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6 Wood Discoloration

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4.5 Lignin Degradation 99<br />

The accessibility of the cellulose for cellulases can be improved by several<br />

pretreatments of wood (Chap. 9): for example, soaking increases the pore areas,<br />

and chemical pretreatments decrease the lignin content.<br />

4.5<br />

Lignin Degradation<br />

Next to cellulose lignin is the most abundant polymeric organic substance<br />

in plants. Of about 10 11 t of annual production of terrestrial biomass, about<br />

2×10 10 t are lignin (Jennings and Lysek 1999). Lignin is contrary to linear<br />

polysaccharides, like cellulose, a complex, stereoirregular, three-dimensional<br />

macromolecule (see Fig. 4.4, Nimz 1974; Higuchi 2002) in the range of 100 kDa<br />

(Abreu et al. 1999) and is highly hydrophobic reducing the hygroscopicity of<br />

wood. Lignin functions as a binding and encrusting material in the cell wall<br />

distributed with hemicelluloses in the spaces of inter-cellulose microfibrils in<br />

the cell wall. It acts as a cementing component to connect cells and harden<br />

Fig.4.4. Structural scheme of beech lignin (modified from Nimz 1974)<br />

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