Whey protein interactions in acidic cold-set gels at different pH values

Cold-set whey protein isolate gels (7% w/w) were produced by the addition of different amounts of glucono-delta-lactone to thermally denatured protein solutions. After 48 h of incubation at 10 degrees C, different final pH values were obtained (5.2 to 3.9). The gels were analysed by uniaxial compression measurements, water-holding capacity and protein solubility. The water-holding capacity of the gels decreased at pH values near the pI, this being associated with the lower protein solubility at this pH value. Protein solubility in the presence or absence of denaturing and reducing agents indicated that electrostatic interactions were responsible for the maintenance of the acidified gel structure at pH values from 5.2 to 4.6, but at pH 4.2, more hydrophobic interactions were present in relation to other final gel pHs, in spite of beta-lactoglobulin (beta-Lg) and BSA being far from their pI values. Complementary PAGE assays showed that disulphide bonds were associated with internal stabilisation of the protein aggregates formed during heat treatment. The mechanical properties of the gels were influenced by the final system pH value, showing that the strongest network was observed at pH 5.2. The results allowed the conclusion that although the interactions amongst the aggregates in the network of cold-set whey protein gels were essentially the same at the pH values evaluated, beta-Lg played an important role in gel stabilisation due to its high concentration in WPI, strengthening the structure at its pI and, at the same time, providing good water-retention capacity.