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.

H Bonding:

Some occurs between the individual caseins in the micelle but not much because there is no secondary structure in casein proteins.

Disulphide Bonds:

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.

Hydrophobic Interactions:

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.

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:

No success in relating these forces to micellular stability.

Steric stabilization:

As already noted, the hairy layer interferes with interparticle approach.

There are several factors that will affect the stability of the casein micelle system:

Salt content:

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 isoelectric point (pH 4.6), all phosphate is dissolved and the caseins precipitate.

Temperature:

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.

Heat Treatment:

whey proteins become adsorbed, altering the behaviour of the micelle.

Dehydration:

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.