Rheological study on the fractal nature of the protein gel structure

The effects of ionic strength on fractal structures in heat-induced gels prepared from globular proteins were investigated in the framework of a fractal aggregation of colloidal particles. All gels formed at 90 degrees C exhibited power law relationships between the storage shear modulus (G') and protein concentration. At 25 mM NaCl, the fractal dimension, d (similar to 2.2), calculated based on the value of the power law exponent agreed with those for reaction-limited cluster-cluster aggregation. Further addition of NaCl (50, 80, 500, 1000 mM) decreased the values of d (similar to 1.8), which agreed with d for diffusion-limited cluster-cluster aggregation. These results suggest that the predominant effect of an increase in ionic strength on globular protein gelation is ascribed to shielding charges on the surface of the proteins, thereby increasing the reaction probability of protein aggregation. The effective structure-determining rheological properties of heat-induced protein gels are characterized by fractal dimensions deduced from the rheological model, which also suggests that the size of primary flocs building fractal structures is one of the important factors that determines the linear viscoelastic properties of the gels.