Heat-induced gelation of mixtures of casein micelles with whey protein aggregates
Résumé
Gelation of aqueous solutions of casein micelles (MC) in mixtures with fractal aggregates of whey protein isolate (WPI) has been studied first by addition of different amounts of aggregates at a fixed concentration of micelles and then by substitution of micelles with aggregates at a fixed total protein concentration. Addition of small amounts of aggregates (1–5 g/L) to suspensions of micelles (20–60 g/L) at a fixed pH led to a strong decrease of the gelation temperature (Tg) and to an increase of the elastic modulus (G′) of the mixed gels. A minimum in Tg and a maximum in G′ as a function of the WPI fraction were obtained in mixtures with fixed total protein concentration and fixed pH and explained by the opposing influence of increasing WPI aggregate concentration and decreasing concentration of micelles. Increasing the total protein concentration at the same WPI fraction caused a decrease of Tg and increase of G′. Decreasing the pH at the same protein composition led to a decrease of Tg but caused an increase of G′. Tg decreased sharply if a small amount of CaCl2 was added, but if more than about 30 mM of calcium was added, Tg increased again. Gelation of mixtures did not depend significantly on the size of the WPI aggregates, but if fractal aggregates were replaced by microgels, weaker gels were produced and the temperature of gelation was higher. Potentiometric titration curves showed that protonation of the micelles and of the WPI aggregates in the mixtures was independent. It is concluded that MC and WPI aggregates co-aggregate during gelation and there is a transition between gelation at a critical temperature for pure MC suspensions and gelation controlled by an activation energy for pure WPI aggregate suspensions.