Relationship between viscoelastic properties and starch structure in rice from Thailand

This study continues the studies of Noosuk et al. to investigate the properties of four Thai rice starches with different amylose content published in Starch/Starke (2003, 55, 337-344). The composition, gelatinisation behaviour and paste viscosity of these starches has been discussed previously. Here the amylopectin fine structure, amounts and size of the amylose are reported and discussed with reference to the viscoelastic behavior of the gels. The viscoelastic response was measured for 6 to 15% (w/w) pastes (60 degrees C) and gels (25 degrees C) by oscillation (pastes and gels) and creep (gels). Viscoelasticity was shown by all gels at concentrations > 6% and the creep compliance could be represented by a four element Burger model. G' from oscillation, the instantaneous elastic modulus and Newtonian viscosity from creep, increased strongly with both starch and amylose concentration. Starch gels (30%, w/w) were monitored over 100 h, by Fourier Transform Infra Red spectroscopy (FTIR) and dynamic oscillatory rheology on storage at 25'C. Changes were seen at similar rates, using the Avrami model, for the two high-amylose (> 20%) samples, but not for the other starches. Development of crystallinity, at both 25 and 5 degrees C, was monitored by X-ray diffraction. Both the kinetics of retrogradation and extent of crystallinity suggest that amylopectin retrogradation is occurring in the high-amylose starch gels on storage but not in the lower amylose samples. It is suggested that amylopectin retrogradation is promoted by retrograded amylose. The fine structure of amylopectin may also be important, because samples that showed no retrogradation on storage had an amylopectin with a high proportion of amylopectin chains of DP 3-10 and a lower portion of chains of DP 11-22. In gels from waxy wheat it has been reported that retrogradation does occur, which may be related to the lower proportion of the shorter amylopectin chains in this material compared with low amylose rice.