Gelation properties of soya and whey protein isolate mixtures

Gelation properties of commercially available soya and whey protein isolates (15–18% w/w) were tested in isolation and when mixed in ratios ranging from 17:1 to 1:17 soya/whey to give a final protein concentration of 18% protein (w/w) in distilled water. The isolates and mixtures were investigated by both small and large deformation rheological analysis, which indicated phase separation of the two proteins systems. Phase inversion occurred at 5:1 ratio (soya/whey) where the lowest shear modulus value, by large deformation testing, was 3.5 compared to 5.5 for 18% (w/w) soya and 10.7 for 18% (w/w) whey proteins in distilled water. The G′ value obtained by small deformation testing, also showed the lowest value at the 5:1 ratio (soya/whey proteins). In addition, gels exhibited maximum syneresis at this ratio indicating instability at the point of inversion. Electron microscopy in combination with immunocytochemistry using anti-soya or anti-whey antibodies also indicated phase separation of the two types of proteins. Phase separation was considered to occur due to differences in the molecular sizes of the proteins and physical–chemical differences including hydrophobicity as verified by the cis-parinaric fluorescent probe technique.