Lignin, a very complex polymer, playing a cementing role to connect cells, increases the mechanical strength properties, and makes plant resistant against diseases and biodegradation by microorganisms. Lignin is sometimes referred as glue between hemicellulose and cellulose components; while sometimes the hemicellulose is referred as glue between lignin and cellulose. Anyway, hemi — cellulose and lignin are known to cover the surface of cellulose which adds structural strength to the cellulose matrix (Perez and Samain 2010). Softwoods (25-40 %) contain higher lignin than hardwoods (18-25 %) and agriculture resi­dues (10-20 %) (Fengel and Wegener 1984; McMillan 1992); however, the lignin content is not the only difference between softwoods and other lignocelluloses. The main distinction is originated from the difference in monomeric units and linkage types in lignin. This dissimilarity in the lignin content may result in significant differences in susceptibility of various pretreatment techniques between hardwoods and softwoods. Pretreatment of hardwoods and agriculture residues is usually less harsh than softwoods. The reason is the presence of higher number of vessels in the hardwoods and agriculture residues which permit greater heat and mass transfer into the biomass matrix (Cochard and Tyree 1990; Hepworth et al. 2002; Kim et al. 2011). Generally, easier penetration of chemicals, enzymes, and heat makes the hardwoods and agriculture residues easier for pretreatment than softwoods.

Lignin is a cross-linked polymer of hydroxyphenylpropanoid units connected by C-C and C-O-C linkages, in which over 10 inter-phenylpropane linkage types have been detected. There are several monomeric units and linkage types in lignin. There are two major classes of lignin, guaiacyl lignins (G-lignin) and syringyl lignins (S-lignin). They contain guaiacyl (G), Syringyl (S), and hydroxybenzal — dehyde (H) units (Lewis and Yamamoto 1990). Different lignocellulose materials with different age and cultivation conditions have different ratios of G, S, and H. The lower accessibility of plant vessels is partially the result of the occurrence of guaiacyl lignin type in the vessel walls. Therefore, not only the amount of lignin, but also the guaiacyl to syringyl ratio in lignin can affect the swelling of the cellulosic residue (Ramos et al. 1992). The principal structural elements in lignins have been largely investigated; however, many aspects of the lignin chemistry are still unclear.

For lignin synthesis in woody materials, a series of secondary reactions are recognized leading to cross-linking between lignin and hemicelluloses (Lee 1997). Biodegradation of lignin is a secondary metabolic process, occurring only under low levels of nitrogen (Lee 1997).

During plant biosynthesis, it is believed that lignin is not simply deposited between cellulose and hemicellulose, but is linked with at least part of them. These linkages are termed as lignin-polysaccharide complex (LPC) or lignin-carbohy­drate complex (LCC) (Chesson 1988). Thus, as a result of these linkages, it is almost impossible to completely separate or purify cellulose or hemicellulose from lignin, and to have lignin free of polysaccharides. Furthermore, lignin has a ten­dency of recondensation during delignification processes (Kim et al. 2003). Not only are van der Waals and H-bond involved, but chemical bonds such as covalent bonds are also detected between lignin and polysaccharides (Besombes and Mazeau 2005).

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