Acid-induced gelation properties of heated whey protein−pectin soluble complex (Part II): Effect of charge density of pectin

As a direct continuation of the first part of our study, here we investigate the effect of charge density of pectin on acid-induced gelation properties of heated whey protein–pectin soluble complex which was formed by heating protein and pectin together at pH 7.0 and 85 °C for 30 min. The gelation properties of complex were also compared with polymer/pectin, where whey protein and pectin were heated separately and then mixed together before acid-induced gelation. Four different types of pectin with charge density ranging from low to high were chosen. For pectin with high charge density, complex gels showed enhanced gel strength and water holding capacity compared to polymer/pectin gels. For those with low charge density pectin, polymer/pectin could not form gel network at highest pectin concentrations, whereas complex still formed gel at high pectin concentrations and showed improved water holding capacity at low pectin concentrations. Confocal micrograph showed pronounced difference in gel microstructure between complex and polymer/pectin gels. An obvious increase in the coarsening of the protein network was observed as pectin concentration increased from 0.1% to 0.5% for polymer/pectin gels. Complex gels showed much smoother network with less phase separation. At 0.25% pectin a change from a homogeneous to a coarse strand, phase separated microstructure upon decreasing charge density of pectin was observed for polymer/pectin gels, while complex gels were homogeneous even with low charged pectin. Results from scanning electron microscopy confirmed that complex gels were composed of larger protein strands and higher interconnectivity between biopolymers due to higher degree of interaction between protein and pectin.