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.

Heat-induced aggregation of beta-lactoglobulin as a function of pH.

The effect of pH in the range 6.0-8.0 on the denaturation and aggregation of beta-lactoglobulin (beta-lg) was investigated. Results were interpreted in terms of the reaction scheme for the denaturation and aggregation of beta-lg proposed by Roefs and De Kruif (Eur. J. Biochem. 1994, 226, 883-889). The rate of conversion of native beta-lg increased strongly at higher pH values, whereas the molecular mass of the aggregates decreased strongly. In the pH range 6.4-8.0 aggregates were formed mainly by intermolecular disulfide bonds, but even at pH 6.0, thiol/disulfide exchange reactions were involved, although to a lesser extent. The time course of the exposure of the thiol group in native beta-lg upon heating and the subsequent disappearance of this group through the formation of disulfide-linked aggregates was investigated by reaction with 5,5'-dithiobis(2-nitrobenzoic acid) and varied strongly with pH. These observations could be used, in combination with the reaction steps of the reaction scheme, to describe qualitatively the strongly pH-dependent isothermal calorimetry curves, measured at 65 degrees C.

Gelation of proteins from milk.

We give a quantitative treatment of the destabilization of three types of milk protein dispersions. For this we consider the proteins as adhesive-hard-sphere bio-colloids. If the attractive interactions become strong enough the system passes the percolation threshold and gels. Macroscopic properties of these gels are studied by measuring viscoelasticity and permeability coefficients. These coefficients are related to structural (fractal) properties of the gels which were measured using scattering and confocal scanning laser microscopy (CLSM) techniques. The behaviour of the protein gels can be understood on a qualitative level.