Behaviour of calcium and phosphate in bovine casein micelles

The behaviour of calcium and phosphate in casein micelles was examined using Ca-45 and P-32 by means of ultrafiltration and high performance gel chromatography on a TSK-GEL G4000SW column in the presence of 6 M urea. Approximately 30-40 % of the phosphate present in the colloidal phase of a casein micelle dispersion (CMD) was estimated to be hard to exchange with the phosphate present in soluble phase. The distribution patterns of Ca-45 and P-32 between soluble and colloidal phases in CMD did not coincide with the protein elution patterns from high performance gel chromatography in the presence of urea. Only two peaks appeared in the distribution patterns of Ca-45 and P-32 in CMD. The proportions of Ca-45 and P-32 in the fraction of casein aggregates cross-linked by micellar calcium phosphate (MCP) increased with the length of holding time after adding of Ca-45 and P-32 to CMD. The proportion of Ca-45 in the fraction of casein aggregates cross-linked by MCP decreased slightly on cooling at 4 degrees C. The proportion of Ca-45 and P-32 distributed in casein aggregates crosslinked by MCP increased significantly on heating at 90 degrees C. The results suggested that calcium and phosphate in CMD could be divided into three types. Copyright (C) 1996 Published by Elsevier Science Limited