Effect of β-Lactoglobulin A and B Whey Protein Variants on the Rennet-Induced Gelation of Skim Milk Gels in a Model Reconstituted Skim Milk System

The effect of fortifying reconstituted skim milk with increasing levels of the beta-lactoglobulin (beta-LG) genetic variants A, B, and an A-B mixture on rennet-induced gelation was studied by small deformation-sensitive rheology. Free-zone capillary electrophoresis and high-sensitivity oscillatory rheology were used to elucidate the role of potential heterotypic associative interactions between whey proteins and casein in a mixed colloidal system, subjected to moderate heating (65 degrees C for 30 min) prior to renneting, on the gelling properties of the system. Increasing levels of added whey protein, in the concentration range of 0.225 to 1.35% of added protein, led to a concomitant progressive increase in the equilibrium shear storage modulus, G' (recorded after approximately 10,800 s), in the order beta-LG B > beta-LG A and beta-LG A-B, as the general expected consequence of the setup of denser casein gel networks. The preferential effect of beta-LG B over beta-LG A on the mechanical strength of the gels may be due to the formation of cross-links and aggregates involving whey proteins and rennet hydrolysis products or an increase in the size of the casein micelle caused by the grafting of beta-LG B to its surface, or both. The results of free-zone capillary electrophoresis were consistent with the notion that beta-LG B (and not beta-LG A) binds to the casein micelle under an optimal stoichiometry of 1:0.045 (mg/mg), even in the absence of heat treatment. The liquid-like character of the gel networks formed, tan delta, was a parameter sensitive to the level of addition of beta-LG A in particular. At low concentrations (up to 0.45%) of beta-LG A, tan delta increased by almost twice as much, which was interpreted as a result of the increase in the loss modulus, G'', of the sol fraction because of the presence of unbound beta-LG A. At greater incremental concentrations of beta-LG (>0.45%), the formation of smaller whey protein aggregates confined to the sol fraction may have led to a progressive decrease in tan delta. The critical gel time, t(gel), was also affected by the concentration of added whey protein and described 3 zones of behavior, irrespective of the type of whey protein variant. The critical gel time was slightly shorter for beta-LG B than for beta-LG A at 0.45% of added whey protein, but this difference became larger at 0.67%. Even when only beta-LG B was found to associate with casein prior to renneting, both beta-LG A and beta-LG B, either alone or mixed, had a profound influence on the mechanical strength and coagulation kinetics of the rennet-induced casein gels. This knowledge is expected to be useful to exert better control and optimize processing conditions during the manufacturing of cheese and cheese analogs.