000 cells ml

1

)

in milk but increase, and are therefore a useful

marker, in the diseased udders of unhealthy cows.

0040The gases carbon dioxide, nitrogen, and oxygen

make up 5–6% by volume in fresh milk and exist

dissolved in the milk, bound and nonseparable from

the milk, or dispersed in the milk. They can represent

a problem during processing, causing burning to

heated surfaces.

Effects of Storage

0041Three chemical processes – oxidation, lipolysis, and

hydrolysis – can affect the fat and protein in milk and

MILK/Physical and Chemical Properties 3961

milk products during storage, generating either off-

flavors (as in milk or butter) or ‘mature’ flavors (as in

cheese).

0042 Oxidation of fat, principally the unsaturated fatty

acids, is a major cause of deterioration in milk and

milk products, leading to a metallic flavor in milk or

an oily tallow taste in butter. This oxidative rancidity

can be accelerated by iron or copper salts, high dis-

solved oxygen levels, or exposure to light. The pres-

ence of reducing agents, e.g., from microorganisms,

pasteurization, or added antioxidants, can help offset

oxidation. With advanced oxidation the fatty acids

can ultimately be degraded into aldehydes and

ketones, resulting in oxidation rancidity in fat-

containing dairy products.

0043 Oxidation of protein by exposure to light results in

the conversion of the amino acid methionine to

methional. This can then react with riboflavin (vita-

min B

) and ascorbic acid (vitamin C) to form

3-mercapto-methylpropionaldehyde, the main com-

ponent of ‘sunlight flavor’. The overall dynamics of

the reactions are factors such as the light intensity and

duration, the condition of the milk, and the pack-

aging of the milk.

0044 Lipolysis, or degradation of fat into glycerol and

free fatty acids, causes a rancid smell and taste due to

the release of butyric and caproic acids. The degrad-

ation is due to the action of lipase, an indigenous

enzyme normally associated with the casein micelle.

This enzyme attacks the triglyceride in fat globules

when the milk fat globule membrane is damaged.

High storage temperatures and routine dairy process-

ing (e.g., homogenization, pumping, and stirring) all

contribute to the breakdown of the milk fat globule

membrane and subsequent lipolysis. The lipases can

be inactivated by high-temperature pasteurization.

0045 Hydrolysis of milk proteins during storage can be

caused by either indigenous proteinases, such as plas-

min or cathepsin D, or enzymes derived from intro-

duced sources, such as rennet or bacteria. Plasmin is

fairly heat-stable, appears to be associated with the

casein micelle during rennet coagulation and cheese

manufacture, and preferentially attacks the b-casein

to yield the g-caseins and proteose-peptones. It con-

tributes to the primary proteolysis of caseins in high-

cook cheese and may affect the age gelation of ultra

heat-treated milk. Introduced proteinases, e.g., from

starter bacteria in cheese, are normally used in con-

trolled conditions to produce a wide range of desired

flavors or other properties in various milk products.

Effects of Heat

0046 Milk and milk products are routinely subjected to

various heat treatments during processing to elimin-

ate any potentially pathogenic microorganisms (e.g.,

pasteurization). Specific heating regimes of time and

temperature are required to destroy these micro-

organisms – regimes that can also result in detrimen-

tal or advantageous changes to other constituents in

the milk.

0047Pasteurization (heating at 70–80



C for 15 s) results

in the ‘cream plug phenomenon’, where some fat

gobules are ruptured, allowing the free fat to bind

other intact fat globules to form the plug. This can

be alleviated by homogenization. At high tempera-

tures (above 135



C), the proteins in the milk fat

globule membrane partially aggregate, creating a

denser, less permeable membrane.

0048Whilst the caseins are considered heat-stable

within the normal pH, salt, and protein concentration

ranges, the whey proteins, being globular proteins,

are much more susceptible to heat denaturation. De-

naturation starts between 60 and 65



C and is char-

acterized by an uncoiling of the proteins, in particular

b-lactoglobulin, followed by the interchange of disul-

fide bonds to form binding of the b-lactoglobulin to

the k-casein of the casein micelle. Total denaturation

is achieved with heating between 90 and 95



C for

5 min. Alteration of the k-casein in this way inhibits

the renneting ability of milk, resulting in a softer

coagulum. Thus, milk for cheese manufacture should

not be subjected to heating regimes harsher than



C for 15–20 s. In cultured milk products, such as

yogurt, however, the whey protein–casein inter-

actions result in reduced syneresis and improved vis-

cosity, which improves the final product.

0049Of the other whey proteins, a-lactalbumin is more

heat-stable than b-lactoglobulin, undergoing revers-

ible denaturation up to approximately 95



C. The

immunoglobulins are denatured at temperatures

above approximately 70



C, resulting in the loss of

their immune properties. Most of the indigenous

enzymes in milk are also denatured and lose their

activity on high heat treatments.

0050Above 100



C, lactose reacts with the milk proteins

in the Maillard reaction to cause nonenzymatic

browning. A chain of reactions between the amino

groups (principally the e-NH

group of lysine) of the

proteins and the carbonyl group of lactose produces

brown-colored melanoidin polymers, off-flavors,

a loss of nutritional value (through loss of lysine),

and loss of solubility in milk powders.

Physical Properties

Appearance

0051The white-yellow color of milk is dependent on the

amount of the hydrocarbon carotene. Its opaque ap-

pearance is caused by suspended particles of fat and

3962 MILK/Physical and Chemical Properties

casein micelles. The greenish color of milk serum and

whey is due to the presence of riboflavin.

Density

0052 The density of milk is dependent on its composition

and temperature and is generally between 1.028 and

1.038 g cm

3

. At 15.5



C, the following formula can

be used:

Density ð15:5



CÞ¼

100

0:93

SNF

1:608

þ water

where F ¼% fat, SNF ¼% solids nonfat, and water

% ¼100 [F þSNF].

Freezing Point

0053 The freezing point of milk can vary from 0.53 to

0.59



C and is used to check for adulteration of milk

with water. After high-temperature processing, e.g.,

ultra heat treatment or sterilization, the freezing point

will rise due to precipitation of some phosphates.

Acidity

0054 Milk is usually slightly acidic, with a pH value be-

tween 6.5 and 6.7. A decrease in the pH is often an

indication of bacterial activity and spoilage.

Viscosity

0055 Under moderate shear rates (> 10 s

1

), fat contents

below 40% and at temperatures above 40



C (i.e.,

the fat is a liquid), milk displays Newtonian rheo-

logical properties. The coefficient of viscosity at



C is approximately 2.127 mPa s

1

. Processing,

such as homogenization, heating, and cold agglutin-

ation, all alter the viscosity to varying degrees.

See also: Butter: The Product and its Manufacture;

Cheeses: Types of Cheese; Cream: Types of Cream;

Dairy Products – Nutritional Contribution; Food

Intolerance: Milk Allergy; Infant Foods: Milk Formulas;

Milk: Liquid Milk for the Consumer; Processing of Liquid

Milk; Analysis; Dietary Importance; Powdered Milk: Milk

Powders in the Marketplace; Characteristics of Milk

Powders; Recombined and Filled Milks; Whey and

Whey Powders: Production and Uses; Yogurt: The

Product and its Manufacture