**2. Lignocellulose**

Cellulose and hemicellulose, like starch, are made up of sugars. However, most of the cellulose in the nature is in the form of lignocellulose. Lignocellulose is a complex structure of natural materials found in plants. It represents the most abundant source of renewable organic matter on the earth. Cheap lignocellulosic biomass resources can be forestry, agricultural, and agro-industrial wastes. A variety of such materials can be mentioned here including sawdust, poplar trees, sugarcane bagasse, brewer's residue, grasses and straws, stems, leaves, husks, shells, and peels from grains, corn, sorghum, and barley. In contrast to a desire of utilizing these materials to produce valuable products, lignocellulose wastes are still accumulated every year in large quantities, causing environmental problems [3].

Lignocellulose consists of cellulose, hemicellulose, and lignin and always exists beside other extracts and mineral traces. The general composition of lignocellulose is presented in **Table 2**. In lignocellulose, cellulose fiber strands are formed by cellulose linking to each other via hydrogen bonding. The cellulose structure within the polymer is not homogenous. Crystalline regions are where cellulose nano-fibrils are organized in order and compact, while amorphous regions are disordered and easier to be hydrolyzed [1]. Cellulose fibers are like skeletons surrounded by hemicellulose and lignin (**Figure 1**). This structure naturally protects the polysaccharides from hydrolysis by enzymes and chemicals, thus raising a difficulty in both chemical and bioconversion of lignocellulose to other products, i.e., ethanol.

*Bioethanol Production from Lignocellulosic Biomass DOI: http://dx.doi.org/10.5772/intechopen.86437*

In lignocellulose, besides cellulose, hemicellulose is also a noticeable polysaccharide. Hemicellulose is a linear and branched heterogeneous polymer typically made up of five different sugars—L-arabinose, D-galactose, D-glucose, D-mannose, and D-xylose. The backbone of the chains of hemicelluloses can be either a homopolymer or a heteropolymer (mixture of different sugars). Hemicelluloses differ from cellulose not only by the different sugar units but also by their molecular morphology of being amorphous, where shorter chains are branching from the main chain molecules. As a result of this chemical characteristic, hemicellulose is easier to be hydrolyzed than cellulose [3].

Coating the fibrils and playing the role as an adhesive bunching the fibers in lignocellulose, lignins are high-molecular-weight, hydrophobic heteropolymer with complex and variable structures. Lignins are composed of phenylpropanoid alcohols, mainly coniferyl, sinapyl, and coumaryl alcohols with hydroxyl, methoxyl, and carbonyl functional groups (**Figure 2**). The ratio of these three monomers in lignins varies between angiosperms and gymnosperms and


#### **Table 2.**

*Typical chemical compositions of lignocellulosic biomass [4, 5].*

#### **Figure 1.**

*Lignocellulose and its components (Image: USDA Agricultural Research Service).*

**Figure 2.**

*The typical aromatic alcohols as monomers of lignins.*

between different plants. Lignins play a role as cross-linking cellulose and hemicellulose in the matrix. Lignins can be dissolved in acidic and alkaline solutions with the solubility depending on their precursors. These properties of lignins make the lignocellulose structure more robust and hard in nature, resisting hydrolysis agents. Therefore, pretreatment to remove lignins from lignocellulose and enhance the penetration of hydrolysis agents is a vital step in the process of converting biomass to bioethanol [2].
