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Bioethanol: Market and Production Processes 75

3.4.3 Enzymatic hydrolysis of starch
Enzymatic hydrolysis has several advantages compared to acid hydroly-
sis. 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 -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 coprod-
uct, 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. Sacchari-
fication and fermentation of dry-milled corn result in ethanol and dis-
tillers’ dried grains (DDG). When DDG are combined with fermentation
liquids and dried, they result in DDGS as the major feed by-product [10].

Reducing
end

a-amylase
b-amylase
Glucoamylase
Pulluanases and isoamylases
Figure 3.4 Mechanism of action of amylase on starch.