ESTUDIO FISICOQUÍMICO, NUTRICIONAL Y ESTRUCTURAL DEL ALMIDÓN DE VARIEDADES DE ARROZ (Oryza sativa subespecie indica) CULTIVADAS EN MÉXICO

Abstract

The quality of the rice grain is influenced by the physicochemical properties and structure of starch. In Mexico, there is not information regarding the features of starch present in rice varieties. The objective of this work was to analyze the physicochemical, nutritional and structural properties of rice starch (Oryza sativa subspecies indica), and study the relationship among them. Nine rice varieties, representing the three types of rice sold in Mexico, were used. The shape of the grains of all varieties studied was medium. Protein, lipid and ash contents were 6.99-11.0%, 0.47-1.22%, 0.48-1.18%, respectively and were significantly higher (=0.05) for A06 variety. Amylose content ranged 24.3-30.4%, based on this parameter rice varieties were classified as intermediate and high amylose content. According to their peak gelatinization temperature (Tp) varieties were classified in two groups, the group A (A06, Campechano, Cotaxtla and Filipino) with Tp<65 °C and group B (A92, A98, Champoton, Culiacan and Huimanguillo) with Tp>74 °C. Gelatinization enthalpy (ΔHG) was significantly higher for rice varieties of group B. Tp and ΔH of amylose-lipid complex type I during gelatinization ranged between 96.9- 101.5 °C and 0.5-1.3 J/g, respectively, while in amylose lipid complex type II Tp and ΔH were lower. Onset temperature of retrogradation (ToR) was higher for rice varieties of group A, but the gelatinization range and percent of retrogradation were lower in comparison with those of group B. Pasting temperature was similar than TpG, pasting viscosity ranged between 2699-5839 cP, 936-3272 cP, 1928-4739 cP and 3453-8156 cP for peak, breakdown, setback and final, respectively; A06 variety had the lowest viscosities. Hardness and stickiness of rice flour gels after 1 day of storage were higher for Huimanguillo variety (5.2 and 2.7 gf, respectively), after 7 days of storage hardness increased for all varieties, but stickiness did not show any trend. Cooking time followed this order: Morelos type varieties (27 min), Milagro type varieties (25 min) and Sinaloa type varieties (25 min). The evaluation of nutritional important starch fractions in raw rice showed that group A had more rapidly digestible starch (RDS) and lower slowly digestible starch (SDS) in comparison with those of group B. When samples were cooked, RDS increased for all samples, while SDS and RS (resistant starch) decreased, no differences were observed between groups. Rice starch was isolated from the varieties studied and its structure was analyzed. Cristallinity was higher for starches of group B. The amylopectin average chain length was higher for those starches of group B. Starches from group A showed higher proportion of chains A (DP 6-12) and lower proportion of chains B1 (DP 13-24), starches from group B had the opposite trend. The statistically correlation analysis showed that peak viscosity, setback viscosity, and final viscosity were affected by the amylose content, and breakdown viscosity was influenced by protein content and the proportion of amylopectin chains type A and B1. It was found that raw starch digestibility is affected by the amylopectin chain length distribution, but this relationship disappears when rice was cooked. The physicochemical and nutritional properties of rice starch showed a significantly effect of the amylopectin chain length distribution.