Grinding and cooking dry-mill germ to optimize aqueous enzymatic oil extraction

Authors: Dickey, Leland; Kurantz, Michael; Johnston, David; McAloon, Andrew; Moreau, Robert

Submitted to: Industrial Crops and Products
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/22/2010
Publication Date: 4/22/2010
Citation: Dickey, L.C., Kurantz, M.J., Johnston, D., Mcaloon, A.J., Moreau, R.A. 2010. Grinding and cooking dry-mill germ to optimize aqueous enzymatic oil extraction. Industrial Crops and Products. 32:36-40.

Interpretive Summary: Dry grind production of ethanol from corn creates a byproduct, distillers’ dried grains, DDG. In the US, DDG contains roughly 480 million gallons of corn oil that can be converted into a similar volume of biodiesel. The embryo of the corn, also called the germ, contains > 90% of the oil in the kernel and can be separated prior to fermentation by several new processes. Corn oil can then be separated from the germ and made available for uses other than animal feed, keeping the vegetable oil price stable despite increased consumption for biodiesel production. Sale of germ as a byproduct, in place of some of the DDG, should significantly increase the overall ethanol production revenues and thereby benefit this industry. More dry-grind ethanol production will bolster the U.S. corn market, reduce petroleum imports and slow depletion of world petroleum reserves. Corn germ price and its impact on dry grind profitability will increase if the cost of separating the oil from the germ can be reduced. Using an aqueous enzymatic method, instead of hexane extraction, is an approach to cheaper oil separation. Aqueous extraction was previously shown to be economically attractive starting with wet-mill germ. This work extends aqueous extraction to dry-mill germ, a fraction containing 15-20% oil that can be separated from corn at dry grind plants for a low retrofitting cost.

Technical Abstract: The many recent dry grind plants that convert corn to ethanol are potential sources of substantial amounts of corn oil. This report describes an aqueous enzymatic extraction (AEE) method to separate oil from dry-mill corn germ (DMG). The method is an extension of AEE previously developed for wet-mill germ (WMG). Oil dispersed in lipid bodies throughout the germ was converted to oil droplets suspended in an aqueous solution and then to drops of oil large enough to be separated from the solution as a continuous, buoyant phase (free oil). A microwave oven was used to cook the germ to a temperature, just short of charring. Thereafter the germ was extracted using the method developed for WMG: mix the heated germ with water and cook it under pressure, followed by colloid milling and enzymatic digestion of the milled germ particles overnight. A foam fraction was removed from the digested dispersion by bubbling nitrogen through a short column connected to the mixing tank. The foam fraction was then centrifuged to separate free oil. An estimate of the AEE economics of a plant extracting 24 million kg of DMG per year [40 million gal/y ethanol], showed that income from the unrefined oil streams and a stream sending the rest of the germ to the fermentation process was roughly equal to the estimated operating cost with an investment of $2.6 million. Recycle of the enzyme may reduce the estimated enzyme cost of $750,000/year.