Effect of Moisture Content at Harvest and Degree of Milling (Based on Surface Lipid Content) on the Texture Properties of Cooked Long-Grain Rice

Cereal Chem. 84(2):119–124 The effects of the degree of milling (based on surface lipids content [SLC]) on cooked rice physicochemical properties were investigated. Head rice yield (HRY), protein, and SLC decreased with increasing milling, while the percent of bran removed and whiteness increased. Results showed that SLC significantly (P < 0.05) affected milled as well as cooked rice properties across cultivar, moisture content (MC) at harvest, and location (Stuttgart, AR, and Essex, MO). Cooked rice firmness ranged from 90.12 to 111.26 N after milling to various degrees (SLC). The decrease in cooked rice firmness with increasing milling was attributed to the lowering of total proteins and SLC. Cooked rice water uptake increased with increasing degree of milling. Water uptake by the kernel during cooking dictated the cooked rice firmness. The increase in cooked rice stickiness with increasing degree of milling was attributed to an increase in starch leaching during cooking because of the greater starch granule swelling associated with a greater water uptake. Rice is usually used as milled whole rice after removing the outer hull and the bran layers of the rough rice. The measure of how much bran is removed during the milling process is known as the degree of milling (DOM) (Perdon et al 2001). However, the amount of bran removed varies depending on dehulling and milling processes. Therefore, variations in DOM result in changes in the rice kernel gross composition. For example, Juliano (1985) indicated that >98% of the SLC and >50% of proteins are usually removed by milling. The removal of the outer layers of rough rice, due to milling, was reported to cause disproportionate losses of lipids, proteins, reducing sugars, and minor components that increase the total amount of starch in milled rice (Singh et al 1998; Azhakanandam et al 2000; Park et al 2001). In addition to milling conditions, the chemical composition of the rice grain also depends on the cultivar, variability in the environment (McClung 2004; Copeland and Dang 2005; Counce et al 2005), cultural practices, and soil type (Park et al 2001). Starch, proteins, lipids, and their interaction have long been reported to influence rice functional properties (Eliasson and Krog 1985; Chrastil 1993; Vandeputte et al 2003). For instance, Wang et al (2003) reported that although pasting properties of rice flour were affected by the drying regimes of rough rice, there were no indications of changes in starch molecular size distribution, pointing out the important role of other grain components (i.e., proteins and lipids) in determining the functional properties of rice kernels. Rice MC at harvest, as an indicator of rice kernel development, is another important factor that affects rice quality (Siebenmorgen et al 2004). For instance, draining and harvesting date, which influence rice MC at harvest, have been reported to affect rice metabolic processes and starch and protein composition as well as structure (Champagne et al 2005). Although starch and protein synthesis is thought to be complete when rice moisture content reaches 27–29%, a slight decrease in rice proteins and lipids and an increase in amylose content have been reported (Siebenmorgen et al 2004; Champagne et al 2005). This could be because a bulk of rice at 27–29% still contains a significant proportion of immature kernels. Because small variations in rice chemical composition may result in changes in rice physicochemical properties (Chrastil 1990), variations in rice harvest MC are expected to lead to variation in functional properties such as cooked rice texture. Various methods have been employed to mill rice samples to various DOM levels. Perdon et al (2001) used milling durations of 15, 30, 45, and 60 sec in a McGill No. 2 laboratory mill to produce samples with varying DOM, while other authors (Kim et al 2001) used the percentage of bran removed after milling as an indication of DOM. However, milling for set durations can yield rice of varying DOM because of factors such as rice cultivar, rice MC, rice temperature, or mill settings (Andrews et al 1992; Bennett et al 1993; Archer and Siebenmorgen 1995). Practices for target DOM seem to vary quite extensively throughout the world, probably because of varying consumer preferences in specific markets. A higher DOM tends to make rice whiter, which is often associated with higher quality rice but may also negatively impact the sensory quality of cooked rice (Kwon and Jeon 1991). Therefore, the optimum DOM is usually determined based on the eating preference for cooked rice by specific consumer groups. A thorough investigation of the effect of rice milling on texture properties is important to understand the relationship between DOM, rice cooking, and cooked rice texture properties. Therefore, the objective of this study was to investigate the effect of DOM (based on SLC) of rice harvested at various MC on the instrumental texture properties of cooked rice. MATERIALS AND METHODS Rice Samples and Rice Fractionation Two long-grain rice cultivars (Francis and Wells) (≈200 lb of hand-harvested samples from each cultivar before size fractionation) harvested in the fall of 2004 from two different locations (Stuttgart, AR, and Essex MO) at low MC (12.8–16.3%) and high MC (21.0–22.0%) were used in this study. Freshly harvested samples were kept in a refrigerated room overnight and transported the next day to the University of Arkansas rice processing laboratories. Rough rice samples were air-dried to ≈12% MC and then size-fractionated using a precision sizer (CarterDay Co., Minneapolis, MN) to large, medium, and thin thickness fractions. Only rice kernels of the medium thickness fraction (1.69–1.72 mm of milled rice) were used in this study to provide a uniform kernel size across samples because kernel size tends to affect uniaxial compression tests results (Matsue et al 2001; unpublished data). Preliminary Milling Experiment Because rice cultivars mill differently (i.e., milling duration varies between cultivars to achieve a target DOM), a preliminary milling experiment was conducted to determine the milling duration necessary to achieve target DOM levels of 0.2, 0.3, 0.4, 0.5, and 0.6% (as-is basis) SLC for each rice cultivar at each location 1 Department of Food Science, University of Arkansas, 2650 N. Young Avenue, Fayetteville, AR 72704. 2 Corresponding author. Phone: 479-575-6822. E-mail: [email protected] doi:10.1094 / CCHEM-84-2-0119 © 2007 AACC International, Inc.