Lignocellulosic biomass is made up of very complex biopolymers that are not used in human food. The main components of lignocellulosic biomass are cellu­lose, hemicellulose, and lignin in addition to a small amount of extractives, acids, and minerals. For its conversion into ethanol, a complex process of pretreatment and hydrolysis is done in order to transform the carbohydrate polymers (cellulose and hemicellulose) into fermentable sugars.

Cellulose is a P-glucan, i. e., a polymer composed of glucose molecules linked by P(1,4) bonds. It can be considered that the cellulose is a linear polymer made up of cellobiose monomers as shown in Figure 3.6. The polymerization degree of cellulose is about 7,000 to 15,000. Due to its linear nature and to the interactions by hydrogen bonds between the OH groups of a same chain or of different chains, cellulose molecules are oriented by length leading to the formation of very stable crystalline structures. These structures allow the bundles of cellulose chains to form rigid, difficult to break microfibers. For this reason, the main function of cellulose in plants is structural, which explains its majority presence in the cell wall. In general, the cellulose composes 40 to 60% of dry matter of lignocellu — losic biomass (Hamelinck et al., 2003).

Hemicellulose composes 20 to 40% of lignocellulosic biomass and consists of short, very branched chains of sugars (200 sugars on average). Among these sugars are, in their order, xylose and arabinose (both 5-carbon sugars or pentoses), and galactose, glucose, and mannose (these latter sugars are hexoses). Other carbo­hydrate-related compounds like glucuronic, methyl glucuronic, and galacturonic acids are also present in hemicellulose structure. Furthermore, hemicellulose con­tains, in a lower proportion, acetyl groups esterified to some OH groups of its dif­ferent sugars. Due to the predominance of xylose, hemicellulose can be considered as a xylan. For lignocellulosic materials derived from hardwood, the xylan back­bone is composed of xylose units linked by P(1,4) bonds that branch through a(1,2) bonds with the methyl glucuronic acid (Figure 3.7a). In the case of xylan from softwood, the acetyl groups are less frequent, but there exist more branches due to the presence of a(1,3) between the xylose backbone and arabinofuranose units (Figure 3.7b). Considering its branched structure, hemicellulose does not form crystalline structures, but amorphous ones. Thus, this biopolymer is more soluble in water and has a higher susceptibility to the hydrolysis (Hamelinck et al., 2003).

The lignin comprises from 10 to 25% lignocellulosic biomass. This component is a very complex phenolic polymer composed of phenyl propane units linked by C-C and C-O-C bonds forming a three-dimensional amorphous structure (Lee, 1997). The structural units of lignin are the cinnamyl alcohols, which are differenti­ated by the various substitutions that the aromatic ring presents (Oliva, 2003). Thus, p-hydroxyphenyl units are derived from the p-coumaryl alcohol, the guaiacyl units are derived from the coniferilyc alcohol, and the syringyl units are derived from the sinapyl alcohol (Figure 3.8). The lignin has hydrophobic character and its main func­tion is as incrustive material of a cell wall, i. e., as a sort of cement between the cells.

The interaction and combination between the hemicellulose and lignin provide a covering shell to the cellulose making its degradation more difficult (Figure 3.9). Precisely, the main aim of biomass pretreatment is to break the lignin seal and significantly reduce the proportion of crystalline cellulose in such a way that the enzymes hydrolyzing the cellulose (cellulases) can have greater access to this polysaccharide and convert it into fermentable sugars.