2009_Book_FoodChemistry

10.1 Milk 509
Table 10.14. Composition and size of casein micelles
isolated by centrifugation
Centrifugation
time (min) a
Composition of the
sediment(%)
α s1 β κ Others
0 b 50 32 15 3
0−7.5 47 34 16 3
7.5−15 46 32 18 4
15–30 45 31 20 4
39–60 42 29 26 3
Fig. 10.5. Electron micrograph of the casein micelles in
skim milk (according to Webb, 1974). The micelles are
fixed with glutaraldehyde and then stained with phosphomolybdic
acid
Table 10.12. Composition of casein micelles (%)
Casein 93.2 Phosphate
Ca 2.9 (organic) 2.3
Mg 0.1 Phosphate
Na 0.1 (inorganic) 2.9
K 0.3 Citrate 0.4
Table 10.13. Typical distribution of components in
casein micelles
Component Ratio numbers
α s1 3 6 9 12
β 1 1 4 4
γ 1 1 1
κ 1 3 3 3
(1.9g water/g protein) and hence are porous.
The monomers are kept together with:
• Hydrophobic interactions that are minimal at
a temperature less than 5 ◦ C.
• Electrostatic interactions, mostly as calcium or
calcium phosphate bridges between phosphoserine
and glutamic acid residues (Fig. 10.6).
• Hydrogen bonds.
On a molecular level different micelle models
have been proposed which to a certain extent
explain the experimental findings. The most
probable model is shown in Fig. 10.7. This model
comprises subunits (submicelles, M r ∼ 760,000)
Serum casein 39 23 33 5
a Centrifugation speed 10 5 × g.
b Isoelectric casein.
which consist of ca. 30 different casein monomers
and aggregate to large micelles via calcium phosphate
bridges. Two types of subunits apparently
exist: one type contains κ-casein and the other
does not. The κ-casein molecules are arranged
on the surface of the corresponding submicelles.
At various positions, their hydrophilic C-termini
protrude like hairs from the surface, preventing
aggregation. Indeed, aggregation of the submicelles
proceeds until the entire surface of
the forming micelle is covered with κ-casein,
i. e., covered with “hair”, and, therefore, exhibits
steric repulsion. The effective density of the hair
layer is at least 5 nm. A small part of the κ-casein
is also found inside the micelle.
10.1.2.1.3 Gel Formation
The micelle system, can be destabilized by
the action of rennin or souring. Rennin attacks
κ-casein, eliminating not only the C-terminus in
the form of the soluble glycopeotide 106–169,
but also the cause of repulsion. The remaining
paracasein micelles first form small aggregates
with an irregular and often long form, which
then assemble with gel formation to give a three
dimensional network with a pore diameter of
a few µm. The fat globules present are included
in this network with pore enlargement. It is
assumed that dynamic equilibria exist between
casein monomers and submicelles, dissolved and
bound calcium phosphate, and submicelles and
micelles.