Moving boundary electrophoresis of native and rennet-treated casein micelles

Abstract

Summary: The electrokinetic potential of bovine casein micelles was determined under a variety of conditions using moving boundary electrophoresis. The technique used allowed sharp stable boundaries to be formed without addition of density gradient-forming materials. Casein micelles are negatively charged and in any one milk sample all micelles had similar electrophoretic mobilities. The electrokinetic potential of micelles was markedly reduced by the action of rennet and to a lesser extent by a decrease in temperature from 30 to 6°C. Addition of sucrose to milk caused a small increase in the electrokinetic potential. Addition of anionic detergent to milk increased both the electrokinetic potential and the rennet coagulation time. Addition of cationic detergent caused a reduction in both these quantities, promoted syneresis of the rennet coagulum and caused coagulation in the absence of rennet if present in concentrations exceeding about 10 mM. Alteration of cation activities by addition of salts to milk caused complex changes, either increasing or decreasing the electrokinetic potential. The data obtained are most readily explained by a model for the casein micelle in which some, but not necessarily all, of the κ-casein is located at the micelle surface.