Commercial production of fuel ethanol in the U.S. involves breaking down the starch present in corn into simple sugars (glucose), feeding these sugars to yeast (fermentation), and then recovering the main product (ethanol) and byproducts (animal feed and carbon dioxide). Ethanol is an alcohol produced by yeast from sugars. Fuel ethanol is ethanol that has been highly concentrated to remove water and blended with other compounds to render the alcohol undrinkable. Ethanol production is based upon a process called “fermentation,” in which yeast eat simple sugars and produce carbon dioxide and ethanol as waste products. For each pound of simple sugars, yeast can produce approximately 1/2 pound (0.15 gallons) of ethanol and an equivalent amount of carbon dioxide.
The value of corn as a feedstock for ethanol production is due to the large amount of carbohydrates, specifically starch. Starch can be easily processed to break down into simple sugars, which can then be fed to yeast to produce ethanol. Modern ethanol production can produce approximately 2.8 gallons of fuel ethanol per bushel of corn. The drymill ethanol production process uses only the starch portion of the corn, which is about 70% of the kernel. All the remaining nutrients: protein, fat, minerals, and vitamins, are concentrated into distillers grain, a valuable feed for livestock. About 40% of the United States’ corn crop is used to produce ethanol.
Ethanol is a renewable fuel. Because it is domestically produced, ethanol helps reduce America’s dependence upon foreign sources of energy. In 2014, 14.3 billion gallons of American ethanol displaced the need for 512 million barrels of crude oil, which is more than the amount of oil imported annually from Saudi Arabia.
The energy used to produce ethanol includes fuel for tractors, combines, and transportation of the grain to the ethanol plant, as well as the energy in processing the corn to ethanol. However, the largest portion of the total energy present in corn is solar energy captured by the corn plant and stored in the grain as starch. When these amounts are totaled, the energy in the ethanol exceeds the fossil fuel energy used to grow and process the corn by 20 to 40%. Most of the energy for processing corn to ethanol is spent on the distillation and DDGS drying steps of the process. When wet distillers’ grain can be fed to livestock close to the ethanol plant, the savings in natural gas for drying can be as high as 20% of the total energy cost for processing corn to ethanol. Corn ethanol has some lifecycle greenhouse gas benefits compared to regular gasoline. The general consensus among researchers is that corn ethanol provides a lifecycle reduction of 10–20% in greenhouse gases compared to traditional gasoline. In 2014, this was the equivalent to taking 20 million vehicles off of the road.