Casein - whey protein interactions in heated milk

Abstract

Heating of milk is an essential step in the processing of various dairy products, like for example yoghurt. A major consequence of the heat treatment is the denaturation of whey proteins, which either associate with the casein micelle or form soluble whey protein aggregates. By combination of enzymatic fractionation and capillary electrophoresis we were able to quantitatively determine the distribution of denatured whey proteins after heat treatment. This thesis describes the relation between these quantitative studies and the acid-induced gelation properties and textural gel properties of milk derived products. In chapter 3 it was demonstrated that more severe heat treatment caused more denaturation and that the whey proteins both associate with the casein micelle and form whey protein aggregates. The formation of these aggregates was visualised and the size was estimated. We clearly demonstrated that at the natural pH of milk the ratio of denatured whey proteins associated with the casein micelle and present in aggregates remained constant and that the observed shift in gelation pH of heated milk is linearly correlated with the two fractions of denatured whey proteins. The shift in gelation pH was more thoroughly studied in chapter 6 and was directly related to whey protein denaturation. It was shown that b-lactoglobulin was principally responsible for the shift in gelation pH. a-lactalbumin caused neither alone nor in combination with b-lactoglobulin an effect on the gelation pH. Chapter 4 reports the effect of pH-adjustment of milk (pH range 6.9 to 6.35) prior to heat-treatment (10 min at 80?C) on the distribution of denatured whey proteins and on the homogeneity of the whey proteins coating of the casein micelles. After heat treatment at pH 6.9 most whey proteins are present in soluble whey protein aggregates while heating at pH 6.55 and lower causes association of all whey proteins with the casein micelle. Heating of milk at pH 6.35 causes a clearly more inhomogeneous coating than heating at pH 6.55. This pH-dependent whey protein denaturation is schematically depicted in a model and related to acid and rennet-induced gelation properties. In Chapter 7 we studied the formation of disulfide linked protein structures during the acidification step at ambient temperature. The time dependent formation of these structures attributed significantly to the mechanical properties of acid milk gels, resulting in gels with an increased storage modulus and hardness. The mechanical properties are shown to be the result of the contribution of denatured whey proteins to the protein network as such and the additional formation of disulfide bonds. Surprisingly, the formation of these disulfide bonds take place at ambient temperature and under acidic conditions. Therefore, the disulfide cross-linking is highly relevant also for textural properties of acid-milk products, like yogurt. In conclusion this work showed that seemingly minor variations in milk treatment may lead to considerable changes in the properties of the end product. Quantitative description will allow better control and tuning of the final gel properties.