Acid-induced gelation of heat-treated milk studied by diffusing wave spectroscopy.

Fresh skim milk is a stable colloidal system containing casein micelles and whey proteins. By decreasing the pH, the casein micelles become unstable and a gel is formed. During heat treatment at temperatures higher than 70 degrees C, the major whey proteins, e.g. alpha-lactalbumin and beta-lactoglobulin denature and start to interact with each other and with casein micelles. This changes the colloidal properties of the casein micelles. In this article, the pH-induced gel formation of heat-treated milk and the role of whey proteins was studied. Heat treatment in the range 70-90 degrees C induced a shift in gelation pH of skim milk to more alkaline pH values. This shift was directly related to whey protein denaturation. By using WPF milk it was shown that beta-lactoglobulin is principally responsible for the shift in gelation pH. alpha-lactalbumin caused neither alone nor in combination with beta-lg, an effect on the gelation pH. Heat treatment of milk for 10 min at 90 degrees C resulted in complete denaturation of the beta-lg present in skim milk but it is estimated that the casein micelles are coated only up to 40% by whey proteins when compared with pure whey protein aggregates.