Heat-induced gelation of globular proteins .2. Effect of environmental factors on single-component and mixed-protein gels

In the first paper in this series, we investigated heat-induced gelation of single component systems of the globular protein, bovine serum albumin. A series of rheological experiments revealed that we can describe gelation time in terms of two physical parameters, concentration and temperature. In this paper we examine more complex systems, including the effect of pH and ionic strength, on heat-induced gelation of bovine serum albumin (BSA), and another globular protein, beta-lactoglobulin (beta-Lg). These factors significantly influenced protein gelation, a result which has not been reported in detail before. However, no detailed analysis was possible, because the behavior is very complex. We also consider the extension of both model and experiments to (quasi-) binary mixed systems, viz. the gelation of binary systems chosen from three globular proteins: BSA, beta-Lg, and alpha-lactalbumin (alpha-La). We have obtained reliable results only for the BSA and beta-Lg system because of experimental difficulties. From the data of the BSA/beta-Lg system, it was found that gelation behavior changed significantly with the ratio of the two proteins. The change in the gelation time and temperatures was not expressed as a linear equation but required a cross-term. This strongly suggests that current models for the concentration dependence of modulus for mixed biopolymer gels will need to be modified.