Amylose Content and Amylopectin Fine Structure of Thai Rice Starches

The amylose content and amylopectin fine structure of five Thai rice starches were investigated. Amylose determined by the Con A method correlated well with the water-soluble amylose (AM) determined by the iodine method, but was less than the amylose equivalent (AE) also determined by the iodine method. The difference between AE and AM is referred to as waterinsoluble amylose (AELBCA). Both AE and AELBCA increased exponentially with AM (AM range 2.56 23.08%). In contrast, neither AE nor AELBCA related to AM for Indian rice having medium amylose content of AM of 9.4-18.2%. The distribution profiles of amylopectin chain length of different rice starches clearly shows that higher amylose rice starches had fewer chains of DPn 3-11 than low amylose and medium amylose rice starches, but the former contained more chains of DPn 12-24 than the latter. The rice starches with low amylose and medium amylose contents had an amylopectin chain ratio (ACR) of 0.25 (S-type amylopectin), while the rice starches with high amylose contents had an ACR of 0.19 (L-type amylopectin). Introduction Amylose and amylopectin are the main component of starch, affecting the physical properties of starch such as gelatinization, retrogradation, viscosity and viscoelasticity. Amylose is quantitatively determined by several methods, and variable results have been reported. The iodine binding has been most commonly used for this purpose because it is specific, sensitive, and easy to determine qualitatively and quantitatively by a spectrophotometer. However, these methods are subject to uncertainties. Amylopectin-iodine complexes also form. These lead to an overestimation of amylose, requiring corrections to be applied. The specific formation of amylopectin complexes with the lectin concanavalin A (Con A) offers an alternative approach to amylose measurement in starches that is not subject to these uncertainties. Under defined conditions of pH, temperature and ionic strength, Con A specifically complexes branched polysaccharides based on -D-glucopyranosyl or D-mannopyranosyl units at multiple non-reducing end groups with the formation of a precipitate. Rice amylopectins have -amylolysis limits of 56-59% and widely different DP and linear chain distribution values. Low average DP values are observed for indica (4,700), as compared to those for Japonica rice (12,800) and waxy rice (18,500) amylopectin. The chain length distribution of rice amylopectin is analyzed by conventional GPC, SEC-MALLS-RI or high-performance-anionexchange chromatography system equipped with pulsed amperometric detection after enzymatic branching. The amylopectin having more long cluster-chains was designated as an L-type amylopectin and that with more short cluster-chains in the latter group as an S-type amylopectin. The difference in structure between L-type and S-type amylopectins was attributed to difference in the lengths of A and B1 chains constituting the unit cluster of the amylopectin molecule The objective of this study was to investigate the amylose content and amylopectin fine structure of five Thai rice starches. Materials and Methods Materials Fresh rices (RD6, Jasmine rice named Khao Dawk Mali 105 and Supanburi 1) were stored at 33 2 C for a designated time of 0 and 5 months, then starches were extracted by alkaline method (Yamamoto et al., 1973 and Juliano, 1984b). Two commercial rice starches samples were also studied.