Gelation of the heat-induced complex between kappa-casein and alpha-lactalbumin.

Whey is a by-product of the manufacture of cheese from milk. The usual practice is to dispose of it, the usage of whey being not sufficiently developed, though it contains proteins of excellent quality such as beta-lactoglobulin and alpha-lactalbumin. Interaction between kappa-casein and whey protein was examined in order to form new food materials. Gelation of the heat-induced complex between kappa-casein and alpha-lactalbumin is described in this paper. alpha-Lactalbumin was easily separated from whey protein by the gel filtration technique on a Toyopearl HW-50 column at pH 6.0. Heat treatment facilitated the hydrolysis of a mixture of kappa-casein and alpha-lactalbumin by trypsin, alpha-chymotrypsin and pronase. Heat treatment at above 75 degrees C and a protein level of over 5% were needed to form gel in 35 mM phosphate buffer, pH 7.6, containing 0.4 M NaCl. The temperature was in agreement with that at which alpha-lactalbumin denatured and formed the complex with kappa-casein. Decrease of soluble protein concentration and increase of turbidity were induced with gelation. Gel was not formed when only kappa-casein or alpha-lactalbumin was heated under the appropriate conditions. It was considered that a kappa-casein-alpha-lactalbumin gel was formed from a complex of the two proteins by heat treatment. The breaking stress of kappa-casein-alpha-lactalbumin gel was less than that of kappa-casein-beta-lactoglobulin gel. If the pH was reduced to 5.8 after complex formation by the two proteins, gel was formed at a low protein concentration compared with that with no alteration of pH.