Effect of the pH and NaCl on the microstructure and rheology of mixtures of whey protein isolate and casein micelles upon heating

Thermal gelation of aqueous solutions of micellar casein (MC), whey protein isolate (WPI) and mixtures was investigated as a function of the total protein concentration and the composition at different pH between 5.8 and 6.6. The sol-gel state diagram was determined visually and the mechanical properties of the gels were studied by oscillatory shear rheology. The evolution of the shear moduli was measured as a function of heating time at different temperatures. Strong synergy between the two types of milk proteins was observed during gelation of mixtures. MC gels are formed above a critical temperature that increases with increasing concentration. The MC gels are reinforced by co-aggregation of denatured whey proteins. Hybrid gels display stronger elastic moduli and less syneresis than pure MC gels. The microstructure of the gels was assessed using confocal laser scanning microscopy and showed that hybrid gels were more homogeneous than pure MC gels. The structure of rapidly formed MC gels was not modified by subsequent co-aggregation with denatured WPI. The effect of electrostatic interactions was probed by comparing gelation of salt free solutions with that at 0.1 M NaCl. Addition of salt led to an increase of the minimal gel concentration of MC. (C) 2017 Elsevier Ltd. All rights reserved.