Acid-induced gelation properties of heated whey protein–pectin soluble complex (Part I): Effect of initial pH

Heated whey protein and pectin soluble complexes were formed at near neutral pH (6.2–7.0) by heating the mixtures (5% w/w protein and 0.05–0.5% w/w pectin) at 85 °C for 30 min, and acid-induced gels were formed by addition of glucono-delta-lactone. Gel properties were characterized by large and small deformation and microstructural analysis as well as water holding capacity and turbidity measurements. Gel properties were compared to those made from whey protein polymers and pectin where proteins were heated alone to form the polymers and mixed with pectin before acid-induced gelation. Both small and large deformation results showed that gels made by heated soluble complexes at pH 7.0 had significantly improved gel strength than the traditional polymer and pectin gels (polymer/pectin gels). No difference in gel strength was observed between complex gels and polymer/pectin gels when solutions were heated at pH 6.5 or pH 6.2. However, complex gels showed significantly improved water holding capacity at all pH values, especially at higher pectin concentrations. The enhanced electrostatic interactions between whey proteins and pectin during heating led to finer gel microstructure formation with less porosity and smoother gel network compared to polymer/pectin gels which had limited interaction between the biopolymers. The interaction between protein and pectin increased with lowering the initial pH of the soluble complexes. As a result, a change from a coarse strand, phase separated to a homogenous microstructure upon lowering the initial pH from 7.0 to 6.2 was observed for complex gels.