19 Chemistry and Biochemistry of Milk Constituents 441-casein exists as monomers, but associates to form
micelles if the concentration or temperature is in-
creased. In the presence of Ca, for example, calcium
caseinate, casein forms large aggregates of several
million Daltons. In milk, the caseins exist as very
complex structures, known as casein micelles, which
are described in the next section, Casein Micelles.
The function of the caseins appears to be to sup-
ply amino acids to the neonate. They have no bio-
logical function stricto sensu, but their Ca-binding
properties enable a high concentration of calcium
phosphate to be carried in milk in a “soluble” form;
the concentration of calcium phosphate far exceeds
its solubility, and without the “solubilizing” influ-
ence of casein it would precipitate in the ducts of the
mammary gland and cause atopic milk stones.
-lactoglobulin has a hydrophobic pocket within
which it can bind small hydrophobic molecules. It
binds and protects retinol in vitro and perhaps func-
tions as a retinol carrier in vivo. In the intestine, it
exchanges retinol with a retinol-binding protein. It
also binds fatty acids and thereby stimulates lipase;
this is perhaps its principal biological function. All
members of the lipocalin family have some form of
binding function (see Akerstrom et al. 2000).
-lactalbumin is a metalloprotein: it binds one Ca
atom per molecule in a peptide loop containing four
Asp residues. The apoprotein is quite heat labile, but
the metalloprotein is rather heat stable; when stud-
ied by differential scanning calorimetry, -la is, in
fact, observed to be quite heat labile. However, the
metalloprotein renatures on cooling, whereas the
apoprotein does not. The difference in heat stability
between the halo- and apoprotein may be exploited
in the isolation of -la on a large (i.e., potentially
industrial) scale.
As discussed in the introduction to the Lactose
section, -la is an enzyme modifier protein in the
lactose synthesis pathway; it makes UDP-galactosyl
transferase highly specific for glucose as an acceptor
for galactose, resulting in the synthesis of lactose.
CASEINMICELLES
s1-,s2- and-caseins, which together represent
about 85% of total casein, are precipitated by calcium
at concentrations6 mM at temperatures20°C.
Since milk contains approximlately 30 mmol/L Ca, it
would be expected that most of the caseins would
precipitate in milk. However,-casein is soluble at
high concentrations of Ca, and it reacts with and sta-
bilizes the Ca-sensitive caseins through the forma-
tion of casein micelles.
Since the stability, or perhaps instability, of the
casein micelles is responsible for many of the unique
properties and processibility of milk, their structure
and properties have been studied intensively. It has
been known for nearly 200 years that the casein in
milk exists as large colloidal particles that are re-
tained by porcelain-Chamberlain filters. During the
early part of the 20th century there was interest in
explaining the rennet coagulation of milk and hence
in the structure and properties of the Ca-caseinate
particles (for review see Fox and Kelly 2003). The
term casein micelle appears to have been first used by
Beau (1920). The idea that the rennet-induced coagu-
lation of milk is due to the destruction of a protective
colloid (Schutzcolloid) dates from the 1920s; ini-
tially, it was suggested that the whey proteins were
the “protective colloid.” The true nature of the protec-
tive colloid, the structure of the casein micelle, and
the mechanism of rennet coagulation did not become
apparent until the pioneering work on the identifica-
tion, isolation, and characterization of-casein by
Waugh and von Hippel (1956). Since then, the struc-
ture and properties of the casein micelle have been
studied intensively. The evolution of views on the
structure of the casein micelle has been described by
Fox and Kelly (2003). Current knowledge on the
composition, structure, and properties of the casein
micelle and the key features thereof are summarized
below.
The micelles are spherical colloidal particles, with
a mean diameter of nearly 120 nm (range, 50–600
nm). They have a mean particle mass of about 10^5
kDa, that is, there are about 5000 casein molecules
(20–25 kDa each) in an average micelle. On a dry
weight basis, the micelles contain approximately
94% protein and 6% nonprotein species, mainly cal-
cium and phosphate, with smaller amounts of Mg and
citrate and traces of other metals; these are collec-
tively called colloidal calcium phosphate (CCP).
Under the conditions that exist in milk, the micelles
are hydrated to the extent of about2gH 2 O/g pro-
tein. There are approximately 10^15 micelles/mL milk,
with a total surface area of approximately 5 104
cm^2 ; the micelles are about 240 nm apart. Owing
to their very large surface area, the surface proper-
ties of the micelles are of major significance, and
because they are quite closely packed, even in