Abstract
Fracture properties of heat-induced whey protein gels were evaluated by applying torsional deformation. The torsional deformation method was employed since it generates equivalent magnitudes of the principle normal stress and the principle shear stress within gels. Gels with added NaCl showed a sharp increase in fracture stress values to a maximum at 100mM NaCl and a sharp decrease in fracture strain values to a minimum at 100mM NaCl with increasing NaCl concentration. Further increasing NaCl concentration caused a sharp decrease in fracture stress values and a gradual increase in fracture strain values. These results suggest that fine-stranded networks were formed at low levels of added NaCl (30-50mM) and particulate gels were formed at high NaCl concentrations (300-500mM). At lower pHs than the isoelectric points of whey proteins, brittle gels, with very low fracture stress and fracture strain values, were formed, suggesting that gels formed at low pH should exhibit remarkable differences in texture or sensory attributes, compared with gels formed at neutral pH. However, storage modulus values of gels formed at pH 3 or 4 were larger than those of gels formed at pH 7. It thus appeared difficult to predict textural properties of gels based only on dynamic viscoelastic properties determined in the linear strain region.
