Comparison Of Extraction Systems For Measuring Surface Lipid

Comparison Of Extraction Systems For Measuring Surface Lipid

RICE QUALITY AND PROCESSINGComparison of Extraction Systems forMeasuring Surface Lipid Content of Milled Rice 1C.A.Rohrer, A.L. Matsler, and T.J.SiebenmorgenABSTRACTThe surface lipid contents (SLCs) of two varieties of long-grain rice, ‘Cypress’and ‘Drew’, were compared using three extraction methods: the Soxtec system, acceleratedsolvent extraction, and supercritical fluid extraction. This study was conducted toprovide additional methods for accurate, time-saving measurements of milled rice degreeof-millingby utilizing new extraction technology. Prior to milling, rough rice was separatedinto three thickness fractions (

B.R. Wells Rice Research Studies 2002quality, appearance, and stability due to the oil content of the bran. Thorough millingproduces rice that is essentially free of bran; however, over-milling to minimize remainingbran generally results in head rice yield reduction. Head rice yield refers to theweight (%) of rough rice that remains as whole kernels (>75% of the original kernellength) throughout the milling process. The degree of milling (DOM) of rice is theextent to which bran has been removed from the surface of rice kernels during themilling operation (Bennett et al., 1993).Limited research has been conducted on evaluating kernel thickness effects onDOM. Since rough rice is comprised of kernels of various sizes, each size fraction couldmill differently, affecting the DOM. Previous research has included separating kernelsinto thickness fractions after milling and determining the SLC as an index for quantifyingDOM (Chen et al., 1998). Thin kernels behaved differently than thicker ones in thattheir SLC was higher than thicker kernels, especially at low overall DOM levels. Thefindings of Chen et al. (1998) indicated that measurements involving SLC of kernelsshould 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 commonmethod of evaluating DOM of milled rice is through solvent (petroleum ether) extraction,such as Soxhlet extraction. An updated Soxhlet, the Soxtec system, has recentlybeen used to determine oil content in soybeans (Morrison, 1990) with no significantdifference found compared to Soxhlet extraction.One limitation of extraction using the Soxtec system is that the presence of theoperator is required in order to complete the extraction; whereas with newer extractiontechnology–accelerated solvent extraction (ASE)–the samples can be loaded and leftovernight to complete the full extraction. Accelerated solvent extraction can be used toextract oil from oilseed crops by using elevated temperatures and pressures relative tothe Soxtec system. The entire extraction system is fully automated, allowing a reducedvolume of solvent, shortened analysis duration, and minimal handling by the operatorwhen compared to the Soxtec. Another fully automated technique–supercritical fluidextraction (SFE)–has also been developed for use in extraction of various compoundsfrom numerous materials. Similar to the ASE, SFE is capable of high temperatures andpressures. The SFE uses supercritical carbon dioxide (CO 2 ) as the extraction solvent.Currently, traditional solvent extraction methods, such as the Soxhlet, are used tomeasure SLC for determining milled rice DOM; however, these methods may not be themost convenient techniques. The objective of this study was to compare the ASE andSFE with the Soxtec system in measuring SLCs for quantifying the DOM of thicknessfractionatedrice.MATERIALS AND METHODSSamplingTwo long-grain rice cultivars, Cypress and Drew, obtained from the NortheastResearch and Extension Center, Keiser, AR, were harvested at moisture contents (MCs)391

AAES Research Series 504of 17.2 and 18.5%, respectively. Immediately after harvest, the rice was cleaned andthen dried to approximately 12% MC by placing the rice onto screens in a controlledtemperature and relative humidity chamber (21°C, 53% RH). Following drying, roughrice samples were separated into three thickness fractions (1.84 mm and1.98 mm) using a precision sizer (Carter-Day Co., Minneapolis, MN).Samples of each thickness fraction were milled for three progressive 10 s durations(10, 20, 30 s) and the head rice collected from each milling duration. Head riceyields 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 durationfrom each thickness fraction was then measured using the Soxtec, ASE, and SFE systems.Surface Lipid ExtractionSurface lipids were extracted from head rice using a Soxtec Avanti 2055 extractionunit (Foss Tecator, Eden Prairie, MN) with petroleum ether as the solvent and a totalextraction length of 50 min/sample; an Accelerated Solvent Extractor (ASE 200, Dionex,Sunnyvale, CA) using petroleum ether with an extraction length of 30 min/sample usinga pressure of 1500 psi; and a supercritical fluid extractor (SFE 3560, Isco Inc., Lincoln,NE) using supercritical CO 2 at a pressure of 7500 psi with 15% petroleum ether as amodifier and a 30 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 (100°C) for30 min to evaporate any residual petroleum ether. Following drying, the cups weretransferred to a desiccator to cool for 30 min, and the weight of the remaining lipids inthe cups was used to calculate SLCs by expressing as a percentage of the original head rice.RESULTS AND DISCUSSIONFigure 1 shows the average SLC for thickness-fractionated Cypress rice witheach fraction milled for three successive 10 s durations. To determine differences amongthe three extraction systems, and the kernel thickness fractions within each millingduration, an analysis of variance was performed using a Student’s t-test p

B.R. Wells Rice Research Studies 2002durations increased from 10 to 30 s for all thickness fractions of Drew rice. These trendsof lower SLC in thinner kernels and decreasing SLC as milling durations increased weresimilar for all three methods for both Cypress and Drew.ComparisonFigure 1 illustrates the average SLC of Cypress head rice of the three thicknessfractions milled for three durations using the three different extraction systems. Thelevels of SLC were significantly higher (p1.98 mm) at 10 and 20 s milling and for thinnest kernels (

AAES Research Series 5041.6Surface Lipid Concentrations (%) Fraction (mm)1.6Surface Lipids Concentrations (%) Fraction (mm)1.61.4Surface Lipid Concentrations (%) Fraction (mm)Fig. 1. Surface lipid concentration (SLC) of thickness-fractionatedCypress rice with each fraction milled for 10 x (A), 20 s (B), and 30 s (C)durations using three different extraction methods. Points oneach graph represent the average of six extracted rice samples.394

B.R. Wells Rice Research Studies 20021.61.4(A)Surface Lipid Concentrations (%) Fraction (mm)1.61.4Surface Lipid Concentrations (%) Fraction (mm)1.61.4Surface Lipid Concentrations (%) 2. Surface lipid concentration (SLC) of thickness-fractionatedDrew rice with each fraction milled for 10 s (A), 20 s (B), and 30 s (C)durations using three different extraction methods. Points on thegraph represent the average of six extracted rice samples.395

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