Stable and unstable casein micelles

In milk, casein occurs as colloidal particles with an average size of about 100 nm. These are stabilized against flocculation by an outer layer of several thousands of kappa-casein molecules. Stability of micelles is characterized by the magnitude of the Smoluchowskian flocculation rate constant, which during the renneting of milk nearly approaches the diffusion-controlled limit. The processes of the clotting of milk by rennet and the phenomena of age-thinning and age-thickening of ultra-high temperature-sterilized, concentrated milks bear interesting kinetic resemblances. Both processes are characterized by a lag phase during which viscosity decreases, followed by an explosive increase in viscosity. In the milk-clotting process, the decrease can be explained by the proteolytic action of the renneting enzyme. This strongly suggests that age-thinning and age-thickening are also caused by the action of a protease that survived the sterilization process. A quantitative check of this theory is difficult because of the apparently small amount of enzyme.