RICE QUALITY AND PROCESSING Comparison of Extraction Systems for Measuring Surface Lipid Content of Milled Rice C.A.Rohrer, A.L. Matsler, and T.J.Siebenmorgen ABSTRACT The surface lipid contents (SLCs) of two varieties of long-grain rice, Cypress and Drew, were compared using three extraction methods: the system, accelerated solvent extraction, and supercritical fluid extraction. This study was conducted to provide additional methods for accurate, time-saving measurements of milled rice degreeof-milling by utilizing new extraction technology. Prior to milling, rough rice was separated into three thickness fractions (<.84 mm,.84-.98 mm, and.98 mm) and samples from each thickness fraction milled for three successive s durations. Collected head rice after each s duration was extracted with each of the three methods. Results showed that regardless of the extraction method, thinner kernels contained lower SLCs and longer milling durations resulted in lower SLCs. Accelerated solvent extraction was shown to produce SLCs at least as great as the, suggesting that the former method is as thorough in extracting lipids as commonly used methods with the advantages of shorter extraction lengths and full automation. INTRODUCTION Rice milling produces white rice kernels that have had the bran layers and germ removed (Juliano and Bechtel, 985). Bran comprises the rice germ and several sub layers and contains 2-23% oil and accounts for over 6% of the nutrients found within each rice kernel. Any bran remaining on rice kernels after milling can result in lowered This is a completed study. 39
B.R. Wells Rice Research Studies 22 quality, appearance, and stability due to the oil content of the bran. Thorough milling produces rice that is essentially free of bran; however, over-milling to minimize remaining bran generally results in head rice yield reduction. Head rice yield refers to the weight (%) of rough rice that remains as whole kernels (>75% of the original kernel length) throughout the milling process. The degree of milling (DOM) of rice is the extent to which bran has been removed from the surface of rice kernels during the milling operation (Bennett et al., 993). Limited research has been conducted on evaluating kernel thickness effects on DOM. Since rough rice is comprised of kernels of various sizes, each size fraction could mill differently, affecting the DOM. Previous research has included separating kernels into thickness fractions after milling and determining the SLC as an index for quantifying DOM (Chen et al., 998). Thin kernels behaved differently than thicker ones in that their SLC was higher than thicker kernels, especially at low overall DOM levels. The findings of Chen et al. (998) indicated that measurements involving SLC of kernels should account for varying kernel sizes. Several chemical analyses have been employed to determine DOM; however, they are typically time-consuming and require large volumes of solvent. A common method of evaluating DOM of milled rice is through solvent (petroleum ether) extraction, such as Soxhlet extraction. An updated Soxhlet, the system, has recently been used to determine oil content in soybeans (Morrison, 99) with no significant difference found compared to Soxhlet extraction. One limitation of extraction using the system is that the presence of the operator is required in order to complete the extraction; whereas with newer extraction technology accelerated solvent extraction () the samples can be loaded and left overnight to complete the full extraction. Accelerated solvent extraction can be used to extract oil from oilseed crops by using elevated temperatures and pressures relative to the system. The entire extraction system is fully automated, allowing a reduced volume of solvent, shortened analysis duration, and minimal handling by the operator when compared to the. Another fully automated technique supercritical fluid extraction () has also been developed for use in extraction of various compounds from numerous materials. Similar to the, is capable of high temperatures and pressures. The uses supercritical carbon dioxide (CO 2 ) as the extraction solvent. Currently, traditional solvent extraction methods, such as the Soxhlet, are used to measure SLC for determining milled rice DOM; however, these methods may not be the most convenient techniques. The objective of this study was to compare the and with the system in measuring SLCs for quantifying the DOM of thicknessfractionated rice. MATERIALS AND METHODS Sampling Two long-grain rice cultivars, Cypress and Drew, obtained from the Northeast Research and Extension Center, Keiser, AR, were harvested at moisture contents (MCs) 39
AAES Research Series 54 of 7.2 and 8.5%, respectively. Immediately after harvest, the rice was cleaned and then dried to approximately 2% MC by placing the rice onto screens in a controlled temperature and relative humidity chamber (2 C, 53% RH). Following drying, rough rice samples were separated into three thickness fractions (<.84 mm, >.84 mm and <.98 mm, >.98 mm) using a precision sizer (Carter-Day Co., Minneapolis, MN). Samples of each thickness fraction were milled for three progressive s durations (, 2, 3 s) and the head rice collected from each milling duration. Head rice yields were determined using a Grainman shaker table with a 4.76 mm screen size (Grainman Machinery Mfg. Corp., Miami, FL). Head rice SLC from each milling duration from each thickness fraction was then measured using the,, and systems. Surface Lipid Extraction Surface lipids were extracted from head rice using a Avanti 255 extraction unit (Foss Tecator, Eden Prairie, MN) with petroleum ether as the solvent and a total extraction length of 5 min/sample; an Accelerated Solvent Extractor ( 2, Dionex, Sunnyvale, CA) using petroleum ether with an extraction length of 3 min/sample using a pressure of 5 psi; and a supercritical fluid extractor ( 356, Isco Inc., Lincoln, NE) using supercritical CO 2 at a pressure of 75 psi with 5% petroleum ether as a modifier and a 3 min/sample total extraction length. After extraction with each instrument, excess petroleum ether was evaporated from collection cups under nitrogen flow. Collection cups containing the extracted lipids were placed in a drying oven ( C) for 3 min to evaporate any residual petroleum ether. Following drying, the cups were transferred to a desiccator to cool for 3 min, and the weight of the remaining lipids in the cups was used to calculate SLCs by expressing as a percentage of the original head rice. RESULTS AND DISCUSSION Figure shows the average SLC for thickness-fractionated Cypress rice with each fraction milled for three successive s durations. To determine differences among the three extraction systems, and the kernel thickness fractions within each milling duration, an analysis of variance was performed using a Student s t-test p<.5 with a one-way analysis of variance (JMP IN 5.., SAS Inc., Cary, NC). The SLCs extracted by the system were significantly lower (p<.5) for the thinnest kernels for all three milling durations compared to the mid-thickness and thicker kernels of Cypress rice. For all thickness fractions of Cypress rice, SLC decreased as the milling duration increased from to 3 s when using the system. For example, the SLC of midthickness (.84 to.98 mm) kernels was % at s milling and decreased to.755% at the longest milling duration, 3 s. The results for Drew rice were similar to those from Cypress in that significantly lower (p<.5) SLCs were found for the thinner kernels compared to thicker kernels (>.84 mm) across all milling durations (Fig. 2). The SLCs also decreased as milling 392
B.R. Wells Rice Research Studies 22 durations increased from to 3 s for all thickness fractions of Drew rice. These trends of lower SLC in thinner kernels and decreasing SLC as milling durations increased were similar for all three methods for both Cypress and Drew. Comparison Figure illustrates the average SLC of Cypress head rice of the three thickness fractions milled for three durations using the three different extraction systems. The levels of SLC were significantly higher (p<.5) in Cypress rice when using for thickest kernels (>.98 mm) at and 2 s milling and for thinnest kernels (<.84 mm) at the 2 s milling duration. Surface lipids extracted using the three extraction systems for thickness-fractionated Drew rice are shown in Fig. 2. The SLC from the thinner kernels (<.84 mm) at s milling durations was significantly higher (p<.5) when extracted from the when compared to and. Along with this advantage of, considerably less manual operation is needed so that samples extracted can be left overnight, optimization is faster, and extraction lengths are shorter, i.e., 3 min with compared to 5 min extraction with the. Accelerated solvent extraction of surface lipids of milled rice could be used in place of conventional solvent extraction methods that generally require longer extraction length and higher consumption of solvents. SIGNIFICANCE OF FINDINGS This study indicates that the provides surface lipid determinations that are in good agreement with those obtained by or. This would imply that the provides SLCs at least as great as the, suggesting that it is as thorough in extracting lipids as commonly used methods with the advantages of shorter extraction lengths, full automation, reduction in the amount of organic solvents required for extraction, and less handling required by the operator. LITERATURE CITED Bennett, K.E., Seibenmorgen, T.J.,and Mauromoustakos, A. 993. Effects of McGill No. 2 miller settings on surface fat concentration of head rice. Cereal Chem. 7(6):734-739. Chen, H., Siebenmorgen, T.J., and Griffen, K. 998. Quality characteristics of longgrain rice milled in two commercial systems. Cereal Chem. 75(4):56-565. Juliano, B.O. and Bechtel, D.B. 985. The rice grain and its gross composition. In: Juliano, B.O. (ed.) Rice Chemistry and Technology. 2 nd Ed. Am. Assoc. Cereal Chem. St. Paul, MN. pp. 7-57. Morrison, W.H. 99. An evaluation of the System for oil determination in soybeans. J. Am. Oil Chem. Soc. 67(7):43-432. 393
AAES Research Series 54 (A) <.84.84-.98 >.98 Surface Lipids Concentrations (%) (B) <.84.84-.98 >.98 (C) <.84.84-.98 >.98 Fig.. Surface lipid concentration (SLC) of thickness-fractionated Cypress rice with each fraction milled for x (A), 2 s (B), and 3 s (C) durations using three different extraction methods. Points on each graph represent the average of six extracted rice samples. 394
B.R. Wells Rice Research Studies 22 (A) <.84.84-.98 >.98 (B) <.84.84-.98 >.98 (C) <.84.84-.98 >.98 Fig. 2. Surface lipid concentration (SLC) of thickness-fractionated Drew rice with each fraction milled for s (A), 2 s (B), and 3 s (C) durations using three different extraction methods. Points on the graph represent the average of six extracted rice samples. 395