MOLECULAR-ORIGIN FOR THE THERMAL-STABILITY OF RICE AMYLOPECTIN

The non-Newtonian behavior and dynamic viscoelasticity of Takanari and Reimei amylopectin solutions were measured with a rheogoniometer. The Takanari and Reimei amylopectin showed plastic behavior at a concentr ation above 2.0% at 25 degrees C. The viscosity of Takanari amylopecti n decreased a little with increasing temperature at 2.0%. However, a l ittle increase in the viscosity was observed with increasing temperatu re from 0 to 15 degrees C, then it stayed at a constant value with fur ther increase in the temperature up to 80 degrees C at a concentration above 4.0%. An increase in the viscosity was also observed in Reimei amylopectin solution at various concentrations. The dynamic viscoelast icity of Takanari amylopectin increased with increasing concentration at low temperature (0 degrees C) and it stayed at a constant value wit h increasing temperature up to 80 degrees C. On the other hand, dynami c viscoelasticity for Reimei amylopectin showed a weak sigmoid curve. The tan delta of both amylopectins showed low values, 0.32-0.38, at lo w temperature range and kept constant with increasing temperature up t o 80 degrees C. A little decrease of dynamic modulus of Takanari and R eimei amylopectin was observed upon addition of urea (4.0 M). The dyna mic modulus of Takanari and Reimei amylopectin solution decreased rapi dly when the temperature reached 45 and 60 degrees C, which was estima ted to be a transition temperature, in 0.10 N NaOH solution. The molec ular origin for the thermal stability of rice amylopectin (Takanari an d Reimei) was essentially attributed to intramolecular associations in aqueous solution. Possible mode of intramolecular hydrogen bonding an d van der Waals forces of attraction of amylopectin molecules are prop osed.