Concentration dependence of dynamic moduli of heat-induced soy protein gels

The concentration dependence of dynamic moduli of soy protein gels was studied for different protein preparations (soy protein isolate), purified glycinin and a β-conglycinin rich fraction) at various pHs and salt concentrations. The concentration dependence of the storage modulus of glycinin and β-conglycinin gels was similar to that of SPI gels. For SPI, the critical protein concentration for gelation was estimated to be between 3 and 5% at pH 7.6 (0.2 M NaCl), and between 6.5 and 8% at pH 7 (0 M NaCl), and 0% at pH 5.2 and pH 3.8 (0.2 M NaCl). At the high pHs the critical protein concentration decreased with the stage in the heating cycle. Relating the experimental data to a fractal model, both rheological and permeability measurements resulted in a consistent value for the fractal dimensionality Df (=2.3) for SPI gels at pH 3.8 and 0.2 M NaCl. At pH 5.2 and 7.6 (0.2 M NaCl), and pH 7 (0 M NaCl), the concentration dependence of the modulus could not be analysed theoretically. Likely this is due to rearrangements in the spatial structure of the network starting directly after a percolating network was formed. It causes the concentration dependence to depend on ageing time.