Effects of Rough Rice Storage Conditions on the Amylograph and Cooking Properties of Medium-Grain Rice cv. Bengal

Cereal Chem. 74(6):864–867 Rough rice (cv. Bengal) was stored at four moisture contents (8.8, 10.7, 12.9, and 13.6% MC) and three temperatures (3, 20, and 37°C) for up to six months. The amylograph overall paste viscosity of the milled rice increased during storage. This increase was most apparent in all samples stored at 37°C. For rice stored at 20 and 37°C at all MC levels, a 30–50% increase in peak viscosity (PV) was observed during the first three months of storage. PV subsequently leveled off for rice stored at 12.9 and 13.6% MC but declined for samples stored at 8.8 and 10.7% MC. The final viscosities also increased during storage. The water-absorption ratio of the samples during cooking in excess water increased by an average of 15% over six months of storage. The amylograph and cooking properties were significantly affected (P < 0.05) by rough rice storage duration, temperature, MC, and their respective interactions. Previous research has shown that the physicochemical properties of rice change during storage. This phenomenon is known as aging. As rice ages, head rice yield (HRY) increases (Villareal et al 1976), water absorption during cooking increases, and cooked rice texture becomes fluffier and harder (Villareal et al 1976; Indudhara Swamy et al 1978; Chrastil 1990, 1992). Additionally, Perez and Juliano (1981) showed that at 15°C these changes were most significant during the first three to four months of storage. One of the most sensitive indexes of the aging process in rice is the change in pasting properties, as measured by an amylograph. The overall viscosity of rice paste increases dramatically during storage (Villareal et al 1976, Indudhara Swamy et al 1978, Hamaker et al 1993). These changes depend on storage temperature and duration. Viscosity increases at higher storage temperatures during the first three months and tends to level off afterward. However, an extended 48-month storage study showed that viscosity began to decrease after 24 months (Indudhara Swamy et al 1978). The effect of rice moisture content (MC) during storage has not been studied as extensively as storage temperature and duration. The MC may be one of the factors that explains the differences observed in paste viscosity after prolonged storage. Attempts to explain these functionality changes have focused on the properties of rice components, such as starch, protein, and lipids, and the interactions among them during storage (Chrastil 1994). As with functionality, changes in starch and protein components were most apparent at temperatures >35°C (Chrastil 1994). Using the published data, modeling physicochemical changes as functions of time and temperature should be possible. In addition, the effect of rice MC during storage should be included. A descriptive model would be a useful tool in the food-processing industry, where consistent behavior of raw material is required. This study was part of an overall research program aimed at quantitatively modeling changes in the physicochemical properties of rice as functions of storage history. The specific objective was to evaluate the effects of rough rice MC, storage temperature, and storage duration on the amylograph, and the cooking properties of rice cv. Bengal (Linscombe et al 1993), which currently accounts for the majority of the medium-grain rice production in Arkansas. MATERIALS AND METHODS Sample Preparation Rough rice (cv. Bengal) was harvested from the University of Arkansas Rice Research and Extension Center, Stuttgart, AR, during September 1995. Immediately after harvest, the rough rice samples were cleaned in a dockage tester (Carter-Day Co., Minneapolis, MN) and immediately air-dried at room temperature (≈20°C) over a 14-day period. Samples were taken after different drying durations to yield rough rice with 8.8, 10.7, 12.9, and 13.6% MC (wb). The MC of the rice was measured by drying duplicate samples for 24 hr in an air oven at 130°C (Jindal and Siebenmorgen 1987). Each of the four lots of rice at the different MC levels was split into three 10-kg portions using a Boerner divider (model 34, Seedburo Equipment Co., Chicago, IL) and placed in sealed plastic buckets. The buckets were held at –10°C for four months before storage at different treatment temperatures. It was assumed that this temporary, low-temperature storage of the dry rice resulted in no significant changes in functional properties. One bucket of rice at each MC was stored in temperaturecontrolled chambers set at 3, 20, and 37°C, for a total of 12 lots. Subsamples (≈600 g) were removed from each lot at 0, 1, 2, 3, 4, 5, and 6 months. Each subsample was allowed to equilibrate to room temperature before milling and subsequent analyses. At each sampling time, the rough rice was hulled and milled in a laboratory milling system. A 150-g portion of rough rice was dehulled in a McGill sample sheller (Rapsco, Brookshire, TX). The resulting brown rice was milled in a McGill No. 2 mill operated with a 1.5-kg weight positioned 15 cm from the mill saddle centerline on the mill lever arm. Head rice was separated from the broken rice in a Seedburo sizer fitted with two 4.0-mm (/64 in.) sizing plates. Because MC affects the degree of milling, milling times were established by milling samples from each lot after different drying durations and analyzing the total lipids with a Soxtec fat extractor (Tecator AB, Hoganäs, Sweden) with petroleum ether as the solvent (Soxtec 1983). Total lipids were plotted against milling times for each lot. From these plots, the milling durations that yielded rice with 0.75% total lipids were calculated and used for testing. These milling durations ranged from 35 sec for rice at 13.6% MC to 100 sec for rice at 8.8% MC. HRY was computed as the mass percentage of head rice relative to 150 g of the starting rough rice. Amylography, using a Brabender Viscograph-E (C. W. Brabender, South Hackensack, NJ), was conducted in duplicate according to approved method 61-01 for milled rice (AACC 1995), using a heating rate of 3°C/min and a cooling rate of 3°C/min. The head rice was ground in a cyclone mill (Udy Corp., Fort Collins, 1Published with the approval of the Director, Arkansas Agricultural Experiment Station, manuscript 97050. 2Research assistant, assistant professor, professor, and graduate assistant, respectively, Biological and Agricultural Engineering Department, University of Arkansas, Fayetteville, AR 72701. 3Corresponding author: E-mail: [email protected] Publication no. C-1997-1030-02R. © 1997 by the American Association of Cereal Chemists, Inc.