Starch is a polymer of glucose sugar, meaning that it is composed of many glucose molecules linked in a chain. Plants store starch instead of sugars because starch is less susceptible to pathogens. Maize kernels, for example, contain about 71% starch and only 1% to 3% simple sugars.
Conversion of maize kernels into ethanol begins with dry- or wet-milling. Dry-milling grinds the entire kernel. Wet-milling first removes valuable components of the kernel before ethanol conversion. The process involves steeping kernels in a dilute sulfuric acid bath at about 52°C (125°F) for a day or longer and then coarsely grinding and separating them into the outer skin (bran or hull), which contains most of the fiber; the germ, which is rich in oil; and the endosperm, which is mostly starch and protein. Centrifugation of the endosperm separates starch from protein.
Hydrolysis of starch is the next step in maize biofuels production. During hydrolysis, enzymes break down starch into its glucose subunits. This process proceeds in several stages, beginning with the conversion of raw starch into gelatinized starch that has shorter chains of glucose. This, in turn, is converted into liquefied starch, which has even shorter chains, and then into glucose syrup. Once hydrolysis is complete, yeast are added to ferment the glucose syrup to 10% ethanol or higher. Fermentation followed by distillation and dehydration results in a product that is purer than 99% ethanol.
Processing maize kernels to ethanol is less efficient than processing sugarcane because of the extra steps involved in converting starch to sugar. Starch extraction from the plant biomass, hydrolysis from starch to sugar, fermentation of the sugar, and distillation and dehydration of the ethanol has an efficiency of about 66% in terms of the energy in the final product (ethanol) versus the energy in the original kernel. 
 Patzek, T. W. (2004) Thermodynamics of the corn-ethanol biofuel cycle. Critical Reviews in Plant Sciences 23:519-567; updated July 22, 2006.
This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.
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