Food Biochemistry and Food Processing (2 edition)

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24 Chemistry and Biochemistry of Milk Constituents 443

the minor constituents of milk are even greater than those of

the macro constituents. The composition of milk also changes

markedly during lactation, reflecting the changing nutritional re-

quirements of the neonate and during mastitis and physiological

stress. In this chapter, the typical characteristics of the princi-

pal, and some of the minor, constituents of bovine milk will

be described. The milk of the principal dairying species, cow,

buffalo, sheep and goat are generally similar but differ in detail.

The milks of several non-bovine species are described by Park

and Haenlein (2006) and Fuquay et al. (2011); the latter includes

articles on monotremes and marsupials, marine mammals and

primates.

Sheep and goats were domesticated around 8000bcand their

milk has been used by humans since. However, cattle, espe-

cially breeds ofBos taurus, are now the dominant dairy animals.

Total recorded world milk production is approximately 600×

106 tonnes/annum, of which approximately 85% is bovine,

11% is buffalo and 2% each is from sheep and goats. Camels,

mares, reindeer and yaks are important dairy animals in lim-

ited geographical regions with specific cultural and/or climatic

conditions.

Milk is a very flexible raw material; several thousand dairy

products are produced around the world in a great diversity

of flavours and forms, including about 1400 varieties/variants

of cheese. The principal dairy products and the percentage of

milk used in their production are: liquid (beverage) milk, ap-

proximately 40%; cheese, approximately 35%; butter, approxi-

mately 32%; whole milk powder, approximately 6%; skimmed

milk powder, approximately 9%; concentrated milk products ap-

proximately 2%; fermented milk products, approximately 2%;

casein, approximately 2% and infant formulae, approximately

0.3%. The flexibility of milk as a raw material is a result of the

properties, many of them unique, of its principal constituents;

many of these are very easily isolated, permitting the production

of valuable food ingredients. Milk is free of off-flavours, pig-

ments and toxins, which greatly facilitates its use as a food or as

a raw material for food production.

The processability and functionality of milk and milk products

are determined by the chemical and physicochemical properties

of its principal constituents, that is lactose, lipids, proteins, and

salts, which will be described in this chapter. The exploitation

and significance of the chemical and physico-chemical prop-

erties of milk constituents in the production and properties of

the principal groups of dairy foods, that is liquid milk products,

cheese, butter, fermented milks, functional milk proteins and

lactose will be described in Chapter 25. Many of the principal

problems encountered during the processing of milk are caused

by variability in the concentrations and properties of the principal

constituents arising from several factors, including breed, indi-

viduality of the animal (i.e., genetic factors), stage of lactation,

health of the animal, especially mastitis, and nutritional status.

Synchronised calving, as practised in New Zealand, Australia

and Ireland, to avail of cheap grass as the principal component

of the cow’s diet, has a very marked effect on the composition

and properties of milk (see O’Brien et al. 1999a, b, c, Mehra

et al. 1999). However, much of the variability can be offset by

standardising the composition of milk using various methods

(e.g., centrifugation, ultrafiltration or supplementation) or by

modifying the process technology.

The chemical and physico-chemical properties of the princi-

pal constituents of milk are well characterised and described.

The very extensive literature includes the following textbooks:

Walstra and Jenness (1984), Wong et al. (1988), Fox (1992,

1995, 1997, 2003a), Fox and McSweeney (1998, 2003, 2006),

Walstra et al. (1999, 2005) and McSweeney and Fox (2009).

SACCHARIDES

Lactose

Introduction

Lactose is a reducing disaccharide comprising glucose and

galactose, linked by aβ1–4-O-glycosidic bond (Fig. 24.1). It

is unique to milk and is synthesised in the mammary gland

from glucose transported from the blood; one molecule of glu-

cose is epimerised to galactose, as UDP-galactose (Gal),viathe

Leloir pathway and is condensed with a second molecule of

glucose by a two-component enzyme, lactose synthetase. Com-

ponent A is a general UDP-galactosyl transferase (UDP-GT; EC

2.4.1.2.2), which transfers galactose from UDP-Gal to a range

of sugars, peptides or lipids. Component B is the whey pro-

tein,α-lactalbumin (α-La), in the presence of which, theKM

of UDP-GT for glucose is reduced 1000-fold and lactose is the

principal product synthesised. There is a good correlation be-

tween the concentrations of lactose andα-La in milk. Lactose

is responsible for approximately 50% of the osmotic pressure of

milk, which is equal to that of blood and varies little; therefore,

the concentration of lactose in milk is tightly controlled and is

independent of breed, individuality and nutritional factors, but

decreases as lactation advances and especially during mastitis,

in both cases due to the influx of NaCl from the blood. The

physiological function ofα-La is probably to control the syn-

thesis of lactose, and thus maintain the osmotic pressure of milk

relatively constant.

The concentration of lactose in milk ranges from approxi-

mately 0, for some species of seal, to approximately 10% in

the milk of some monkeys. The concentration of lactose in the

milk of the principal dairy species is quite similar (cow, 4.8%;

buffalo, 4.3%; sheep, 4.6%; goat, 4.9%; camel, 5.1%), excep-

tions are the horse (6.1%), donkey (6.9%) and reindeer (2.5%);

human milk contains about 7.0% lactose. The lactose content of

bulk herd milk from randomly calved cows varies little through-

out the year but differences can be quite large when calving of

cows is synchronised, for example in Ireland, the level of lac-

tose in creamery milk varies from approximately 4.8% in May

to approximately 4.2% in October.

Chemical and Physico-Chemical Properties of Lactose

Among sugars, lactose has a number of distinctive character-

istics, some of which cause problems in milk products during

processing and storage; however, some of its characteristics are

exploited to advantage.