On solid-like rheological behaviors of globular protein solutions

Dynamic viscoelastic and steady flow properties of beta -lactoglobulin, bovine serum albumin, ovalbumin, and alpha -lactalbumin aqueous solutions were investigated at 20 degreesC. When a sinusoidal strain in the linear viscoelastic region was applied, the solutions of the globular proteins except for alpha -lactalbumin showed typical solid-like rheological behavior: the storage modulus G ' was always larger than the loss modulus G in the entire frequency range examined (0.1- 100 rad/s). Under a steady shear flow, strong shear thinning behavior was observed with increasing shear rate from 0.001 to 800 s(-1), for the globular proteins except for alpha -lactalbumin. The values of the steady shear viscosity eta were lower than those of the dynamic shear viscosity eta* at a comparable time scale of observation, violating the Cox-Merz rule, and thus suggesting that a solid-like structure in a globular protein solution was susceptible to a steady shear strain. During isothermal gelation of the protein colloids at 70 degreesC, no crossover between G ' and G was observed so that the gelation point was judged by an abrupt increase in the modulus or a sudden decrease in tan delta. (C) 2001 Elsevier Science Ltd. All rights reserved.