Casein Micelle Stability
Colloidal calcium phosphate (CCP) acts as a cement between the hundreds or even thousands of submicelles that form the casein micelle. Binding may be covalent or electrostatic. Submicelles rich in kappa-casein occupy a surface position, whereas those with less are buried in the interior. The resulting hairy layer, at least 7 nm thick, acts to prohibit further aggregation of submicelles by steric repulsion. The casein micelles are not static; there are three dynamic equilibria between the micelle and its surroundings:
- the free casein molecules and submicelles
- the free submicelles and micelles
- the dissoved colloidal calcium and phosphate
The following factors must be considered when assessing the stability of the casein micelle:
Role of Ca++:
More than 90% of the calcium content of skim milk is associated in some way or another with the casein micelle. The removal of Ca++ leads to reversible dissociation of ß -casein without micellular disintegration. The addition of Ca++ leads to aggregation.
Some occurs between the individual caseins in the micelle but not much because there is no secondary structure in casein proteins.
alpha(s1) and ß-caseins do not have any cysteine residues. If any S-S bonds occur within the micelle, they are not the driving force for stabilization.
Caseins are among the most hydrophobic proteins and there is some evidence to suggest they play a role in the stability of the micelle. It must be remembered that hydrophobic interactions are very temperature sensitive.
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:
No success in relating these forces to micellular stability.
As already noted, the hairy layer interferes with interparticle approach.
There are several factors that will affect the stability of the casein micelle system:
affects the calcium activity in the serum and calcium phosphate content of the micelles.
lowering the pH leads to dissolution of calcium phosphate until, at the isoelectric point (pH 4.6), all phosphate is dissolved and the caseins precipitate.
at 4° C, beta-casein begins to dissociate from the micelle, at 0° C, there is no micellar aggregation; freezing produces a precipitate called cryo-casein.
whey proteins become adsorbed, altering the behaviour of the micelle.
by ethanol, for example, leads to aggregation of the micelles.
When two or more of these factors are applied together, the effect can also be additive.