19 Chemistry and Biochemistry of Milk Constituents 427The concentration of lactose in milk ranges from
almost 0 (in marine mammals) to about 10% in the
milk of some monkeys; midlactation bovine milk
contains about 4.8% lactose and human milk about
7.0%.
Lactose is the principal sugar in milk, but the milk
of most, if not all, species also contains oligosaccha-
rides, up to hexasaccharides, derived from lactose (the
nonreducing end of the oligosaccharides is lactose,
and many contain fucose and N-acetylneuraminic
acid). About 130 oligosaccharides have been identi-
fied in human milk; the milk of elephants and bears
also contains high levels of oligosaccharides. The
oligosaccharides are considered to be important
sources of certain monosaccharides, especialy fucose
and N-acetlyglucosamine, for neonatal development,
especially of the brain (Urashima et al. 2001).
CHEMICAL ANDPHYSICOCHEMICAL
PROPERTIES OFLACTOSE
Among sugars, lactose has a number of distinctive
characteristics, some of which cause problems in
milk products during processing and storage; how-
ever, some of its characteristics are exploited to its
advantage.
The functional aldehyde group at the C-1 position
of the glucose moiety exists mainly in the hemiac-
etal form and, consequently, C-1 is a chiral, asym-
metric carbon. Therefore, like all reducing sugars,
lactose can exist as two anomers, and , which
have markedly different properties. From a function-
al viewpoint, the most important of these properties
are differences in solubility and crystallization char-
acteristics between the isomers; -lactose crystal-
lizes as a monohydrate, while crystals of -lactose
are anhydrous. The solubility of - and -lactose in
water at 20°C is about 7 and 50 g/100 mL, respec-
tively. However, the solubility of -lactose is much
more temperature-dependent than that of -lactose,
and the solubility curves intersect at approximately
93.5°C (see Fox and McSweeney 1998).
At equilibrium in aqueous solution, lactose exists
as a mixture of - and -anomers in the approxi-
mate ratio 37:63. When an excess of -lactose is
added to water, approximately 7 g/100 mL dissolve
immediately, some of which mutarotates to give an
:ratio of 37:63, leaving the solution unsaturated
with respect to both - and -lactose. Further -
lactose then dissolves, some of which mutarotates to
-lactose. Solubilization and mutarotation continue
until two conditions exist, that is, approximately 7
g/100 mL of dissolved -lactose and an :ratio of
37:63, giving a final solubility of about 18.2 g/100
mL. When-lactose is added to water, approximately
50 g/100 mL dissolve initially, but about 18.5 g ofFigure 19.1.Structures of lactose.