Fuels and Chemicals from Hemicellulose Sugars - Biotechnology in China: Biofuels and Bioenergy - page 201

Table 1 Composition of various agricultural lignocellulosic biomass (adapted from [ 11 ])

Composition (%, dry basis)
Cellulose Hemicellulose Lignin

Corn fiber a

15 35 8

Corn cob 45 35 15

Corn stover 40 25 17

Rice straw 35 25 12

Wheat straw 30 50 20

Sugarcane bagasse 40 24 25

Switchgrass 45 30 12

Coastal bermuda grass 25 35 6

a

Contains 20% starch

hemicelluloses [ 13 ]. Hemicelluloses are heterogeneous polymers of pentoses
(xylose, arabinose), hexoses (mannose, glucose, galactose), and sugar acids.
Unlike cellulose, hemicelluloses are not chemically homogeneous [ 11 ]. The most
relevant hemicelluloses are xylans and glucomannans; hardwood hemicelluloses
contain mostly xylans, whereas softwood hemicelluloses contain mostly gluco-
mannans [ 14 ].
Xylans are the main hemicellulose components of secondary cell walls, con-
stituting about 20-30% of the biomass of hardwoods and herbaceous plants. In
some tissues of grasses and cereals, xylans can account for up to 50% [ 15 ]. Xylans
are usually available in huge amounts as byproducts of forest, agriculture, pulp and
paper industries. Xylans are heteropolysaccharides with homopolymeric backbone
chains of 1,4-linked b-D-xylopyranose units. Besides xylose, xylans may contain
arabinose, glucuronic acid or its 4-O-methyl ether, and acetic, ferulic, and
p-coumaric acids. The frequency and composition of branches are dependent on
the source of xylan [ 11 ]. The backbone consists of O-acetyl, a-L-arabinofuranosyl,
a-1,2-linked glucuronic or 4-O-methylglucuronic acid substituents. However,
unsubstituted linear xylans have also been isolated from some agricultural resi-
dues. Xylans can thus be categorized as linear homoxylan, arabinoxylan, glucu-
ronoxylan, and glucuronoarabinoxylan. About 80% of the xylan backbone is
highly substituted with monomeric side-chains of arabinose or glucuronic acid
linked to O-2 and/or O-3 of xylose residues, and also with oligomeric side chains
containing arabinose, xylose, and sometimes galactose residues (Fig. 1 ).

2.2 Hemicellulose Hydrolysis

Hemicellulose can be converted to various products, using biological or chemical
methods. However, these often require prior hydrolysis of the polysaccharides to
their sugar constituents. The yield, rate of hydrolysis, and type of sugar recovered
depend on the hydrolysis method, as well as the source of substrate and its
composition [ 10 ].