Rice starches. III. Structural aspects provide insight in amylopectin retrogradation properties and gel texture

Abstract

Abstract The long term amylopectin retrogradation behaviour of five waxy (low peak gelatinisation temperature ( T p )) and 10 normal (low (three), intermediate (four) and high (three) T p )) rice starches was investigated by differential scanning calorimetry (DSC) rescan following gelatinisation in excess water (1:2 w/w starch/water). Textural attributes of rice starch gels (stored during 2 days or 2 weeks at 6 °C) were studied following gelatinisation (8% dry matter starch) in a Brabender visco-amylograph. Both amylopectin retrogradation and gel textural characteristics were related to absolute (AAM), free (FAM) and lipid-complexed (LAM) amylose contents and amylopectin chain length distributions. Upon storage at ambient temperature (2 and 4 weeks), retrograded amylopectin DSC conclusion temperatures were not related to AAM and FAM contents. Upon 4 weeks storage at 6 °C, however, retrograded amylopectin conclusion temperatures and enthalpies of the investigated normal intermediate and high T p rice starches were negatively related to both AAM and FAM contents. LAM was positively correlated with retrograded amylopectin conclusion temperatures and enthalpies in the case of 4 weeks at ambient temperature and 6 °C, suggesting heterogeneous nucleation. Relative amounts of amylopectin chains with DP 6-9 and DP>25 decreased amylopectin retrogradation conclusion temperatures and enthalpies, whereas opposite results were observed for chains of DP 12-22. Higher correlations were found with increasing storage temperatures and times. Firmness and force after relaxation increased and elasticity (2 days of storage at 6 °C) decreased with increasing AAM and FAM contents, whereas no correlations with LAM were observed. No significant correlations between amylopectin chain length distribution and gel texture were observed.