All potable alcohol and most fermented industrial alcohol in the United States are currently made from grains. Typically, corn contains 15% moisture, 7.7% protein, 3.3% oil, and 62% starch. Fermentation of starch from corn is somewhat more complex than fermentation of sugars because starch must first be converted to sugar by hydrolysis, which is typically carried out enzymatically by diastase present in sprouting grain or by fungal amylase. The resulting dextrose is fermented to ethanol with the aid of yeast producing. A second coproduct of unfermented starch, fiber, protein, and ash known as distillers grain (a high-protein cattle feed) is also produced. The main component of interest in ethanol production is the starch content of corn. Starch is a polymer of glucose subunits (and C12H16O5 repeat unit) linked via α-1,4 linkages, with some branches formed by α-1,6 linkages (Figure 5.3). The starch polymer is highly amorphous, making it readily digested by human and animal enzyme systems for hydrolysis into glucose units. There are two distinct methods for processing corn, wet milling and dry milling, and each method generates unique coproducts.
Ethanol burns more cleanly in air than petroleum, producing less carbon (soot) and carbon monoxide.
The use of ethanol as opposed to petroleum could reduce carbon dioxide emissions, provided that a renewable energy resource was used to produce crops required to obtain ethanol and to distil fermented ethanol.…