There are various raw materials that contain starch and are suitable for ethanol production. Corn is the most widely used on an industrial scale for this purpose. However, there are several other cereals, such as wheat, rye, barley, and sorghum, and crop roots such as potato and cas­sava, which are used as raw materials for ethanol production. The cereals contain about 60-70% starch, 8-12% proteins, 10-15% water, and small

amounts of fats and fibers. The compositions of the crop roots are almost identical to those of the cereals on a dry basis, but the water content of the roots is usually 70-80%. The exact composition of each raw mate­rial depends on the type and variety of materials used and can be found in literature (e. g., [7]). Starch from these materials is used as a carbon and energy source, and part of the proteins as a nitrogen source, by the microorganisms.

Starch contains two fractions: amylose and amylopectin. Amylose, which typically constitutes about 20% of starch, is a straight-chain polymer of a-glucose subunits with a molecular weight that may vary from several thousands to half a million. Amylose is a water-insoluble polymer. The bulk of starch is amylopectin, which is also a polymer of glucose. Amylopectin contains a substantial number of branches in the molecular chains. Branches occur from the ends of amylose seg­ments, averaging 25 glucose units in length. Amylopectin molecules are typically larger than amylose, with molecular weights ranging up to 1-2 Mg. Amylopectin is soluble in water and can form a gel by absorb­ing water.

For ethanol production, hydrolysis is necessary for converting starch into fermentable sugar available to microorganisms. Traditional con­version of starch into sugar monomers requires a two-stage hydrolysis process: liquefaction of large starch molecules to oligomers, and sac­charification of the oligomers to sugar monomers. This hydrolysis may be catalyzed by acid or amylolytic enzymes.