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Molecular Characteristics Influencing Retrogradation Kinetics of Rice Amylopectins

¹Department of Food and Nutrition, Providence University, Shalu, Taichung 43301, Taiwan. ²Department of Food Science, National Chung-Hsing University, Taichung 40227, Taiwan. ³Institute of Chemistry, Academia Sinica, Nankang, Taipei 11529, Taiwan. Graduate Institute of Food Science and Technology, National Taiwan University, Taipei 10764, Taiwan. Corresponding author. E-mail: cylii@chem.sinica.edu.tw. Fax: 886-2-2783-1237.

The enthalpy changes (ΔH) for melting of crystallites formed during retrogradation of 60% (w/w) amylopectins (AP) aged at 4°C were investigated using AP from 13 rice cultivars with well-known structural properties. According to the Avrami equation, the resultant kinetic parameters for AP retrogradation were obtained in relation to structural factors. Generally, the AP systems studied showed two stages of retrogradation behavior during early (≤7 days) and late (≥7 days) storage. The Avrami exponent for early-stage kinetics (n₁, 1.04–5.54) was greater than the corresponding value for late-stage kinetics (n₂, 0.28–1.52). While the Avrami K constant of the early-stage kinetics (K₁, 1.0×10^-5 to 2.3×10^-1 day^-n) was lower than the corresponding value of late-stage kinetics (K₂, 4.4×10^-2 to 1.4 day^-n). The ΔH values for late and infinite retrogradation stages showed a significantly positive correlation with the proportions of short chain (chain length [CL] ≤ 15 glucose units) and long chain (CL = 16–100 glucose units) fractions, respectively. Retrogradation of AP with a higher number-average degree of polymerization, greater proportion of short chain fractions, and shorter average chain lengths revealed significantly greater n₁ values and smaller K₁ values. Values for n₂ and K₂ showed little influence from the molecular properties except for the proportion of extra long (CL>100 glucose units) and long chain fractions on K₂. The negatively linear relationships between log K and n suggest the importance of some nonstructural factors for AP retrogradation mechanisms in various starch systems.