Fine Structures of Starches from Long-Grain Rice Cultivars with Different Functionality

Cereal Chem. 79(3):465–469 The structural features of rice starch that may contribute to differences in the functionality of three long-grain rice cultivars were studied. Dried rough rice samples of cultivars Cypress, Drew, and Wells were analyzed for milling quality, grain physical attributes, and starch structures and physicochemical properties. Drew was lower in head rice yield and translucency and higher in percentage of chalky grains compared with Cypress and Wells. Apparent amylose content (21.3–23.1%), crude protein (8.3–8.6%), and crude fat (0.48–0.64%) of milled rice flours were comparable, but pasting properties of rice flours as measured by viscoamylography, as well as starch iodine affinity and thermal properties determined by differential scanning calorimetry were different for the three cultivars. Drew had higher peak, hot paste, and breakdown viscosities, and gelatinization temperature and enthalpy. Molecular size distribution of starch fractions determined by high-performance sizeexclusion chromatography showed that the three samples were similar in amylose content (AM) (20.0–21.8%) but differed in amylopectin (AP) (64.7–68.3%) and intermediate material (IM) (10.9–13.5%). Drew had highest AP and lowest IM contents, whereas Cypress had the lowest AP and highest IM contents. High-performance anion-exchange chromatography of isoamylase-debranched starch indicated that the AP of Drew was lower in A and B1 chains but higher in B2, B3, and longer chains. Long-grain rice cultivars constitute a majority of the rice acreage in Arkansas and the rest of the southern United States (USDA 2001). These cultivars are generally characterized by a minimum grain-to-width ratio of 3.0:1, intermediate apparent amylose content (21–24%), intermediate alkali spreading values (3–5), and dry, fluffy, and separate grains when cooked (Gravois and Webb 1997). However, as a result of increasing crossbreeding activities, secondary differences in the cooking, eating, and processing characteristics among cultivars from the same grain type and the same apparent amylose class have been reported (Juliano 1998). It is not surprising that some U.S. rice processors have complained about rices with similar quality parameters but different in processing behavior (Kohlwey 1994; Juliano 1998). There is a dire need for alternative parameters that could adequately explain the complexities in rice functionality, especially within similar grain type. Empirical tests like gel consistency and amylograph viscosity have been employed to discriminate secondary differences in rice functionality, but these tests mainly serve as quality indices rather than explaining the cause. To better understand the determinants of rice quality, the chemical basis needs to be further defined; past research has demonstrated that the quality aspects of rice are due to multiple factors. The major component of rice grain is a complex carbohydrate, starch, which constitutes ≈90% of milled rice on a dry weight basis (dwb). Starch owes much of its functionality to two major components, amylose and amylopectin, as well as to the physical organization of these macromolecules in the granule structure. To gain a better understanding about the starch structure-functionality mechanisms in rice, we examined the structural features of the starches isolated from three long-grain rice cultivars, Cypress, Drew, and Wells, which exhibit different grain quality and functional properties. Our ultimate goal was to understand whether these cultivars showed distinct differences in starch molecular size distribution and amylopectin branch chain-length distribution, which may substantially explain observed variations in the functionality of long-grain rices such as milling quality, chalkiness, gelatinization, pasting behavior, and the like. A more thorough characterization of starch fine structures and other physicochemical properties of rice cultivars Cypress, Drew, and Wells would likewise enhance their utilization in food and other value-added applications. MATERIALS AND METHODS Materials Rough rice samples of cultivars Cypress, Drew, and Wells were obtained from the 1999 crop of the University of Arkansas Rice Research and Extension Center at Stuttgart, AR. Samples were dried in a conditioning chamber controlled at 21°C and 50% rh and equilibrated to a target moisture content of 12% (wb). Dried samples were stored in self-sealing plastic bags under ambient conditions before analyses. Milling Quality Duplicate samples of 150 g of rough rice were dehulled in a dehusker (THU-35, Satake, Hiroshima, Japan). The brown rice was weighed and polished for 30 sec in a friction mill (McGill Miller #2, Rapsco, Brookshire, Texas). Resulting milled rice was weighed and separated into head rice and broken kernels on a double-tray shaker table (GainMan Machinery, Miami, FL) with 4.76-mm indentation on both trays. Brown rice, milled rice, and head rice yields were calculated as percentage by weight of rough rice. Physical Attributes of Rice Grain In 100-grain duplicate samples, the dimensions (length, width, and thickness) of head rice were obtained with a rice image analyzer (Satake) equipped with a NaiS image checker 30R. Chalky kernels (%) were measured by ocular inspection of 50 g of duplicate head rice samples (Patindol 2000). Grain translucency and whiteness were measured with a Satake milling meter (model MM-1B). Physicochemical Properties of Rice Flour Head rice was ground into flour with a cyclone sample mill (Udy Corp. Ft. Collins, CO) fitted with a 100-mesh sieve. Duplicate 10-g samples were used to determine moisture content on an infrared moisture balance (model MB200, Ohaus, Florham PK, NJ). Apparent amylose content was determined by iodine colorimetry (Juliano et al 1981). Crude protein was measured according to Approved Method 46-13 (AACC 2000) and crude fat by Soxhlet extraction with petroleum ether (HT1043 Extraction Unit, Tecator). Pasting Properties of Rice Flour Pasting properties of rice flour were characterized according to Approved Method 61-01 (AACC 2000) with a Viskograph-E (C.W. Brabender Instruments, South Hackensack, NJ). The gelatinized flour pastes prepared by the Viskograph-E were used to measure gel strength with a TA-TX2 texture analyzer (Texture Technologies, Scarsdale, NY) after five days of storage at 5°C. 1 Department of Food Science, University of Arkansas, Fayetteville, AR 72704. 2 Corresponding author. E-mail: [email protected] Phone: 479-575-3871. Fax: 479-