| Abstract: | Single-component gels were prepared by cold-setting aqueous preparations of thermally processed milk and soya proteins. Small deformation mechanical measurements on soya protein samples showed a strong elastic response (G′) even at the hydration temperature (50°C). Both proteins produced an initial monotonie increase in G′ on cooling, followed by a relatively constant modulus during a subsequent time sweep at the setting temperature (5°C). Networks were fully reversible on heating; the milk protein gels melting out completely at temperatures >60°C, whereas the soya protein gels maintained significant structure even at the highest accessible temperature (95°C). The lack of thermal hysteresis or of sharp, cooperative melting was also confirmed by differential scanning calorimetry. Further investigation of the macromolecular properties of the gels, comprising G′ dependence as a function of frequency of oscillation and creep experiments, suggests that gels remain stable within the time scale of the measurements (90 min). Finally, under increasing amplitude of oscillation, networks withstood structural breakdown up to strain levels of ~70%; behaviour anticipated for biopolymer gels. Mixed gels were studied using a fixed amount of milk protein (10% w/w) with soya protein concentrations from 6 (minimum gelling requirement) to 16% w/w (solubility limit). Comparison of melting profiles (G′ vs. T) for the phase separated systems with those obtained for the individual components indicated phase-inversion from a milk protein continuous network to a soya continuous system at a soya protein concentration of ~11%. Analysis of solvent partition between the constituent phases utilized classical theory of network deswelling for polymer combinations below the phase inversion point and phase equilibria treatment for the soya continuous network with milk protein inclusions. In the case of equilibrium separation of the two components, results were expressed in terms of a single adjustable parameter, p (the ratio of solvent to polymer in one phase divided by the corresponding ratio in the other phase), indicating a soya hydrophilicity of ~1.25 times that of milk protein. |
