Heat-induced protein interactions in mixed reconstituted skim milk powder-sodium caseinate systems
The formation of heat-induced aggregates of [kappa]-casein and denatured whey proteins was investigated in mixed reconstituted skim milk-sodium caseinate (NaCN) systems. The serum and pellet of the ultracentrifuged milk-caseinate mixtures, heated for various time/temperature combinations, were analyzed by Size-Exclusion Chromatography (SEC). SEC fractions were collected and analyzed for composition. It was found that analysis of the serum fraction was beneficial over the analysis of the pellet, as small changes were more easily observed and the results had less variability. Quantification of the material in the soluble aggregates indicated that there is a fixed stoichiometry associated with the complexes. The composition of the complex was strongly dependent on the relationship between the temperature and time of heating, rather than the initial ratio of casein to whey protein in the mixture. Results also showed that even with a significant increase in the quantity of soluble [kappa]-casein, that both micelle-bound and soluble complexes still formed; and that the amount of soluble complexes was not greatly affected by the increase in soluble [kappa]-casein. There was an increase in the amount of soluble casein in the mixtures with added NaCN. However, during heating a loss in solubility of this casein was observed. Model systems showed that during heating the caseinate interacted with the micellar pellet, although this interaction appeared to be limited and did not cause significant increases in casein micelle size.