Cellulose is a hygroscopic polyglycan. The chemical formula of cellulose is (C6H10O5)n where n=10,000 to 15,000 linked together by P(1^4)-glycosidic linkages between the first and fourth carbon atoms of adjacent glucose units. Cellulose molecules are linear glucans ranging from 300,000 Da to 500,000 Da in molecular size. Cellulose may occur in crystalline or amorphous forms where crystalline structures are highly ordered and poorly depolymerized by cellulase when compared to the amorphous forms. The solubility of cellulose is highly dependent on the degree of polymerization (DP). Crystalline and amorphous cellulose structures are governed by inter and intra-hydroxyl groups. Amorphous cellulose has intra-polymer linkages with hemicellulose through H-bonding that is weaker than the H-bonding found in crystalline cellulose. Cellulose microfibrils are usually 5-50 nm in diameter with a few microns in length (Harmsen et al. 2010; Moon et al. 2011).

Several polymorphs of crystalline cellulose including I, II, III and IV have been studied. Crystalline cellulose I is the most common variety found in lignocellulosic biomass. Cellulose I can be converted to cellulose II by regeneration and mercerization processes. However, cellulose III can be generated from cellulose I or II by aqueous ammonia pretreatment. Cellulose IV is generated by thermal pretreatment of cellulose III (Moon et al. 2011).

Table 2. Primary production of biomass derived bioproducts via chemical (C), fermentation (F), enzymatic (E), and natural (N) processes (Klass 1998). Lignocellulosic Biomass C_ sugars C, sugars Cellulose Lignins Others C F F C E C C C&N Xylitol Lipids* Acetaldehyde Hydroxymethyl furfural Fructose Cellulose esters Vanillin Alkaloids Ethanol Acetic Acid Sorbitol Cellulose nitrates Lignin sulfonates Glycerides Acetone Cellulose ethers Gutta Glycerol Cellulose xanthogenates Phenols n-Butanol Resins n-Butyric acid Rubber Amyl alcohols Saponins Oxalic acid Sterols Lactic acid Tail oils Citric acid Tannins Amino acids Tarpenes Antibiotics Vitamins Ethanol Lipids* Waxes * Reference: Fall et al. 1984