Molecular characteristics influencing retrogradation kinetics of rice amylopectins

The enthalpy changes (Delta H) for melting of crystallites formed during re trogradation of 60% (w/w) amylopectins (AP) aged at 4 degrees C were invest igated using AP from 13 rice cultivars with well-known structural propertie s. According to the Avrami equation, the resultant kinetic parameters for A P retrogradation were obtained in relation to structural factors. Generally , the AP systems studied showed two stages of retrogradation behavior durin g 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 f or late and infinite retrogradation stages showed a significantly positive correlation with the proportions of short chain (chain length [CL] less tha n 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 an d 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 relat ionships between log K and n suggest the importance of some nonstructural f actors for AP retrogradation mechanisms in various starch systems.