426 Part IV: Milk
taurus, are now the dominant dairy animals. Total
recorded world milk production is approximately
600 106 metric tons per year(mt/yr), of which
about 85% is bovine, 11% is buffalo, and 2% each is
from sheep and goats. Camels, mares, reindeer, and
yaks are important dairy animals in limited geo-
graphical regions with specific cultural and/or cli-
matic conditions. This article will be restricted to the
constituents and properties of bovine milk. The con-
stituents of the milk of the other main dairy species
are generally similar to those of bovine milk, al-
though minor differences exist, and the technologi-
cal properties of the milk of these species differ sig-
nificantly.
Milk is a very flexible raw material; several thou-
sand dairy products are produced around the world
in a great diversity of flavors and forms, including
about 1400 varieties/variants of cheese. The princi-
pal dairy products and the percentage of milk used
in their production are liquid (beverage) milk, about
40%; cheese, about 33%; butter, about 32%; whole
milk powder, about 6%; skimmed milk powder,
about 9%; concentrated milk products about 2%;
fermented milk products, about 2%; casein, about
2%; infant formulas, about 0.3%. The flexibility of
milk as a raw material is a result of the properties,
many of them unique, of the constituents of milk,
of which the principal ones are very easily isolated,
permitting the production of valuable food ingredi-
ents. Milk is free of off-flavors, pigments, and tox-
ins, which greatly facilitates its use as a food or as a
raw material for food production.
The processibility and functionality of milk and
milk products are determined by the chemical and
physicochemical properties of its principal con-
stituents, that is, lactose, lipids, proteins, and salts,
which will be described in this chapter. The ex-
ploitation and significance of the chemical and
physicochemical properties 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 20. Many of
the principal problems encountered during the pro-
cessing of milk are caused by variability in the con-
centrations and properties of the principal con-
stituents, arising from several factors, including
breed, individuality of the animal (i.e., genetic fac-
tors), stage of lactation, health of the animal (espe-
cially mastitis), and nutritional status. Synchronized
calving, as practiced in New Zealand, Australia, and
Ireland to take advantage 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. 1999 a,b,c; Mehra et al.
1999). However, much of the variability can be off-
set by standardizing the composition of milk using
various methods (e.g., centrifugation, ultrafiltration,
or supplementation) or by modifying the process
technology.
The chemical and physicochemical properties of
the principal constituents of milk are well character-
ized and described. The very extensive literature
includes textbooks by Walstra and Jenness (1984),
Wong et al. (1988), Fox (1992, 1995, 1997, 2003),
Fox and McSweeney (1998, 2003), and Walstra et
al. (1999).LACTOSE
INTRODUCTIONLactose is a reducing disaccharide comprised of glu-
cose and galactose, linked by a -1-4-O-glycosidic
bond (Fig. 19.1). It is unique to milk and is synthe-
sized in the mammary gland from glucose transported
from the blood; one molecule of glucose is epimer-
ized to galactose, as UDP-galactose, via the Leloir
pathway and is condensed with a second molecule
of glucose by a two-component enzyme, lactose
synthetase. Component A is a general UDP-galacto-
syl transferase (UDP-GT; EC 2.4.1.2.2) that trans-
fers galactose from UDP-galactose to any of a range
of sugars, peptides, or lipids. Component B is the
whey protein, -lactalbumin (-la) , in the presence
of which the KMof UDP-GT for glucose is reduced
1000-fold and lactose is the principal product syn-
thesized. There is a good correlation between the
concentrations of lactose and -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 de-
creases as lactation advances and especially during a
mastitic infection, in both cases due to the influx of
NaCl from the blood. The physiological function of
-la is probably to control the synthesis of lactose
and thus maintain the osmotic pressure of milk at a
relatively constant level.