Effect of size, proteic composition, and heat treatment on the colloidal stability of proteolyzed bovine casein micelles

The casein micelles of reconstituted nonfat milk that have been fractionated by controlled pore glass chromatography showed a relationship between their size and their proteic composition: The fractions containing the smaller particles were richer in κ-casein than the fractions containing the bigger ones, in accordance with the casein micelle model of submicelles. The initial aggregation rate of micelles of different sizes, partially proteolyzed with chymosin (para-casein micelles), was measured in conditions of enzyme excess in which aggregation is the rate-limiting step of enzymatic coagulation, showing higher rates for the smaller micelles with the production of less compact para-casein micelle networks. This behavior could be explained in terms of electrostatic and steric colloidal stabilization due to their lower negative net charge and size and to a higher surface density of hydrophobic “patches” of proteolyzed κ-casein related to a higher probability of effective collisions between particles. Differences in the β-casein content did not seem to affect the initial aggregation rate of the micelles. On the contrary, the modifications of the micelle surface by heating affected the colloidal stability of the hydrolyzed micelles in different ways. The denaturation of the whey proteins and the formation of covalent complexes with κ-casein modify the micelle surface, increasing specially the steric stabilization, and produces a diminution in the number of hydrophobic sites that could be able to give interparticle hydrophobic interactions.