On heating milk, the dissociation of κ-casein from the casein micelles can precede interactions with the denatured whey proteins

When kappa-casein (kappa-CN) was added to milk, and the milk was subsequently pH adjusted (pH6.5-6.9) and heated (90 degrees C/15 min), the serum contained considerably higher levels of denatured whey proteins than the milks without added kappa-CN. When milk at pH 6.5-6.9 was heated at 90 degrees C for different times, kappa-CN was found in the serum in the early stages of heating and before significant levels of whey proteins were denatured. kappa-CN reached its maximum level in the serum before the whey proteins were fully denatured. When milk at pH 6.5-6.9 was heated at 20-90 degrees C for 15 min, kappa-CN dissociated from the casein micelles at all temperatures, with the level in the serum increasing with the temperature and the pH at heating. kappa-CN dissociated from the micelles at temperatures below those at which significant levels of the whey proteins were denatured. When taken together, the results from these experiments strongly indicate that the dissociation of kappa-CN from the micelles can precede the interaction of the denatured whey proteins with kappa-CN, and that there is a preferential interaction of the denatured whey proteins with serum-phase kappa-CN.