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Bioethanol: Market and Production Processes 77

its neighbors so that the basic repeating unit is in fact cellobiose, a
dimer of a two-glucose unit. As glucose units are linked together into
polymer chains, a molecule of water is lost, which makes the chemical
formula C 6 H 10 O 5 for each monomer unit of “glucan.” The parallel
polyglucan chains form numerous intra- and intermolecular hydrogen
bonds, which result in a highly ordered crystalline structure of native
cellulose, interspersed with less-ordered amorphous regions [15, 16].

3.5.2 Hemicellulose
Hemicelluloses are heterogeneous polymers of pentoses (e.g., xylose and
arabinose), hexoses (e.g., mannose, glucose, and galactose), and sugar
acids. Unlike cellulose, hemicelluloses are not chemically homogeneous.
Hemicelluloses are relatively easily hydrolyzed by acids to their monomer
components consisting of glucose, mannose, galactose, xylose, arabinose,
and small amounts of rhamnose, glucuronic acid, methylglucuronic acid,
and galacturonic acid. Hardwood hemicelluloses contain mostly xylans,
whereas softwood hemicelluloses contain mostly glucomannans. Xylans
are the most abundant hemicelluloses. Xylans of many plant materials
are heteropolysaccharides with homopolymeric backbone chains of 1,4-
linked -D-xylopyranose units. Xylans from different sources, such as
grasses, cereals, softwood, and hardwood, differ in composition. Besides
xylose, xylans may contain arabinose, glucuronic acid or its 4-O-methyl
ether, and acetic, ferulic, and p-coumaric acids. The degree of polymer-
ization of hardwood xylans (150–200) is higher than that of softwoods
(70–130) [14, 15].

3.5.3 Lignin
Lignin is a very complex molecule. It is an aromatic polymer constructed
of phenylpropane units linked in a three-dimensional structure.
Generally, softwoods contain more lignin than hardwoods. Lignins are
divided into two classes, namely, “guaiacyl lignins” and “guaiacyl-
syringyl lignins.” Although the principal structural elements in lignin
have been largely clarified, many aspects of their chemistry remain
unclear. Chemical bonds have been reported between lignin and hemi-
cellulose, and even cellulose. Lignins are extremely resistant to chem-
ical and enzymatic degradation. Biological degradation can be achieved
mainly by fungi, but also by certain actinomycetes [15, 17].

3.6 Sugar Solution from Lignocellulosic
Materials

There are several possible ways to hydrolyze lignocellulose (see Fig. 3.5).
The most commonly applied methods can be classified into two groups:
chemical hydrolysis and enzymatic hydrolysis. In addition, there are