Biotechnological processes for conversion of corn into ethanolAbstractBackgroundCurrent ethanol production processesDry grind ethanol productionStarch conversionFermentationDistillation and dehydrationStillage processingFeed products from stillage processingFuture directionsHigh fermentable hybridsFiber conversionNew and higher-value coproductsReferencesAppl Microbiol Biotechnol (2005) 67: 19–25DOI 10.1007/s00253-004-1819-8MINI-REVIEWR. J. Bothast.M. A. SchlicherBiotechnological processes for conversion of corn into ethanolReceived: 15 July 2004 / Revised: 11 October 2004 / Accepted: 15 October 2004 / Published online: 14 December 2004# Springer-Verlag 2004Abstract Ethanol has been utilized as a fuel source in theUnited States since the turn of the century. However, it hasrepeatedly faced significant commercial viability obstaclesrelative to petroleum. Renewed interest exists in ethanol asa fuel source today owing to its positive impact on ruralAmerica, the environment and United States energysecurity. Today, most fuel ethanol is produced by eitherthe dry grind or the wet mill process. Current technologiesallow for 2.5 gallons (wet mill process) to 2.8 gallons (drygrind process) of ethanol (1 gallon = 3.785 l) per bushel ofcorn. Valuable co-products, distillers dried grains withsolubles (dry grind) and corn gluten meal and feed (wetmill), are also generated in the production of ethanol.While current supplies are generated from both processes,the majority of the growth in the industry is from dry grindplant construction in rural communities across the cornbelt. While fuel ethanol production is an energy-efficientprocess today, additional research is occurring to improveits long-term economic viability. Three of the mostsignificant areas of research are in the production ofhybrids with a higher starch content or a higher extractablestarch content, in the conversion of the corn kernel fiberfraction to ethanol, and in the identification and develop-ment of new and higher-value co-products.BackgroundThe production of ethanol from corn for use as atransportation fuel is a mature technology. It was firstintroduced in the United States in the early 1900s. Theearly Ford Model T had a carburetor adjustment thatallowed the vehicle to run on either gasoline or ethanolproduced by American farmers. Henry Ford’s vision wasto build a vehicle that was affordable to the workingfamily and powered by a fuel that would boost the ruralfarm economy (Kovarik 1998).Ethanol was used as a fuel source for cars well into the1930s. Post World War II, however, little interest remainedin using agricultural crops for liquid fuel productionbecause of the abundant and cheap supply of fuel frompetroleum and natural gas. Renewed interest in ethanoldeveloped in the 1970s with oil supply disruptions fromthe Middle East and the phase-out of lead as an octanebooster for gasoline (Hunt 1981). Additional Federal andState tax incentives helped fuel the revitalization of theethanol industry from production volumes of 10×106gallons in 1979 to 2.81×109gallons in 2003 (1 gallon =3.785 l; Anonymous 1999, 1981). Together, the passage ofthe Clean Air Act Amendments by Congress in 1990(which mandated the use of oxygenated fuels), questionsconcerning the oxygen source methyl tert-butyl ether(MTBE), and the opportunity to spur rural developmenthave further accelerated the growth of the domesticethanol industry. With today’s limited oil supplies andthe ever-increasing United States’ dependence on foreignoil (over 62% imported), the need for alternative energysources is receiving a renewed focus. Fuel ethanol remainsan attractive option. Ethanol has strategic value because itis a renewable energy source and reduces the UnitedStates’ dependence on foreign oil imports. It benefitsfarmers by creating a substantial new market for cornsupplies and by creating new jobs in economicallydepressed rural areas and small communities. As a fuelcomponent, it burns cleanly and increases the octane levelof gasoline. Because ethanol has a higher oxygen contentthan MTBE, only half the volume is required to producethe same oxygen level in gasoline; and it is biodegradable(DiPardo 2000).Most of the current ethanol produced in the UnitedStates uses field corn as a feedstock. Corn is the mostimportant and economical source of starch in the UnitedStates. Starch is the major carbohydrate storage product incorn kernels comprising 70–72% of the kernel weight on aR. J. Bothast (*).M. A. SchlicherNational Corn-To-Ethanol Research Center, Southern IllinoisUniversity Edwardsville,400 University Park Drive,Edwardsville, IL 62025, USAe-mail: [email protected].: +1-618-6596737Fax: +1-618-6598762dry weight basis. Starch is readily converted to glucoseand fermented into ethanol. In 2003, conversion to ethanolaccounted for nearly 109bushels of corn, or 10% of theUnited States corn crop. Projections are for ethanoldemand to reach 5×109gallons by 2012. BBI Internationalmaintains records on current and proposed capacity. As ofMay 2004, they cite 76 currently running ethanol plants(with a capacity of over 3×109gallons of ethanol) and 12additional plants under construction, adding an incremen-tal 0.5×109gallons in commercial capacity. Tax incentivesand commercial efficiencies are expected to keep pacewith the expected future demand.The United States Department of Agriculture and theDepartment of Energy at Argonne National Laboratorieshave conducted numerous studies on the energy balance ofethanol production. They both concluded that a gallon ofethanol produces more energy than the fossil inputs toproduce it (Shapouri et al. 1996; Wang et al. 1997). Themost recently published results by Shapouri indicate anaverage (across dry grind and wet mill processes) energyyield of 67% more than the fossil inputs required toproduce it (Shapouri et al. 2004). This has been an area ofsignificant improvement in ethanol production, withenergy requirements at 50% less than what was requiredfor ethanol production in the late 1970s. Alternatively,gasoline yields 20% less energy than the fossil inputsrequired to produce it (Shapouri et al. 1996; Wang et al.1997).Current ethanol production processesToday, most fuel ethanol is produced from corn by eitherthe dry grind (67%) or the wet mill (33%) process. Thekey distinction between wet mill and dry grind facilities isthe focus of the resourcing. In the case of a dry grind plant,the focus is maximizing the capital return per