Steady and dynamic shear rheology of rice starch-galactomannan mixtures

Rheological properties of rice starch-galactomannan mixtures (5%, w/w) at different concentrations (0, 0.2, 0.4, 0.6 and 0.8%, w/w) of guar gum and locust bean gum (LBG) were investigated in steady and dynamic shear. Rice starch-galactomannan mixtures showed high shear-thinning flow behaviors with high Casson yield stress. Consistency index (K), apparent viscosity (eta(a,100)) and yield stress (sigma(OC)) increased with the increase in gum concentration. Over the temperature range of 20-65 degrees C, the effect of temperature on apparent viscosity (eta(a,100)) was described by the Arrhenius equation. The activation energy values (E-a = 4.82-9.48 kJ/mol) of rice starch-galactomannan mixtures (0.2-0.8% gum concentration) were much lower than that (E-a = 12.8 kJ/mol) of rice starch dispersion with no added gum. E-a values of rice starch-LBG mixtures were lower in comparison to rice starch-guar gum mixtures. Storage (G') and loss (G) moduli of rice starch-galactomannan mixtures increased with the increase in frequency (omega), while complex viscosity (eta*) decreased. The magnitudes of G' and G increased with the increase in gum concentration. Dynamic rheological data of In (G', G) versus In frequency (omega) of rice starch-galactomannan mixtures have positive slopes with G' greater than G over most of the frequency range, indicating that their dynamic rheological behavior seems to be a weak gel-like behavior.