Lignin is a phenolic compound composed of three-dimensional amorphous phenylpropane units or C9 units. The most common phenylpropane unit consists of p-courmaryl alcohol, coniferyl alcohol and sinapyl alcohol units (Fig. 1), which are water insoluble. However, the compounds may be significantly dissolved in low-molecular weight organic solvents such as methanol and ethanol (Mantanis 1994). When lignocellulosic compounds are hydrolyzed by strong sulfuric acid, most of the lignin is insoluble in the acid solution and will precipitate as acid-insoluble lignin. However, small quantities of lignin, known as acid-soluble lignin (ASL) remain in solution and may be determined using analytical procedures such as NREL Laboratory Analytical Procedures (NREL LAP-004) to complete the mass balance for total lignin fractions.

Lignin is intermixed with cellulose and hemicellulose polymer chains by intra — and inter-polymer linkages. Lignin polymers are connected to each other mainly through ether bonds and carbon-carbon bonds (Fig. 2). Ether bonds constitute two-thirds of the total bonds between the lignin

monomers. In addition, lignin is connected to cellulose and hemicelluloses via hydrogen bonds and ester bonds (Gupta 2008; Harmsen et al. 2010). These bonds play an important role in selection of pretreatment method to remove lignin from the biomass.

Softwoods contains higher amounts of lignin compared to hardwoods and grasses (Table 1). In hardwoods, lignin is typically composed of guaiacylpropane and syringylpropane, with a small amount of p-hydroxyphenylpropane units (Lee 2005). In softwoods, lignin is typically constituted of guaiacylpropane with traces of p-hydroxyphenylpropane units (Lee 2005). Lignin composition in grasses mainly contains both guaiacylpropane and syringylpropane units with a small amount of p-hydroxyphenylpropane units (Lee 2005).