Correcting Head Rice Yield for Surface Lipid Content (Degree of Milling) Variation

Cereal Chem. 84(1):88–91 Head rice yield (HRY) is the primary parameter used to quantify rice milling quality. However, HRY is affected by the degree of milling (DOM) and thus HRY may not be comparable between different lots if the DOM is different. The objective of this study was to develop a method by which HRY values can be adjusted for varying DOM values when measured by surface lipid content (SLC). Seventeen rough rice lots including long-grain and medium-grain cultivars and hybrids were harvested from two 2003 and five 2004 locations. Duplicate subsamples of each lot were milled in a McGill No. 2 laboratory mill for 10, 15, 20, or 40 sec after zero, one, two, three, and six months of storage. HRY and SLC were measured. The average HRY versus SLC slope across all milling duration data sets was 9.4. As such, it is suggested that, when milling with a McGill No. 2 laboratory mill, the HRY of a rice lot can be adjusted by a factor of 9.4 percentage points for every percentage point difference between the rice lot SLC and a specified SLC. Many factors, including rice moisture content (MC), temperature, and kernel surface topography, affect the relative ease by which bran is removed from rice kernels during milling. Therefore, the rate of bran removal can vary from one lot of rice to another (Siebenmorgen et al 2006). Ideally, rice samples are milled to a consistent, specified DOM, with DOM defined as the extent to which the germ and bran layers of brown rice kernels have been removed during the milling process. There are specific rice DOM grades established by the U.S. Department of Agriculture (USDA) that range from under-milled to hard-milled and are officially determined visually (USDA 1997). However, in commercial practice, DOM is often rapidly and objectively measured using lighttransmittance or reflectance instruments such as milling meters and color meters or with NIR technology. A more time-consumptive, yet presumably more accurate, method of DOM determination measures the milled rice surface lipid content (SLC) through petroleum ether extraction (Mastler and Siebenmorgen 2005). This extraction method measures the amount of lipid remaining on a milled, whole rice kernel and thus is an indication of the amount of bran and germ remaining after milling, as rice bran is composed of ≈20% lipids. Rice milling quality is usually expressed as HRY, the mass percentage of rough rice kernels that are ≥75% of the original kernel length after milling. As milling duration increases, DOM increases (thus SLC decreases) and HRY decreases linearly (Reid et al 1998; Siebenmorgen et al 2006). When comparing the milling quality of rice lots, HRY is the comparison parameter even though the DOM of the lots may be different, which ultimately results in inaccurate HRY comparisons. The ultimate objective of this study was to develop a method by which HRY values can be adjusted for varying DOM values when measured in the form of SLC. One foreseen consideration was the effect of storage duration on HRY. Daniels et al (1998) found that as storage duration increased, milling duration had to be increased to achieve a consistent DOM as measured with a milling meter. Head rice yield also increased as storage duration increased for up to three months postdrying (Daniels et al 1998; Pearce et al 2001). Therefore, a parallel objective of this study was to determine whether storage duration should be considered when adjusting HRY for differences in SLC. MATERIALS AND METHODS Sample Procurement and Pretreatment Conditions Seventeen rough rice lots (Fig. 1) were harvested from two 2003 and five 2004 locations as part of a field-scale variety testing program. Harvest MC range was 18–20%, wet basis. After harvest, samples were dried in a chamber maintained at 21°C and 62% relative humidity, corresponding to a rough rice equilibrium MC of 12.5% (ASAE 2004). Actual dried rough rice MC range was 11.5–13.0%, determined as the average MC of 50 kernels measured with an individual kernel MC meter (CTR 800E, Shizuoka Seiki, Shizuoka, Japan). After drying, samples were stored in Ziplock plastic storage bags at 4°C until treatment. Head Rice Yield Determination Samples were milled after 0, 1, 2, 3, and 6 months of storage. Before milling, samples from each of the 17 lots were removed from 4°C storage and equilibrated to room temperature, while in the sealed plastic bags, for at least one day. Eight rough rice samples (150 g) from each year/location/cultivar/storage period lot were then dehulled in a laboratory sheller (Type THU, Satake, Tokyo, Japan). Pairs (duplicates) of the resultant brown rice sam1 Graduate assistant and university professor, respectively, University of Arkansas, Department of Food Science, 2650 N. Young Ave., Fayetteville, AR 72704. 2 Corresponding author. Phone: 479-575-2841. Fax: 479-575-6936. E-mail: