title: Stable and unstable casein micelles.
abstract: 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.
mesh_terms: D000818:Animals; D002364:Caseins / Q000032:analysis*; D002417:Cattle; D012085:Chymosin / Q000494:pharmacology; D005427:Flocculation; D006358:Hot Temperature; D008823:Micelles; D008854:Microscopy, Electron; D008892:Milk / Q000032:analysis*; D008956:Models, Chemical; D008970:Molecular Weight; D013242:Sterilization; D014783:Viscosity
chemical_list: D002364:Caseins; D008823:Micelles; C025350:rennet; D012085:Chymosin