354 Effects of molecular structure on the retrogradation kinetics of rice amylopectins.

VIVIAN M.-F. LAI (1), Lu, Shin (2) and Cheng-Yi Lii (3). (1) Department of Food and Nutrition, Providence University, Shalu, Taichung 43301, Taiwan; (2) Department of Food Science, National Chung-Hsing University, Taichung 40227, Taiwan; (3) Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan.

The two-stage retrogradation kinetics of fourteen rice amylopectins aged at 5 C was investigated by differential scanning calorimetry (DSC). The retrogradation enthalpies of 60% amylopectins were present in terms of aging time according to the Avrami equation. It appears that the rate constant (k) and the Avrami exponent (n) during aging of short-term periods (0–7 days) differed from those of long term periods (>7 days). The results of correlation analysis suggested that for short-term retrogradation the k significantly increased, but a decreased n value, with increasing the molar fraction of extralong and long chains, the average chain length (CL), and the average external chain length (ECL) of the amylopectins examined. As to the long-term retrogradation, only the molar fraction of short and long chains gave significant influences on k value, rather than on n value. The role of the molecular properties of rice amylopectins in controlling the physical properties of their parent flours and starches or the eating quality of cooked rice were also discussed.