The phase diagram, structural evolution, and kinetics of temperature-induced protein gelation of protein Bovine Serum Albumin (BSA) have been studied as a function of solution $p\mathrm{H}$ and protein concentration. The protein gelation temperature represents the onset of turbidity in the protein solution, which increases significantly with increasing $p\mathrm{H}$ beyond the isoelectric $p\mathrm{H}$ of the protein molecule. On the other hand, the gelation temperature decreases with an increase in protein concentration only in the low-protein-concentration regime and shows a small increasing trend at higher protein concentrations. The structural evolution and kinetics of protein gelation have been studied using small-angle neutron scattering. The structure of the protein molecule remains stable up to temperatures very close to the gelation temperature. On increasing the temperature above the gelation temperature, the protein solution exhibits a fractal structure, an indication of gel formation due to aggregation. The fractal dimension of the gel increases with increasing temperature, suggesting an increase in branching between the aggregates, which leads to stronger gels. The increase in both solution $p\mathrm{H}$ and protein concentration is found to delay the growth in the fractal structure and its saturation. The kinetics of gelation has been studied using the temperature-jump process of heating. It is found that the structure of the protein gels remains invariant after the heating time $(\sim 1\min )$, indicating a rapid formation of gel structure within this time. The protein gels prepared through gradual and temperature-jump heating routes do not always show the same structure. In particular, at higher temperatures (e.g., $85°\mathrm{C}$), while gradual heating shows a fractal structure, there is collapse of such fractal structure during temperature-jump heating.

• Received 11 November 2008

DOI:https://doi.org/10.1103/PhysRevE.79.021912