Properties of Flours and Starches as Affected by Rough Rice Drying Regime

Cereal Chem. 80(1):30–34 Flours and starches from rough rice dried using different treatment combinations of air temperature (T) and relative humidity (RH) were studied to better understand the effect of drying regime on rice functionality. Rough rice from cultivars Bengal and Cypress were dried to a moisture content of 12% by three drying regimes: low temperature (T 20°C, RH 50%), medium temperature (T 40°C, RH 12%), and high temperature (T 60, RH 17%). Head rice grains were processed into flour and starch and evaluated for pasting characteristics with a Brabender Viscoamylograph, thermal properties with differential scanning calorimetry, starch molecular-size distribution with highperformance size-exclusion chromatography (HPSEC), and amylopectin chain-length distribution with high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD). Lower head rice and starch yields were obtained from the batch dried at 60°C which were accompanied by an increase in total soluble solids and total carbohydrates in the pooled alkaline supernatant and wash water used in extracting the starch. Drying regime caused no apparent changes on starch molecular-size distribution and amylopectin chain-length distribution. Starch fine structure differences were due to cultivar. The pasting properties of flour were affected by the drying treatments while those of starch were not, suggesting that the grain components removed in the isolation of starch by alkaline-steeping were important to the observed drying-related changes in rice functionality. For safe storage and milling, rough rice needs to be dried to a moisture content of 12% after harvest. Rice is often air-dried at low (35°C) to high (60°C) temperature, with the level of temperature generally dictated by the type of drying equipment. Regardless of the method used in drying rice, the ultimate goal is that the drying operation itself should be efficient without causing deleterious effects on rice end-use quality. To command a premium price, rice kernels should remain intact after milling, thus it is important to avoid drying conditions that promote breakage. High temperatures can be used in drying rough rice without consequent reduction in head rice yield as long as proper tempering techniques are employed (Cnossen and Siebenmorgen 2000). However, some changes in rice functionality associated with the temperature and time used in drying have been reported (Iwasaki and Tani 1966; Sato et al 1971; Saito et al 1974a; Champagne et al 1998; Meullenet et al 1999; Inprasit and Noomhorn 2001; Wiset et al 2001). Iwasaki and Tani (1966) reported that heating exerted an effect similar to aging on rice texture based on some cooking quality tests. Heatdried rice produced a lower quantity of crackers compared with room temperature-dried rice (Saito et al 1974a). Champagne et al (1998) found that a commercial drying temperature of 60°C resulted in cooked rice with higher cohesiveness based on texture profile analysis with a texture analyzer. Meullenet et al (1999) observed that a high drying temperature (54.3°C) resulted in less firm cooked rice kernels but with higher cohesiveness of mass based on sensory evaluation by a trained panel. Water absorption and stickiness-to-hardness ratio decreased while b-value and hardness of cooked rice increased with drying temperature, exposure and drying duration, longer tempering time, and rate of water removal (Inprasit and Noomhorn 2001). Inprasit and Noomhorn (2001) added that rice cooking and eating qualities were changed by single and multistage commercial drying. Wiset et al (2001) noted that head rice yield, amylose content, gel consistency, water absorption, and volume expansion were affected by the method or temperature used in drying rough rice. The mechanisms governing the changes in rice functionality associated with the conditions used in drying rough rice are not fully understood. Shcherbakov et al (1977) noticed that drying rice at >50°C resulted in a temperature-dependent decrease in soluble proteins, particularly albumins, globulins, and prolamins, and increases in free amino acids and insoluble proteins. Federova et al (1977) used a drying temperature range of 35–120°C and found that high drying temperatures modified rice protein composition, reduced soluble proteins, and increased nonprotein nitrogen compounds. Increasing drying temperature brought about a decrease in free lipids and an increase in bound lipids (Shcherbakov et al 1977). Increasing drying temperature also increased enzymatic activity up to a certain limit, after which it decreased (Federova et al 1977). Saito et al (1974b) observed higher catalase and protease activities in heat-dried grains (30–50°C) than those dried at room temperature but no marked difference was detected in other physical and chemical properties. It is well known that changes in starch properties during cereal processing affect product quality, especially texture, because starch is a main component in cereals. Starch owes much of its functionality to two major components, amylose and amylopectin, as well as to the physical organization of these macromolecules into the granule structure. Literatures on rice starch changes associated with drying are inadequate. A more thorough study of the fine structures and other physicochemical properties of starch may provide a deeper insight about the effect of drying regime on milled rice end-use quality. Hence, this work assessed the structures and physicochemical properties of starches isolated from rough rice dried using different combinations of air temperature and relative humidity. A comparison between the physicochemical properties of flour and starch samples was also made to account for the importance of other grain components that are present in flour but are removed during starch extraction. MATERIALS AND METHODS Materials Rough rice samples from cultivars Bengal (medium-grain) and Cypress (long-grain) were obtained from the 2001 crop of the University of Arkansas Rice Research and Extension Center in Stuttgart, AR. The harvest moisture content (MC) of the samples was 22.0 and 20.5% for Bengal and Cypress, respectively. Drying of Rough Rice Rough rice samples were dried by thin-layer drying as described by Cnossen and Siebenmorgen (2000). One batch was dried with air at 60°C and 17.0% relative humidity (RH), corresponding to an equilibrium moisture content (EMC) of 5.5%, for 40 min. Another batch was dried at 40 C and 12% RH, corresponding to an EMC of 5.8%, for 90 min. Immediately after drying, the samples were tempered in sealed plastic bags for 3 hr at the temperature of the drying air. The samples were then taken out of the sealed bag, spread evenly on meshed trays, and then gently cooled and dried in a chamber 1 Department of Food Science, University of Arkansas, Fayetteville, AR 72704. 2 Corresponding author. Phone: 1-501-575-3871. Fax: 1-501-575-6936. E-mail: [email protected]. 3 Department of Food Science and Human Nutrition, Iowa State University,