Interaction of Lactose and Proteins of Skim Milk during Ultra-High-Temperature Processing

Abstract

Incorporation of [carbon-14]lactose into milk proteins following ultra-high temperature processing was measured. When lactose was heated with casein micelles, o~-lactalbumin, and fi-lactoglobulin in model systems, casein micelles incorporated the greatest amount of lactose. Raw skim milk which had been dialyzed against simulated milk ultrafiltrite and heated with [carbon-14]lactose in capillary tubes to 143.5 C +-2 for 10 s gave a protein-lactose complex. The complex seemed covalently bonded because the label accompanied individual protein fractions during chromatography in the presence of urea and 2-mercaptoethanol on both Sephadex G-10 and O-(diethylaminoethyl) cellulose. Identification and purity of protein fractions were assessed by vertical polyacrylamide gel electrophoresis. The first fraction eluted from the cellulose column exhibited the highest specific radioactivity. It appeared heterogenous as judged by polyacrylamide gel electrophoresis and by comparison of its amino acid composition with known milk proteins. Kappa-casein was identified in the first fraction by the presence of sialic acid, half-cystine, and by sensitivity to rennin. The possible presence of 72-and 73-caseins also was indicated by electrophoresis and amino acid analysis.