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 degrees C showed a pronounced shear-thinning behavior (n = 0.33-0.40) with Casson yield stress (sigma(oc) = 15.9-31.7 Pa). The consistency index (K) and yield stress (sigma(oc)) 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 (eta(a,500)) obeyed the Arrhenius temperature relationship over the temperature range of 10-55 degrees C; 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 delta (G"IG') values decreased, indicating that G' rose more strongly than G" with increased MS.