Hemicellulose
Hemicelluloses are the second most abundant component in biomass and are composed of a combination of several heteropolymers. The most common ones include xylan, glucuronoxylan, arabinoxy — aln, glucomannan, and xyloglucan. These are often branched polysaccharides. In contrast to cellulose, which contains only D-glucose, hemicellulose contains many different sugar monomers. Most of the sugars in hemicelluloses are 5-carbon D-pentose sugars and occasionally small amounts of L-sugars as well. In most cases xylose is the sugar monomer present in the largest amount, although in softwoods mannose can be the most abundant sugar. Not only regular sugars like xylose, but also carboxylic acid group or their derivative containing sugars like glucuronic acid and galacturonic acid can also be present in hemicellulose. Some common molecular motifs found in hemicellulose are shown in Figure 4.7.