Since the 1980s ethanol production has steadily increased in the United States with the need to reduce energy dependence on foreign oil supplies. With falling production costs the momentum of ethanol production has continued. The US Clean Air Act has specified a percentage of renewable fuels to be mixed with gasoline. High oil prices spurred the risk-taking that is needed to develop cellulose-to-ethanol production.
Ethanol production is by one of two methods, i.e. wet milling and dry milling. Wet mills are expensive to construct but versatile in the products they can produce, and have valuable byproducts. Dry milling requires plants that are smaller giving a chance for local farmer cooperatives to flourish. They yield higher quantities of ethanol.
In both processes, the corn is cleaned before it enters the mill. The dry mill process of ethanol production the corn is cleaned first. The dry mill process involves grinding the corn and adding water to the mash. On the other hand, the wet mill process is more complicated as the grain must be separated into its components to begin the process. In dry milling the entire mash is fermented and in wet milling only starch is fermented. After the starch is cooked or liquefied an enzyme is added to hydrolyze the long starch chains. A second enzyme turns the starch into glucose by a process called saccharification. In wet mills this process can take 48 hours and in dry mills the process has been combined with the fermentation step in a process called simultaneous saccharification and fermentation (SSF). Glucose is fermented into ethanol by yeast at 95 degrees.
A distillation step, an energy-consuming process, separates the ethanol from the alcohol-water solution. A two-part process consists of primary distillation and dehydration. Primary distillation produces ethanol that is almost 95 percent water-free and dehydration increases the concentration to 99 percent. Finally, in a step called denaturing, gasoline is added to the ethanol that makes it unfit for human consumption.