Effect of molar substitution on rheological properties of hydroxypropylated rice starch pastes

The steady and dynamic shear rheological properties of hydroxypropylated rice starch pastes (5%, w/w) were evaluated at different molar substitution (MS, 0.030-0.142). The swelling power (35.5-52.8 g/g) and solubility (8.19-10.7%) values of the hydroxyproylated rice starches were higher than those of native rice starch (26.6 g/g and 7.78%) and increased with an increase in MS. The hydroxypropylated starch pastes at 25°C showed a pronounced shear-thinning behavior (n = 0.33-0.40) with Casson yield stress (σ₀₀ = 15.9-31.7 Pa). The consistency index (K) and yield stress (σ₀₀) values of the hydroxypropylated starch pastes were lower than those of the native starch, and increased progressively with an increase in MS. The apparent viscosity (η_a,500) obeyed the Arrhenius temperature relationship over the temperature range of 10-550C; the activation energies (E_a) of the hydroxypropylated starch pastes were in the range of 14.8-18.5 kJ/mol, i.e. higher than that (14.1 kJ/mol) of the native starch. Storage (G′) and loss moduli (G″) of hydroxypropylated starch pastes increased with an increase in MS, while tan δ(G″/G′) values decreased, indicating that G′ rose more strongly than G″ with increased MS.