Thermal gelation of commercial whey protein concentrate: influence of pH 4.6 insoluble protein on thermal gelation

Thermal gels were prepared from solutions of seven commercial whey protein concentrates (WPCs) and were found to vary considerably in strength, as determined by a compressive rheological measurement method. Further studies were carried out on one WPC (WPC5) which had far superior gelling properties to the other six WPCs and revealed that this WPC had a substantial level of protein that was insoluble (sedimentable) at pH 4.6 on centrifugation at 10000 g for 30 min but not insoluble under similar conditions at pH 7.0. When this pH 4.6 insoluble material was removed the gelling properties of WPC5 decreased considerably. Alkaline treatment (pH 9.0 for 3 hours) of a WPC5 supernatant devoid of pH 4.6 insoluble material resulted in a substantial improvement in gelling properties and generation of further pH 4.6 insoluble material. Gel electrophoresis studies and differential scanning calorimetry confirmed that the pH 4.6 insoluble material recovered from WPC5 and the alkaline treated WPC5 supernatant contained denatured protein which was associated via covalent and non-covalent interactions. Transmission electron microscopy of thermal gels prepared from WPC5 solutions containing and devoid of pH 4.6 insoluble material indicated that gel microstructure was dependent on the presence or absence of this denatured aggregated protein material in dispersion prior to heat treatment. Overall, the results suggested that the pH 4.6 insoluble, but pH 7.0 soluble, protein present in a WPC5 dispersion influenced gel microstructure on heating in a manner that had a positive influence on gel rheology.