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Surface Structure of Bovine Casein Micelles Elucidated With Insolubilized Carboxypeptidase A

Department of Food Science, University of Wisconsin, Madison 53706

ABSTRACT

Carboxypeptidase A was insolubilized with glutaraldehyde to form a superpolymer. Bovine casein micelles were obtained from uncooled, raw skim milk centrifuged at 50,000 x g for 60 min and were resuspended in simulated milk ultrafil-trate. Native micelles and micelles cross-linked with glutaraldehyde were reacted with insolubilized carboxypeptidase A at 37 C for periods up to 42 h. Since the enzyme is much larger than the micelle and cannot penetrate it, the C-terminal amino acids released gave an indication of the surface structure of the micelle. The relative rates of hydrolysis were: s₁-casein more than ß-casein more than -casein; although C-terminal amino acids were incompletely released from most casein fractions. After taking into account any solubilization or autolysis of the micelle, analysis of the reaction products revealed all major casein fractions on the surface of the micelle.