Lipid Extraction from Wheat Flour Using Supercritical Fluid Extraction

Cereal Chem. 81(6):693–698 Environmental concerns, the disposal cost of hazardous waste, and the time required for extraction in current methods encouraged us to develop an alternate method for analysis of wheat flour lipids. Supercritical fluid extraction (SFE) with carbon dioxide has provided that medium and the method is fully automatic. Crude fats or nonstarch free lipids (FL) were extracted from 4–5 g of wheat flour by an SFE system. To develop optimum conditions for SFE, various extraction pressures, temperatures, and modifier volumes were tried to provide a method that would produce an amount of lipids comparable to those extracted by the AACC Approved Soxhlet Method and the AOCS Official Butt Method using petroleum ether as solvent. Using several wheat flour samples, the best conditions were 12.0 vol% ethanol (10.8 mol%) at 7,500 psi and 80°C to extract the amount of FL similar to those by the AACC and AOCS methods. Using solid-phase extraction, lipids were separated into nonpolar lipid (NL), glycolipid (GL), and phospholipid (PL) fractions. The mean value of five flours was 1.15% (flour weight, db) by the SFE method, 1.07% by the Butt method, and 1.01% by the Soxhlet methhod. The SFE-extracted lipids contained less NL and more GL than either the Butt or Soxhlet methods. All three methods extracted lipids with qualitatively similar components. The overall benefit for SFE over the Soxhlet or Butt methods was to increase the number of samples analyzed in a given time, reduce the cost of analysis, and reduce exposure to toxic chemicals. The quantity and quality of wheat flour lipids are of great interest to cereal scientists because of their nutritional and functional roles in processing wheat-based food products (Chung et al 2002). The traditional and officially approved method for extracting nonstarch free lipids (FL) has been by exhaustive extraction of the lipids using a Soxhlet apparatus with a nonpolar solvent such as hexane, diethyl ether, or petroleum ether (Approved Method 30-25, AACC 2000). Although the Soxhlet method is the AACC Approved Method for extracting lipids, it has a number of disadvantages. Setting up exhaustive extractions can be too laborious and takes considerable time; if not set up properly, there is a potential for fire (or even an explosion), let alone a failed extraction; and if there is a slight water leak in the condensers or in the water circulating tubing, this could lead to flooding in the laboratory. The American Oil Chemists Society Official Method Aa 4-38 (AOCS 2000) discouraged using syphoning methods like Soxhlet because of problems like variable temperature of extraction. Environmental concerns, exposure to toxic vapors, solvent disposal cost of hazardous solvent waste, and time required for extraction have made it imperative to search for a method that would alleviate these concerns. Supercritical fluid carbon dioxide (SF-CO2) (Friedrich et al 1982; King 1993) was used to extract oil from soybeans (Friedrich and List 1982; Friedrich et al 1982; Friedrich 1983); fat from meat products (King et al 1989); and phospholipids (PL) from canola (Temelli 1992; Dunford and Temelli 1995). Wu et al (1990) have defatted corn distillers’ dried spent grains with SF-CO2 and Hopper et al (1995) employed a multiple supercritical fluid extraction (SFE) approach for food items in their total diet study. SFE technology has been well documented (King 1989, 1993; King and France 1992; King et al 1993; Taylor 1996) and has attracted considerable interest (Latta 1990). This is due to the solvent power of SF-CO2, which increases with density at a given temperature (King 1993; Taylor 1996). In addition, diffusivity, the tendency of a dense gas to penetrate the sample matrix, easily increases with temperature (Taylor 1996). There are no published methods of wheat flour FL extraction using SF-CO2 as far as we know. Therefore, the objective of this study was to develop a method for extracting FL from wheat flour using an SFE system that would give reproducible results that compare reasonably with that of the Soxhlet or Butt extraction methods, while addressing the above concerns. In this study, we investigated different conditions for extraction pressure (2,500– 7,500 lb/sq in. [psi] = 17.2–51.7 MPa), extraction temperature (60–100°C), and modifier concentration (0–19% by volume) to determine the combination of those three variables that would best produce optimum results using one hard red winter (HRW) wheat composite flour. Furthermore, we verified the SFE method using flours from various other wheat classes. MATERIALS AND METHODS Wheat Flours Flour K, a composite of HRW wheat cv. Karl grown at six locations in Kansas, was used for development of the SFE method. An additional eight flours including four HRW wheat cultivars (1039, Karl, Larned, and a flour composite designated as Cargill, obtained from Cargill company); two soft red winter (SRW) wheat cultivars (Excel and 2500); and two hard red spring (HRS) wheat cultivars (AcCora and Grandin) were used to verify the SFE method developed and were compared with the Soxhlet method. In addition, three HRW wheat flours (Karl92, Jagger, and Horizon composite), one HRS wheat flour (Kaskaskia), and one SRW wheat flour (Caldwell) were used for FL extraction by three extraction methods including the SFE, Soxhlet, and Butt, and fractionation to nonpolar lipids (NL), glycolipids (GL), and PL (Tweeten et al 1981; Ohm and Chung 1999). All flour samples were provided by the USDA-ARS Wheat Quality Laboratories; the three HRS wheat flours were provided by the Hard Red Spring and Durum Wheat Quality Lab, Fargo, ND; Caldwell flour was provided by the Soft Wheat Quality Lab, Wooster, OH; and the remaining were provided by the Hard Winter Wheat Quality Lab, Manhattan, KS. 1 Cooperative investigations, U.S. Department of Agriculture (USDA), Agricultural Research Service (ARS), and the Department of Grain Science and Industry, Kansas State University. Contribution No. 03-30-J from the Kansas Agricultural Experiment Station, Manhattan, KS 66506. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product to the exclusion of others that may also be suitable. 2 USDA, ARS, Grain Marketing and Production Research Center, Manhattan, KS 66502. 3 Department of Grain Science and Industry, Kansas State University, Manhattan, KS 66506. 4 Current address: Atrix Labs, 2579 Midpoint Drive, Fort Collins, CO 90525-4417. 5 Corresponding author. Phone: 785-776-2703; Fax: 785-537-5534; E-mail: [email protected] Publication no. C-2004-1013-01R. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. American Association of Cereal Chemists, Inc., 2004.