Alkali cold gelation of whey proteins. Part II: Protein concentration

The effect of the whey protein isolate (WPI) concentration on the sol-gel-sol transition in alkali cold gelation was investigated at pH 11.6-13 using oscillatory rheometry. The elastic modulus increases quickly with time to reach a local maximum (G' _max), followed by a degelation step where the modulus decreases to a minimum value (G' _min). Depending on the pH, a second gelation step will occur. At the end of the first gelation step around G' _max, the system fulfilled the Winter-Chambon criterion of gelation. The analysis of the maximum moduli with the protein concentration shows that (i) there is a percolation concentration above which an elastic response is observed (∼6.8 wt %); (ii) there are two concentration regimes for G _max and G" _max above this concentration, where we have considered power-law and percolation equations; (iii) there is a crossover concentration between the two regimes (at ∼8 wt %) for both G' _max and G" _max when both moduli are equal, and this value is constant under all conditions tested (G' _max = G" _max ≈ 4 Pa). Therefore, alkali cold gelation is better represented using two concentrations regimes than one, as observed for other biopolymers.