Changes in the Protein Composition and Size Distribution of Bovine Casein Micelles Induced by Cooling

Abstract

The changes of bovine casein micelles during cold storage were investigated from the standpoint of the size of micells. Large, medium, and small micelles (>50, 50-30, and <30 nm in radius) were obtained from skim-milk by differential centrifugation. After the definite-sized micelles were equilibrated at 4°C and 37°C, they were fractionated by differential centrifugation and gel chromatography. The casein contents of these fractions were measured by SDS-polyacrylamide gel electrophoresis. A high quantitative separation of caseins was achieved by this method. The liberation of β-casein by cooling occurred easily from the larger-sized micelles. A larger amount of κ-casein was liberated from the smaller-sized micelles. The medium micelle fraction was increased by the supply from large micelles at 4°C, and the medium micelles degraded partly to the small micelle fraction and soluble casein. Small micelles degraded partly to soluble casein. The small micelle fraction was not produced from large micelles. The stability of micelles under cooling was related closely to the miceller size. © 1990, Japan Society for Bioscience, Biotechnology, and Agrochemistry. All rights reserved.