Lignocellulosic materials predominantly contain a mixture of carbohy­drate polymers (cellulose and hemicellulose) and lignin. The carbohydrate polymers are tightly bound to lignin mainly through hydrogen bonding, but also through some covalent bonding. The contents of cellulose, hemi — cellulose, and lignin in common lignocellulosic materials are listed in Table 3.2. Different types of carbohydrates (glucan, xylan, galactan, arabinan, and mannan), lignin, extractive, and ash content of many lig — nocellulosic materials have been analyzed and are available in the lit­erature [2, 11-14] (see Table 3.2).

1.5.1 Cellulose

Cellulose is the main component of most lignocellulosic materials. Cellulose is a linear polymer of up to 27,000 glucosyl residues linked by ^-1,4 bonds. However, each glucose residue is rotated 180° relative to

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 C6H10O5 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].