Gelling properties of whey protein isolate: influence of calcium removal by dialysis or diafiltration at acid or neutral pH

Dialysis of whey protein isolates (WPI) removed much more calcium when carried out at an acid pH (close to 4.0) than at neutral pH. Diafiltration at acid pH was also effective. The characteristics of thermally-induced gels prepared from WPI dialysed at acid or neutral pH were studied at pH 3.75 or pH 7.0, respectively, and at calcium concentrations ranging from 0 to about 60mM (with addition of calcium chloride). The water-holding capacity (WHC) and elasticity of gels increased with decreasing calcium concentration, at both pHs. Gel firmness was maximum at 10–20 mM calcium. The solubility of the protein constituents of WPI gels in a pH 8.0 buffer was high in the case of acid gels (especially at calcium concentrations lower or equal to 20 mM) and low for neutral gels at all calcium concentrations. Protein solubility values in the presence or absence of denaturing and reducing agents reflect the existence of intermolecular disulphide bonds in neutral gels and their absence in acid gels.