By-Products/Coproducts of Corn Ethanol

Ethanol by-products (coproducts) include distillers dried grains or dried distill­ers grains, distillers dried grains with solubles, wet distillers grains, corn bran, corn gluten feed, corn gluten meal (CGM), corn germ meal, and condensed fermented corn extractives. A bushel of corn produces about 2.78 gallons of ethanol. About 5.3 pounds of DDGs, 2.15 pounds of WDGs, and 0.06 gallons of corn oil are also produced per gallon of ethanol as coproducts [14]. Per bushel of corn, the amounts of DDGs and WDGs coproduced are on average about

14.7 and 6.0 pounds, respectively. As of 2010, nearly 3.8 million tons of DDGs (including both DDG and DDGS [dried distillers grains with solubles]) are pro­duced in domestic dry grind ethanol production, that is, dry milling ethanol production. This accounts for more than 98% of the total U. S. DDG and DDGS production and the remaining 1-2% comes from the alcohol beverage indus­tries. DDG is nearly identical to DDGS except that the former does not contain the distillers solubles, which is a "sticky" syrup.

Carbon dioxide is also becoming an important coproduct, as mentioned earlier. Instead of becoming a greenhouse gas emitting industry, carbon dioxide generated in the alcohol fermentation reaction can be relatively easily captured due to its high concentration and purified for manufacture of dry ice or compressed carbon dioxide gas for the food and beverage industries.

Corn oil is very valuable in both food and fuel applications, however, a corn kernel contains only about 3.6-4.0 wt% of corn oil/fat, Due to its low level, any process targeting direct extraction of corn oil alone from the corn kernel would not be a cost-effective solution. In this regard, recent dry mill­ing ethanol plants potentially offer a good opportunity as potentially large sources of corn oil, as long as an economical separation process can be devel­oped and implemented. Coproduction of corn oil is one of the promising options for the corn ethanol refinery to improve the gross margins in the industry.

Commercial processes for separation of corn oil are currently being devel­oped [26, 27]. POET, the largest ethanol producer in the world has been pro­ducing VoilaTM corn oil since the beginning of 2011 for the biodiesel and feed markets. POET in Iowa had been predicted to produce corn oil as feedstock for 12 million gallons of biodiesel per year by the end of 2011 [28]. SunSource technology produces corn oil as an additional coproduct available to etha­nol producers, most likely to be used in biodiesel production. The process uses centrifuge technology to extract the oil from the distillers grains in the evaporation step [27]. They also claim that removing the corn oil from the distillers grains does not lower the value of the grain feed coproduct and instead makes it easier to handle. They claim that the process also reduces volatile organic compounds emission from the dryers [27].

R&D efforts in cost-effective corn oil extraction and purification are under way. Corn oil can be extracted from dry-milled or wet-milled germ by crush­ing for $35-45/tonne or by hexane extraction for $20-40/tonne, which are significant after considering all other associated costs as well as the market price for unrefined corn oil [29]. Quick germs [30] and enzymatically milled germs [31] have been successfully produced in laboratory quantities with 30% and 39% oil, respectively. Oil yields of 65 wt% can be recovered from wet-milled or dry-milled germ by expeller pressing [32]. Oil separation from corn germ using aqueous extraction (AE) and aqueous enzymatic extraction (AEE) was studied and the efficiency of the process evaluated by Dickey, Kurantz, and Parris [33]. A recent AEE study [34] reports 90 wt% oil recovery from wet-milled corn germ, at a 24 g scale.