Effect of acetylation on rheological properties of rice starch

Rheological properties of acetylated rice starch pastes (5%, w/w) as a function of acetyl degree of substitution (0.025-0.104) were evaluated in steady and dynamic shear. The acetylated rice starch pastes at 25 degrees C had high shear-thinning behavior (n = 0.17-0.20) with high Casson yield stress (sigma(oc) = 47.4-84.1 Pa). Magnitudes of consistency index (K) and yield stress (sigma(oc)) increased with increase in acetyl substitution degree. Apparent viscosity (eta(a,100)) obeyed the Arrhenius temperature relationship over the temperature range of 10-55 degrees C; the activation energy (E-a) values of acetylated starch pastes were in the range of 9.46-11.4 kJ/mol. Magnitudes of storage modulus (G'), loss modulus (G") and complex viscosity (eta*) of acetylated starch pastes increased with increase in degree of acetyl substitution, and the magnitudes of G' were also much greater than those of G" and eta* at all values of frequencies (CO). The Cox-Merz superposition rule did not describe the relationship between dynamic (eta*) and apparent viscosity (eta(a)) for all samples.