Enzymatic hydrolysis has several advantages compared to acid hydroly­sis. First, the specificity of enzymes allows the production of sugar syrups with well-defined physical and chemical properties. Second, milder enzymatic hydrolysis results in few side reactions and less “browning” [8]. Different types of enzymes involved in the enzymatic hydrolysis of starch are a-amylase, ^-amylase, glucoamylase, pullua — nases, and isoamylases. The mechanism of action of these enzymes is presented schematically in Fig. 3.4.

There are two popular industrial processes from starch materials, dry milling and wet milling. In the dry-milling process, grain is first ground into flour and then processed without separation of the starch from germ and fiber components. In this method, the mixture of starch and other components is processed. Starch is converted to sugar in two stages: liquefaction and saccharification, by adding water, enzymes, and heat (enzymatic hydrolysis). Dry-milling processes produce a coprod­uct, distillers’ dried grains with solubles (DDGS), which is used as an animal-feed supplement. Without the revenues from that coproduct, ethanol from dry-milled corn processing would not be economically favorable [2]. A dry-milling process for alcohol production processes the whole grain, or components derived from the whole grain. Sacchari­fication and fermentation of dry-milled corn result in ethanol and dis­tillers’ dried grains (DDG). When DDG are combined with fermentation liquids and dried, they result in DDGS as the major feed by-product [10].

In the wet-milling process, grain is steeped and separated into starch, germ, and fiber components. Wet milling is capital intensive, but it gen­erates numerous coproducts that help to improve the overall production economics [2]. Wet mills produce corn gluten feed, corn gluten meal, corn germ, and other related coproducts. In this method, after the grain is cleaned, it is steeped and then ground to remove the germ. Further grind­ing, washing, and filtering steps separate the fiber and gluten. The starch that remains after these separation steps is then broken down into fer­mentable sugars by the addition of enzymes in the liquefaction and sac­charification stages. The fermentable sugars produced are then subjected to fermentation for ethanol production, like the other fermentable sugars.