Gelatinization mechanism of rice starch

The non-Newtonian behavior and dynamic viscoelasticity of rice starch (Niho nbare; amylose content, 15.8%) solutions were measured with a rheogoniomete r. A gelatinization of Nihonbare starch occurred above 3.0% after heating a t 100 degrees C for 30 min. The Nihonbare starch showed shear-thinning beha vior at a concentration of 2.0%, but plastic behavior above 3.0% at 25 degr ees C. The viscosity of Nihonbare starch at a concentration of 2.0% solutio n decreased gradually with increase in temperature from 10 to 55 degrees C, then it stayed at a constant value with further increase in the temperatur e. However, for 4.0% solution, rapid decrease in the viscosity was observed after the temperature reached 25 degrees C up to 50 degrees C, then it sta yed at a constant value. The dynamic modulus of Nihonbare starch stayed at a constant value during increase in the temperature at 4%. The tan delta of the starch showed low values, 0.28, at low temperature range and stayed at a constant up to 30 degrees C, then it increased a little with increasing temperature. A little decrease of dynamic modulus of Nihonbare starch was o bserved at low temperature range upon addition of urea (4.0 M). The dynamic modulus, however, decreased rapidly when the temperature reached 50 degree s C, which was estimated to be a transition temperature. The dynamic modulu s also decreased rapidly in 0.10 M NaOH solution above 50 degrees C. A poss ible mode of intermolecular hydrogen bonding between amylose and amylopecti n molecules of Nihonbare starch is proposed. The short chains (A and B 1) o f the amylopectin molecules may take part in the intermolecular association in aqueous solution.