Molecular origin for the thermal stability of Koshihikari rice amylopectin

The non-Newtonian behavior and dynamic viscoelasticity of Koshihikari rice amylopectin (waxy type) in aqueous solution were investigated. The how curv es, at 25 degrees C, of Koshihikari amylopectin showed plastic behavior at various concentrations. The viscosity increased slightly with increase in t emperature up to 25 degrees C, then it decreased a little with further incr ease in the temperature at a concentration of 6%. The dynamic modulus showe d very large values and stayed constant during increase in temperature at 2 .0 and 4.0%, but a little increase of dynamic modulus was observed at 6.0% solution. The dynamic modulus decreased slightly upon addition of urea (4.0 M) or 85% DMSO and decreased gradually with increase in the temperature. T he dynamic modulus, however, was lower in 0.10 M NaOH solution than in aque ous solution and decreased rapidly when the temperature reached 45 degrees C, which was estimated to be a transition temperature. The results obtained support the mode of intramolecular associations within rice amylopectin mo lecules previously proposed. An anomeric hydrogen atom may take part in an intramolecular van der Waals forces of attraction with an adjacent OH-6 or methylene group of D-glucosyl residue. (C) 2000 Elsevier Science Ltd. All r ights reserved.