Emulsions, Microemulsions, and Lyotropic Liquid Crystals - Surface and Colloid Chemistry: Principles and Applications - page 207

Electrostatic interactions : Some of the subunit interactions may be the result of
ionic bonding, but the overall micellar structure is very loose and open.
Van der Waals forces : There has been some success in relating these forces to
micellar stability. However, the steric stabilization has been found to be also of some
importance. Especially, the hairy layer interferes with the interparticle approach.
There are several factors that will affect the stability of the casein micelle system:
Salt content: It affects the calcium activity in the serum and calcium phosphate
content of the micelles.
pH: Lowering the pH leads to dissolution of calcium phosphate until, at the iso-
electric point (pH 4.6), all phosphate is dissolved and the caseins precipitate.
Temperature: At 4°C, the beta-casein begins to dissociate from the micelle;
at 0°C, there is no micellar aggregation; freezing produces a precipitate
called cryocasein.
The heat treatment leads to whey proteins becoming adsorbed, altering the behavior of
the micelle. Dehydration by ethanol, for example, leads to aggregation of the micelles.
9.5.3.4.1 Casein Micelle Aggregation
Caseins are able to aggregate if the surface of the micelle is reactive. Although the
casein micelle is fairly stable, there are four major ways in which aggregation can
be induced:
Proteolytic enzymes as in cheese manufacturing
Acid
Heat
Age gelation
During the secondary stage, the micelles aggregate. This is due to the loss of steric
repulsion of the kappa-casein as well as the loss of electrostatic repulsion due to
the decrease in pH. As the pH approaches its isoelectric point (pH 4.6), the caseins
aggregate. The casein micelles also have a strong tendency to aggregate because of
hydrophobic interactions. Calcium assists coagulation by creating isoelectric condi-
tions and by acting as a bridge between micelles. The temperature at the time of
coagulation is very important to both the primary and secondary stages. With an
increase in temperature up to 40°C, the rate of the rennet reaction increases. During
the secondary stage, increased temperatures increase the hydrophobic reaction. The
tertiary stage of coagulation involves the rearrangement of micelles after a gel has
formed. There is a loss of paracasein identity as the milk curd firms and syneresis
begins. It has been found that acidification causes the casein micelles to destabilize
or aggregate by decreasing their electric charge to that of the isoelectric point. At
the same time, the acidity of the medium increases the solubility of minerals so
that organic calcium and phosphorus contained in the micelle gradually become
soluble in the aqueous phase. Casein micelles disintegrate, and casein precipitates.
Aggregation occurs as a result of entropically driven hydrophobic interactions. At
temperatures above the boiling point, casein micelles will irreversibly aggregate. On