INFLUENCE OF HEAT ON K-CASEIN

An aqueous solution of κ-casein at pH 7 is not affected by heating (100 C for 5 min). If, however, sodium chloride (0.05 m) or other salts are present the κ-casein is labile to heat, judged by loss of ability to stabilize αs-casein against precipitation by calcium ions. In experiments at several pH values (6.2 to 7.9) in sodium chloride-imidazole buffer the heat lability was somewhat greater at the lower pH values. At pH 6.2 about 75% of the κ-casein precipitated when the solution was heated. This did not occur in sodium chloride alone or in cacodylate buffer at pH 6.2. The κ-casein in solutions (pH 6.2) containing 0.01 m CaCl2 did precipitate when they were heated. Reducing compounds like mercaptoethanol alone had little or no effect on the heat lability of κ-casein; when present with salts, however, the combined effect was greater than that of the salts alone. Mercaptoethanol, cysteine, and dithiothreitol enhanced the heat lability. The dithiothreitol was effective at a concentration of 5 × 10−5 m. κ-Casein, reduced and stabilized by alkylation, was somewhat heat labile in sodium chloride-imidazole buffer, but this lability was not enhanced by addition of mercaptoethanol. © 1969, American Dairy Science Association. All rights reserved.