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Distribution of Glyco--Casein in Bovine Casein Micelles: A Study Using Soluble and Immobilized Proteases

Department of Food Science, University of Illinois, 1302 W. Pennsylvania Avenue, Urbana 61801

ABSTRACT

Bovine casein micelles isolated from raw uncooled skim milk by ultracentrifugation at 50,000 x g for 60 min were treated with chymosin and pepsin in both soluble and immobilized forms. The release of nonprotein nitrogen and sialic acid soluble in 2% and 12% trichloroacetic acid was studied. Soluble chymosin released a maximum amount of 7.56 mg nonprotein nitrogen per gram casein soluble in 2% trichloroacetic acid and 4.67 mg nonprotein nitrogen per gram casein soluble in 12% trichloroacetic acid. The corresponding figures for immobilized chymosin were 6.46 and 4.20. Pepsin generally released higher quantities of nonprotein nitrogen from the micelle. Both glyco--casein and carbohydrate-poor -casein exist on the micelle surface with at least 90% of the micelle's glyco--casein being on its surface. Sialic acid analysis of trichloroacetic acid filtrates corroborated the nonprotein nitrogen data.