Lignin is an aromatic network polymer composed of phenylpropane units [53]. It is generally accepted that lignin is the “glue” that binds cellulose and hemicellulose, imparting both rigidity and moisture resistance to the lig — nocellulosic structure. Lignin has also been implicated as an inhibitor of cellulases; therefore, many of the pretreatment methods currently being ex­plored have tried to decrease the lignin content of the solid substrate while minimizing the degradation of carbohydrates [22]. The amount of lignin that must be dealt with by a particular pretreatment and subsequent hydro­lysis depends on the source of biomass. For example, corn fiber has a low lignin content of 7% (w/w) [23] compared to 30% (w/w) in the case of a soft­wood such as Douglas-fir [51]. In addition to the amount of lignin present in a biomass feedstock, the type of lignin differs between agricultural residues, hardwoods and softwoods [54]. Grasses and agricultural residues contain primarily p-hyroxyphenyl units while hardwoods and softwoods contain greater amounts of syringyl and guaiacyl subunits, respectively [54]. Soft­woods lignin also exhibits a greater degree of cross-linking due to an extra linking site provided by the presence of only a single methoxyl group on the guaiacyl aromatic ring [54]. Another factor that must be considered is the ex­istence of lignin carbohydrate complexes (LCCs) that consist of lignin linked to carbohydrates through bonds such as ester, ether or ketal [55]. LCCs have been shown to be particularly problematic for hydrolysis processes, as access to the carbohydrate fraction is restricted by the attached lignin, therefore pre­treatment processes should either expand the pore structure of the substrate or remove the lignin outright [56].

3.1