On the Stability of Casein Micelles

Abstract

A view of the structure of the casein micelle is given. It is built of submicelles, roughly spherical aggregates of several casein molecules held together by hydrophobic bonds and salt bridges. Regions of amorphous calcium phosphate link the submicelles to each other; the ester phosphate groups form part of this colloidal phosphate. In this way, almost all regions of the casein molecules are severely restricted in mobility. The C-terminal part of the .kappa.-casein is however predominantly present as flexible "hairs" located at the outside of the micelles. There are essentially two types of submicelles, with and without (much) .kappa.-casein. The casein micelles greatly change in properties upon lowering the pH, mostly due to dissolution of colloidal phosphate; at still lower pH, increased formation of salt bridges predominates. Temperature also has pronounced effects: upon lowering it, the micelles become more voluminous, presumably due to protruding hairs of (mainly) .beta.-casein. Also at high temperature (>70.degree.C), parts of the casein molecules become more flexible. Casein micelles are very stable. If conditions are changed they may disintegrate or aggregate. Aggregation mostly leads to formation of a gel. The application of the theory of fractal flox formation to gelation is briefly discussed: it serves to explain the very strong dependence of gelation times on volume fraction of aggregating particles. The stability against aggregation is primarily due to steric repulsion, caused by the hairs of .kappa.-casein, and at low temperature presumably also .beta.-casein. The hairs on different casein micelles may however touch, and this may lead to lasting contact of the micelles, i.e., aggregation. The bonds formed can be salt bridges or, at high temperature, covalent bonds (chemical crosslinks). Hydrophobic bonds are probably not involved. The probability that casein molecules in different micelles may touch each other for a sufficient time for bonds to be formed appears to depend on electrostatic as well as steric repulsion, which thereby affect aggregation rate.