Acid-induced gelation of whey protein aggregates: Kinetics, gel structure and rheological properties

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Acid-induced gelation of fractal aggregates with different sizes formed by preheating whey protein isolate solutions has been investigated in the protein concentration range 10–60 g/L as a function of the net protein charge density (α). Homogeneous gels could be formed in a range of α that depended on the heating time and temperature. Decrease of the negative charge density of the proteins increased the rate of gelation, but lowering α to less negative values than approximately −2.5 by adding HCl increased turbidity of the solutions and led to syneresis. Strong syneresis also occurred at more negative net charge densities (higher pH) if the heat treatment was too severe. The elastic modulus of homogeneous gels increased with increasing protein concentration. The heterogeneity of the gels decreased with increasing protein concentration. The temperature dependence of the gelation rate was characterized by an activation energy of 155 kJ/mol independent of the charge density, protein concentration and aggregate size. Increase of the temperature increased the gelation rate, but did not influence the elastic modulus of the gels. The present findings on acid-induced gelation are compared with gelation of the same WPI aggregates induced by adding CaCl2 reported elsewhere.