Effect of Ultrasonic Treatment of Brown Rice at Different Temperatures on Cooking Properties and Quality

Cereal Chem. 87(5):403–408 This research studied developing quick cooking brown rice by investigating the effect of ultrasonic treatment at different temperatures on cooking time and quality. The medium grain brown rice was ultrasonically treated in water at temperatures of 25, 40, and 55°C for 30 min and then dried by air at 25°C to its initial moisture content (11.0 ± 0.6%, wb) before cooking. The microstructure of rice kernel surface, chemical composition, and optimal cooking time of treated brown rice were determined. The pasting and thermal properties and chemical structure of flour and starch from treated brown rice were also examined. The results showed that the optimal cooking times were 37, 35, and 33 min after treatment at 25, 40, and 55°C, respectively, compared to the control of 39.6 min. The ultrasonic treatment resulted in a loss in natural morphology of rice bran, allowing water to be absorbed by a rice kernel easily, particularly at hightemperature treatment. Even through rice flour still maintained an Apattern in the pasting properties, the crystallinity significantly increased after treatment at 55°C. Ultrasonic treatment increased the peak, hold, and final viscosities and decreased the onset temperature (To) and peak temperature (Tp), significantly. Thus, ultrasonic treatment could be used for reducing cooking time of brown rice. Brown rice is considered more nutritious than white or milled rice because it contains bran and embryo that are rich in fibers and vitamins (Muramatsu et al 2006). However, it normally takes much longer to cook because rice bran layers slow the water absorption during cooking. Several methods have been studied or developed by researchers to reduce cooking time of brown rice. Partial milling has been used commercially for producing quick cooking brown rice, but it causes loss of nutrients and integrity due to partial removal of rice bran. When brown rice is heated and then cooled, the sudden temperature change can cause fissures in the endosperm, resulting in accelerated water absorption during cooking (Tadahiko et al 1986). The disadvantage of this treatment is that brown rice may break during cooking giving low quality appearance. Pregelatinization using water and heat is also reported as effective for reducing cooking time, but the texture and taste of cooked brown rice may not be desirable (Ataullah et al 1960; Freserick et al 1960; Douglas 1976; Afif et al 1992). Germination decreases the cooking time of brown rice, but it could lead to fermentation or other unpleasant odors (Se-Soon et al 1999; Hiromichi et al 2003; Hidechika et al 2004; Sang-You et al 2007). Enzymatic treatment also was studied for degradation of the cellulose in rice bran to reduce the resistance of water absorption during cooking, but the enzymes need to be inactivated after the treatment, which complicates the process (Ruiyu and Jinlin 2006; Mithu et al 2008). Therefore, there is still a very strong interest in seeking new processing methods for producing quick cooking brown rice with high quality. Ultrasonic treatment has been studied for many food processing applications. High-amplitude waves (10 kHz to 1 MHz) generally are best suited for applications such as cleaning, drilling, emulsification, and chemical and biological applications (Koo et al 2002). Mark et al (2007) reported that ultrasonic cavitation produces bubbles in a liquid; the bubbles then collapse and form tiny jets directly interacting with the surface of objects in liquid. Ultrasonic treatment has been tested for degrading cellulose material by changing the morphology of cellulose fibers (Tang et al 2005). If the particles subjected to sonication are rice grains in water, then some destruction of the surface “shell” and grain fragmentation would be anticipated (Mason and Paniwnyk 1996). Moreover, rice starch kept in sufficient water above the glass transition temperature and below the gelatinization temperature introduces annealing effects (Gough and Pybus 1971). A cursory survey of literature reveals no reports about ultrasonic treatment for producing quick cooking brown rice. The objective of this study was to investigate the effect of different water temperatures during ultrasonic treatment on cooking time and quality of brown rice for producing quick cooking brown rice. To understand the mechanism of reduced cooking time of brown rice after ultrasonic treatment, chemical and physical properties of brown rice and its components were also examined. MATERIALS AND METHODS Sample Preparation Brown rice of medium grain cultivar M202, obtained from Pacific International Rice Mill (Woodland, CA) was used in this research. The brown rice had an initial moisture content of 11.0 ± 0.6% wb, which was determined by the air oven method at 130°C for ≈48 hr (S352.2, ASAE 1995). Foreign substances, damaged kernels, and immature kernels were removed manually from the samples before the samples received ultrasonic treatment and produced milled rice sample or its components. Partially and fully milled rice samples were prepared for comparison of cooking properties with those of regular and ultrasonic treated brown rice. Samples were obtained by milling regular brown rice 1–4× using a rice mill (VP-222N, Yamamoto, Japan) with throughput settings at #1 and whitening setting at #5. The milled samples were labeled, respectively, as pass I and pass IV. The pass IV sample represented well-milled white rice as defined by the Federal Grain Inspection Service (FGIS) (USDA 1994). Weight removal was 5.4% of pass I sample and 14.5% of pass IV. Whiteness index was determined using a milling meter (MM1D, Satake, Hiroshima, Japan). Before measuring the whiteness, the milling meter was calibrated with three colored references: white, brown, and black, representing standard color values of 85.9, 0.8, and 1.6, respectively. Brown rice flour was prepared using a disk attrition mill (CEBouer, Springfield, OH) for study of pasting properties and X-ray diffraction. Rice starch was prepared using the method of Wang and Wang (2001) for studying thermal properties. Brown rice flour (100 g) was soaked in 200 mL of 0.0025M NaOH for 18 hr, 1 College of Food Science and Engineering, Northwest Agriculture and Forestry University, Yangling, Shaanxi, 712100, China. 2 Department of Biological and Agricultural Engineering, University of California, Davis, One Shields Avenue, Davis, CA 95616. 3 Processed Foods Research Unit, USDA-ARS-WRRC, 800 Buchanan St., Albany, CA 94710. 4 Corresponding author. Fax: 510-559-5851. E-mail: [email protected] doi:10.1094 / CCHEM-02-10-0034 This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. AACC International, Inc., 2010.