19 Chemistry and Biochemistry of Milk Constituents 439lecular level; their chemical composition is summa-
rized in Table 19.2. The most notable features of the
principal milk-specific proteins are discussed below.
The principal milk-specific proteins are quite
small molecules (molecular weight of 15–25
kDa). All the caseins are phosphorylated, but to dif-
ferent and variable degrees, 1–13 mol P/mol protein;
the phosphate groups are esterified as monoesters of
serine residues.
The primary structures of the caseins have a
rather uneven distribution of polar and apolar re-
sidues along their sequences. Clustering of the phos-
phoseryl residues is particularly marked, resulting in
strong anionic patches in s1-, s2-, and -caseins.
-casein does not have a phosphoseryl cluster, but
the N-terminal two-thirds of the molecule is quite
hydrophobic, while its C-terminal is relatively hy-
drophilic—it contains no aromatic and no cationic
residues. These features give the caseins an am-
phiphatic structure, making then very surface active,
with good emulsifying and foaming properties. The
amphiphatic structure of -casein is particularly sig-
nificant and is largely responsible for its micelle-
stabilizing properties. The distribution of amino
acids in -lg and -la is quite random.
The two principal caseins, s1- and -casein, are
devoid of cysteine or cystine residues; the two minor
caseins, s2- and -casein, contain two intermolecu-
lar disulphide bonds. -lactoglobulin contains two
intramolecular disulphide bonds and one sulphydryl
group, which is buried and unreactive in the native
protein but becomes exposed and reactive when
the molecule is denatured; it reacts via sulphydryl-
disulphide interactions with other proteins, especially
-casein, with major consequences for many impor-
tant properties of the milk protein system, especially
heat stability and cheese-making properties. -la
has four intramolecular disulphide bonds.
All the caseins, especially -casein, contain
a high level of proline (in -casein, 17 of the 209
residues are proline), which disrupts - and -
structures; consequently, the caseins are rather un-
structured molecules and are readily susceptible to
proteolysis. However, theoretical calculations sug-
gest that the caseins may have a considerable level
of secondary and tertiary structure; to explain the
differences between the experimental and theoreti-
cal indices of higher structures, it has been suggest-
ed that the caseins are very mobile, flexible, rheo-
morphic molecules (i.e., the casein molecules, in
solution, are sufficiently flexible to adopt structures
that are dictated by their environment; Holt and
Sawyer 1993).
In contrast, the whey proteins are highly compact
and structured, with high levels of -helices, -
sheets, and -turns. In -lg, the -sheets are in an
antiparallel arrangement and form a -barrel calyx.
-lactoglobulin is a member of the lipocalin family,
which now includes 14 proteins (for review see
Akerstrom et al. 2000)
The caseins are often regarded as very hydropho-
bic proteins, but they are not particularly so; how-
ever, they do have a high surface hydrophobicity,
owing to their open structures. Also due to their
open structure, the caseins are quite susceptible to
proteolysis, which is as would be expected for pro-
teins the putative function of which is to be a source
of amino acids for the neonate. However, due to
their hydrophobic patches, caseins have a high pro-
pensity to yield bitter hydrolyzates, even in cheese
that undergoes relatively little proteolysis. In con-
trast, the highly structured whey proteins are very
resistant to proteolysis in the native state and may
traverse the intestinal tract of the neonate intact.
Some -casein molecules are glycosylated. The
sugar moieties present are galactose, galactosamine,
and N-acetylneuraminic acid (sialic acid), which
occur as trisaccharides or tetrasaccharides. The oli-
gosaccharides are attached to the polypeptide via
threonine residues in the C-terminal region of the
molecule and vary from 0 to 4 mol/mol protein.
Probably because of their rather open structures,
the caseins are extremely heat stable, for example,
sodium caseinate can be heated at 140°C for 1 hour
without obvious physical effects. The more highly
structured whey proteins are comparatively heat
labile, although in comparison with many other
globular proteins, they are quite heat stable; they are
completely denatured on heating at 90°C for 10
minutes.
Under the ionic conditions in milk, -la exists as
monomers with a molecular weight of 14.7 kDa. -
lactoglobulin exists as dimers (molecular weight
36 kDa) in the pH range 5.5–7.5; at pH values 3.5
or 7.5, it exists as monomers, while at pH 3.5–5.5,
it exists as octamers. The caseins have a very strong
tendency to associate. Even in sodium caseinate, the
most soluble form of whole casein, the proteins exist
mainly as decamers or larger aggregates at 20°C. At
a low concentration, for example,0.6%, at 4°C,