Bioethanol from Lignocellulosic Biomass - Biotechnology in China: Biofuels and Bioenergy - page 29

Fig. 2 Repeating units of hemicelluloses (Reprinted from [ 12 ] with permission)

Unlike cellulose which is crystalline and resistant to degradation, hemicelluloses
are random and amorphous, and thus easily hydrolyzed to monomer sugars.
However, hemicelluloses are embedded and interact with cellulose and lignin,
which significantly increase the strength and toughness of plant cell walls.
Xyloglucan and xylans are major hemicelluloses in plant biomass. Xyloglucan is
abundant in the primary walls, with the oligosaccharide composed of xylose (X) and
glucose (G) with various side chains, XXXG or XXGG for vascular plants including
grain crops, as the repeating unit. Xylans are polysaccharides with b-(1?4)-linked
xylose residues as a backbone, which are often acetylated at the O-3 position of
xylose residues and/or modified by a-(1?2)-linked glucuronosyl and 4-O-methyl
glucuronosyl residues. Xylans, also known as glucuronoxylans, are the dominant
noncellulosic polysaccharide in the secondary walls of dicots. A schematic illustra-
tion of xyloglucan and xylans is given in Fig. 3 . The major sugars in the hydrolysate of
hemicelluloses are therefore xylose, arabinose, glucose and galactose.

2.3 Lignin

Although lignin is a non-sugar-based polymer and cannot be used as feedstock for
ethanol production via microbial fermentation, it exerts a significant impact on the
economic performance of the corresponding bioconversion processes, since most
inhibitors of microbial growth and fermentation come from this compound during the
pretreatment that is needed to render cellulose amenable to enzymatic attack. Mean-
while, as the second most abundant component in biomass after cellulose, lignin yields