The effect of casein to whey protein ratio on the heat stability of skim milk

Heating is an integral part of processing milk and the response of milks to heating dictates the processability and functionality of the milks. This thesis examines the effects of changing the ratio of the caseins to whey proteins on the behaviour of skim milk on heating. Modified skim milks with similar serum mineral compositions but with different casein to whey protein ratios (0.03, 1.74, 3.97, 5.27, 7.25) were prepared by blending casein rich and whey protein rich fractions in appropriate amounts and dialyzing against 9% total solids skim milk (pH6.7). These modified milks were freeze dried and used for preparation of 9% total solids milks at various pH (6.2, 6.7 and 7.2). Viscometry and Diffusing Wave Spectroscopy (DWS), both prior to and after heating as well as in situ, were used to investigate the effects of the casein to whey protein ratios on the heat stability of the milks. The pH in situ at the time of heating and Ca activity at 25°C were used to gain insights into changes in the mineral equilibria. The heat-induced changes to protein distribution between the serum and colloidal phases and the type of protein complexes formed after heating were followed using HPLC-SEC and gel electrophoresis. On heating (90°C/10min), milks (pH6.2) with the lowest ratios of casein to whey protein (0.03 and 1.74) gelled while the viscosities of the other milks increased. At pH6.7 and 7.2, the viscosity decreased for all milks except that with the lowest casein to whey protein ratio (0.03) where viscosity was increased. The viscosity increase at the lower pH was attributed to complex formation between the denatured whey proteins and the casein micelles. The decrease in viscosity for the higher pH milks after heating was due to the dissociation of caseins from the micelle and the formation of increased amounts of soluble aggregates as confirmed by the HPLC-SEC and gel electrophoresis. Characterization of the serum protein concentration and gel electrophoresis analysis of the modified milks showed that iIn the presence of casein micelles, the serum protein concentration of the milks increased on heating as a function of pH and increasing casein to whey protein ratio as a result of more dissociation of caseins and the formation of soluble aggregates at higher pH. HPLC-SEC analysis showed that the size of the soluble aggregates decreases as the casein to whey protein ratio increases. The pH in situ decreased during heating (90oC/10min), the magnitude of which was increased with increasing initial milk pH. The changes in pH in situ were independent of the casein to whey protein ratio and were similar for all milks, clearly demonstrating that pH changes are solely governed by the serum mineral composition. The pH and Ca activity after heating werewas largely restored to the initial values for most milks except for the milks with lower casein to whey protein ratios (0.03, 1.74) at high pH (7.2). This demonstrates that in the absence of casein micelles, there were changes to the calcium phosphate equilibria that were not reflected in the overall pH change. DWS measurements as a function of temperature (25-120°C) enabled an in-situ study of (a) the heat-induced changes to the milk proteins and the serum as manifested by the changes to the casein micelles and (b) the temperature of the onset of gelation. Changes to the interactions of the casein micelles as shown by changes in their diffusion coefficients were most pronounced at pH 6.2 where significant changes to the mobility of the casein micelles occurred between 80 to 120oC due to the aggregation of the denatured whey proteins on the casein micelles surface. At pH 6.7 and 7.2, changes to the mobility of the casein micelles were minimal except at high temperatures (105-120oC) due to the decrease in size as a result of increased soluble aggregate formation. At 120oC, with the exception of the milk with the highest casein to whey protein ratio, all pH6.2 milks gelled on heating. The reduction in the low whey protein concentration for the milk with the highest casein to whey protein ratio (7.25) improved heat stability at low pH. Overall, the results showed that the pH and temperature of heating have major effects on the aggregation behavior of the proteins and that the susceptibility of the milks to aggregation was strongly influenced by the casein to whey protein ratio. Heat stability increased with increasing ratio of caseins to whey proteins, especially at low pH. The results of this work have demonstrated that the heat stability of milks can be manipulated by careful control of the protein composition and the pH prior to heating.