Evaluation of corn distillers' dried grains defatted with supercritical carbon dioxide. (1990)

Cereal Chern. 67(6):585-588 Corn distillers' grains (CDG) were extracted with supercritical carbon dioxide (SC-COz) and with SC-COz plus water and/ or ethanol, at pressures of 63,600, 68,000, and 81,600 kPa (9,350, 10,000, and 12,000 psi) and at temperatures around 84 and 102°C. The CDG extracted with Corn is the most common biomass for commercial ethanol production in the United States (Morris, 1983). A protein-rich residue (stillage) remains after ethanol is distilled from the mash. This stillage is screened or centrifuged to yield an insoluble solids fractions, corn distillers' grains (CDG), and a soluble fraction, corn distillers' solubles (CDS). CDG and CDS are usually combined during drying to produce corn distillers' dried grains with solubles. Bookwalter et al (1984) reported that corn distillers' dried grains with solubles had poorer flavor quality than CDG and that CDG flavor was improved by azeotropic extraction with hexane-ethanol (82:18, vjv). Hexane, however, is highly flammable and explosive when mixed with air. An alternative to hexane is carbon dioxide, which, when compressed at high pressure above its critical temperature (31 ° C), has the density of a liquid but diffuses as a gas and can function as a solvent. Supercritical carbon dioxide (SCCOz) is an ideal solvent because it is nontoxic, nonexplosive, low-cost, readily available, and easily removed from extracted products (Friedrich and Pryde, 1984). Christianson et al (1984) used SC-COz to extract dry-milled corn germ. List et al (1984) processed flaked cottonseed with SC-COz. Friedrich and List (1982) and Eldridge et al (1986) extracted soybean flakes with it. Favati et al (1988) used it to extract carotene and lutein from leaf protein concentrates. King et al (1989) extracted fat tissue from meat products with it. The major commercial applications of SC-COz are hops extraction; coffee and tea decaffeination; removal of flavors and other components from basil, vanilla, and ginger; and residuum oil extraction (Parkinson and Johnson 1989). Recently SC-COz has been used to remove nicotine from tobacco for the commercial production of cigarettes with extremely low nicotine content. Since corn distillers' grain products have been incorporated into a number of food systems (Tsen et a11982, 1983; Bookwalter et al 1984; Wall et al 1984; Reddy et al 1986a,b; Wu et al 1987), and one of the major problems is the poor flavor quality of CDG, this article concentrates on the improvement of CDG flavor by SC-COz extraction. The objective of this research is to measure the effects of SC-COz extraction of the CDG samples on the flavor of the CDG in a model system. MATERIALS AND METHODS Materials CDG was from Brown-Forman Distillers Corp. (Louisville, KY) and was stored at 1°C upon arrival. The CDG was ground in an Alpine model 160 Z pin mill to reduce particle size so that all materials passed through a 35-mesh screen (420-j.Lm opening). 'Northern Regional Research Center. Agricultural Research Service, U.S. Department of Agriculture, 1815 N. University St., Peoria, IL 61604. The mention of firm names or trade products does not imply that they are endorsed or recommended by the U.S. Department of Agriculture over other firms or similar products not mentioned. 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., 1990. SC-COz had lower fat and higher neutral detergent fiber contents than untreated CDG. Defatted CDG had acceptable flavor scores. CDG treated with SC-COz may thus have potential as a food ingredient with high protein and dietary fiber contents. Unless otherwise stated, the same batch of CDG was used for all the experiments reported in this article. Extraction Apparatus King et al (1989) described the apparatus in detail (Fig. 1). Highlights are as follows. Carbon dioxide from a cylinder was fed through a check valve and a 5-/lm particulate filter to a compressor (model AGT-62j 152-C; Haskel Engineering Corp., Burbank, CA). Extraction pressure was regulated to ± 1.4 MPa by adjusting the air intake valve to the compressor and the downstream relief valve. The gas was introduced into the extraction tube by four valves that permitted gas flow from either end of the extraction vessel. A Hewlett-Packard 7610 gas chromatograph oven (Palo Alto, CA) enclosed the vertically mounted extraction tube. Carbon dioxide was equilibrated to the oven temperature by passing it through a 3-m coil. The extraction tube, with dimensions of 2.54 cm o.d., 1.75 cm i.d., and 61.0 cm length, was made from No. 316 stainless steel, pressure-rated to 76 MPa at room temperature (Autoclave Engineers, Erie, PA). Pressure gauges monitored the extraction pressure and the pressure drop across the extraction tube. The oven and extractor tube temperatures were measured with thermocouples. The oil-laden carbon dioxide passed through a micrometering valve to a receiver vessel, a modified 300-ml Magnedash autoclave (Autoclave Engineers) slightly above atmospheric pressure. The oil and gas phases separated in the receiver. A flow meter, calibrated for standard liters per minute of carbon dioxide, monitored flow rates (Fischer and Porter Co., Warminster, PA). A Singer model DTM-200 gas totalizer (Singer American Meter Div., Philadelphia, PA) determined total volume of carbon dioxide passed as a function of time. Extraction Procedure The extraction tube containing 50-60 g of CDG was gradually brought to the approximate temperature and pressure before the carbon dioxide flow was started. Extractions were performed at 82-86 or 100-103°C and 63,600, 68,000, or 81,600 kPa. Small volumes (1 or 2 ml) of distilled water or 95% ethanol were introduced into the top glass wool plug of the extractor for some runs. A separate stainless steel tube loaded with glass wool and 10 or 20 ml of distilled water was necessary to add a larger volume of water to the carbon dioxide solvent stream by bubbling carbon dioxide through the stainless steel tube. The weight of oil extracted from the CDG was monitored from time to time, and the extraction was stopped when the additional weight of oil extracted was close to zero. A typical extraction time was 30 min, and weight of the carbon dioxide used was 94 g for CDG extractions with up to 2 ml of water or ethanol added. Extraction time was 20 min and the weight of the carbon dioxide used was 915 g (a much faster flow rate) for CDG extractions with 10 or 20 ml of water added.