Effect of Oxalate and Urea upon Ultracentrifugation Properties of Raw and Heated Skimmilk Casein Micelles

Abstract

The effect of oxalate and urea treatments upon casein micelles sedimented from raw and heated skimmilk was examined by ultracentrifugation, to determine the relative importance of calcium and hydrogen or hydrophobic bonds in determining their structure. A model was proposed for the native casein micelle which consists of numerous loosely packed calcium caseinate complex units joined in association by a combination of calcium and colloidal calcium phosphatecitrate linkages. The colloidal calcium phosphate-citrate is considered to be distributed throughout the micelle rather than as a layer on its outer surface.The disaggregation of skimmilk casein micelles appears to involve at least a three-stage process: a) dissolution of intercomplex colloidal calcium phosphatecitrate linkages, to produce calcium caseinate complex units of less than ∼ 30-mµ diameter, b) removal of intra-complex calcium to form soluble caseinate complex units, and c) disaggregation of soluble caseinate complex units to monomer units by rupturing hydrogen, hydrophobic, or salt linkages. Hydrogen or hydrophobic bonds appear to be somewhat more important than calcium linkages in stabilizing the individual calcium caseinate complex units of the micelle.Heating to temperatures of 120C or less had only a slight effect upon the ability of 6.6 m urea to disaggregate the casein micelles. However, casein micelles from heated skimmilk were less effectively disaggregated by removal of calcium and colloidal calcium phosphatecitrate linkages than those from raw skimmilk, which is probably due to the heat-induced complex formed between β-lactoglobulin and the κ-casein portion of the micelle.