510 10 Milk and Dairy Products
Fig. 10.6.Peptide chain bridg-
ing with calcium ions
Fig. 10.7. Schematic model of a casein micelle;
(a) a subunit consisting ofαs1-,β-,γ-,κ-caseins, (b)
Micelle made of subunits bound by calcium phosphate
bridges (according toWebb, 1974)
The rate of gel formation increases with in-
creasing temperature (Fig. 10.8). It is slow at
T< 25 ◦C and proceeds almost under diffusion
control at T∼ 60 ◦C. It follows that hydrophobic
interactions, especially due to the very hydropho-
bic para-κ-casein remaining on the surface after
the action of rennin, are the driving force for
Fig. 10.8.Temperature dependency of the aggregation
rate of para-casein micelles (rate constant k in fractions
of the diffusion-controlled rate kD; according toDal-
gleish, 1983)gel formation. In addition, other temperature-
dependent reactions play a role, like the binding
of calcium ions and ofβ-casein to the micelles,
and the change in solubility of colloidal calcium
phosphate.
Acid coagulation of casein is also definitely
caused by hydrophobic interactions, as shown
by the dependency of the coagulation rate on
the temperature and pH value (Fig. 10.9). On
acidification, the micelle structure changes due
to the migration of calcium phosphate and
monomeric casein. Since the size of the micelle
remains practically constant, this migration of
components must be associated with swelling.
During coagulation, dissolved casein reassociates
with the micelles, forming a gel network.
The gel structure can be controlled via changes
in the hydrophobicity of the micelle surface.
A decrease in hydrophobicity is possible, e. g., by
heating milk (90◦C/10 min). Covalent bonding
of denatured β-lactoglobulin to κ-casein (cf.
10.1.3.5) occurs, burying hydrophobic groups.