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°C had high shear-thinning behavior (n = 0.17-0.20) with high Casson yield stress (σ_oc = 47.4-84.1 Pa). Magnitudes of consistency index (K) and yield stress (σ_oc) increased with increase in acetyl substitution degree. Apparent viscosity (η_a,100) obeyed the Arrhenius temperature relationship over the temperature range of 10-55°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 (η*) 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 η* at all values of frequencies (ω). The Cox-Merz superposition rule did not describe the relationship between dynamic (η*) and apparent viscosity (η_a) for all samples.