Model mixtures evidence the respective roles of whey protein particles and casein micelles during acid gelation

In acidified milk, heat-induced whey protein aggregates and casein micelle particles assemble to form a soft gel. The present study was set to evaluate the respective roles of whey protein aggregates (WPIA) and native casein micelles (NMC) during acid gelation by means of changing their ratio in model systems. NMC and WPIA were dispersed in milk permeate at different weight ratios ranging from 0% to 100% NMC for a total protein concentration of ∼45 g kg−1. Acidification was performed at 35 °C by addition of glucono-δ-lactone to achieve the same final pH of 4.5 in 6 h. Acid-induced gelation of these systems was followed using small deformation rheology followed by large deformation test and whey retention measurement at pH 4.5, while their microstructure was investigated microscopically. The results showed that higher content in WPIA promoted faster gelation and led to more elastic gels with smaller pore size and increased whey retention. The effects were particularly dramatic up to ∼10% w/w WPIA, where the aggregates were about equimolar to the casein micelles and covered ∼8% of the micellar surface. The results were discussed in terms of the physical interactions between two populations of colloids of different abilities for acid gelation. It seemed likely that a preferred interaction exists between the casein micelles and the aggregates, and directs the structural and mechanical properties of the acid gel.