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

Non-starch polysaccharide compositions of rice grains with respect to rice variety and degree of milling

The chemical compositions of cell wall materials (CWM) in brown and milled rice were investigated using four rice varieties, Taichung Sen 10 TCS 10, indica), Tainung 67 (TNu67, japonica), Taichung Sen Waxy 1 (TCSW 1, indica waxy), and Taichung Waxy 70 (TCW70, japonica waxy). The yield of CWM preparation, equivalent to total dietary fiber content, followed the order of TNu67 > TCS 10 > the waxy cultivars. This order also held for the water solubility and pectic substance content of the CWM preparations and the compositional ratio of arabinose to xylose of all CWM samples. Comparatively, the nonwaxy CWM were rich in pectic substances and glucans; whereas the waxy CWM counterparts were dominant with hemicellulose plus cellulose and arabinoxylan-related polysaccharides. These results were more significant for the hot-water-soluble than insoluble parts and mainly dependent of rice variety rather than the degree of milling. (c) 2006 Elsevier Ltd. All rights reserved

Effect of amylopectin structure on the gelatinization and pasting properties of selected yam (Dioscorea spp.) starches

This study was aimed at elucidating the decisive structural parameters of amylopectin which govern the gelatinization or pasting behavior of yam starch dispersions, exemplified with four selected yam starches from Dioscorea alata and D. batatas with very similar amylose contents (32.2-34.6%). The results indicated that the yam amylopectins examined showed an average degree of polymerization (DP) of 3469-4474 anhydroglucose units (AGU), average chain length (CL) of 18.8-28.5 AGU, average exterior CL (ECL) of 11.8-16.4 AGU, average interior CL (ICL) of 6.0-11.1 AGU, and a noticeable proportion of extralong (> 100 AGU) and long (40-100 AGU) chains that gave the chain ratio r((el+l)/s) (the weight ratio of extralong and long chains to short chains) of 0.88-1.40. Generally, the yam amylopectin with a lower DP and NC possessed a greater CL, ECL, ICL, and r((el+l)/s), associating with a higher blue value (BV) and maximal wavelength (lambda(max)) for their complexes with iodine. D. alata amylopectins exhibited a lower DP longer chain lengths and greater r((el+l)/s) than the D. batata amylopectins. Statistically, r((el+l)/s) was decisive for the BV and lambda(max); while CL and phosphate content were crucial for the gelatinization peak temperature and pasting viscosities of the yam starches examined. The role of swelling power or the volume fraction of dispersed remnants instead in governing the pasting viscosity was also mathematically explored