Molecular characteristics influencing retrogradation kinetics of rice amylopectins

The enthalpy changes (Delta H) for melting of crystallites formed during retrogradation of 60% (w/w) amylopectins (AP) aged at 4 degrees 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 (less than or equal to 7 days) and late (greater than or equal to 7 days) storage. The Avrami exponent for early-stage kinetics (n(1), 1.04-5.54) was greater than the corresponding value for late-stage kinetics (n(2). 0.28-1.52). While the Avrami K constant of the early-stage kinetics (K-1, 1.0x10(-5) to 2.3x10(-1) day(-n)) was lower than the corresponding value of late-stage kinetics (K-2, 4.4x10(-2) to 1.4 day(-n)). The Delta H values for late and infinite retrogradation stages showed a significantly positive correlation with the proportions of short chain (chain length [CL] less than or equal to 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 n(1) values and smaller K-1 values. Values for n(2) and K-2 showed little influence from the molecular properties except for the proportion of extra long (CL > 100 glucose units) and long chain fractions on K-2. The negatively linear relationships between log K and n suggest the importance of some nonstructural factors for AP retrogradation mechanisms in various starch systems.