It is considered that heat treatment affects the properties of milk such as heat-stability and chymosin clottability. The heat-induced interaction between K>casein and a-lactalbumin was investigated under various conditions (buffer, pH, ionic strength, temperature). The results obtained, using Sephacryl S-300 gel filtration and sodium dodecylsulfate-polyacrylamide gel electrophoresis, indicated that a complex of the two proteins was formed in 35mMphosphate buffer, pH 7.6, containing 0.4m NaCl on heat treatment at 90°C for 30 min, while no complex was formed in 10mMimidazole-HCl buffer, pH 7.1, containing 70mMKC1 with the same heat treatment. A K-casein-a-lactalbumin complex was formed at high pH by heat treatment, and was dissociated in the presence of 2-mercaptoethanol. Under complex forming conditions, a change in the higher-order structure of a-lactalbumin was observed by ultraviolet absorption and fluorescence experiments. Though the participation of specific amino acid residues in the complex formation could not be clarified by the chemical modification procedure, the rearrangement of the disulfide cross-links did take place in the reaction. An optical study on the complex formation did not indicate the involvement of a hydrophobic bond. Heat treatment of milk is necessary for its preservation and as a prior treatment for processing. The characteristics of milk are almost all due to milk proteins. Mechanisms of changes in proteins on heat treatment and formations of a precipitate and gel in ultrahigh temperature sterilized milk have not been clarified. Bovine milk contains about 4% protein, consisting of casein components and whey proteins. Casein components form casein micelles in milk in the presence of calcium ions. The major constituents of wheyproteins

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