6 Wood Discoloration

4.1 Enzymes and Low Molecular Agents 91
Table 4.2. Characteristics that make wood recalcitrant to fungi and bacteria
– accessory compounds in the heartwood
– resin excretion of softwoods, wound reactions of parenchyma cells in hardwoods
– polymeric structure of the cell wall components
– extracellular degradation of the nutrients
– small pore sizes within the cell wall
– complex structure of the woody cell wall
– partially crystalline nature of cellulose
– incrustation of the more easily degradable carbohydrates by lignin
– structure and non-water-solubility of lignin
1984; Murmanis et al. 1987). Thus, cellulases are too large for diffusing into
the capillary areas of the cell wall from 0.5–4 nm pore size (average in spruce:
1 nm: Reese 1977; Kollmann 1987) (Keilisch et al. 1970; Flournoy et al. 1991).
This pore size excludes compounds with kDa mass greater than 6. Bailey et al.
(1968) postulated a “pre-cellulolytic phase”. Meanwhile, so-called low molecular
weight agents are known to be involved in the decay of the woody cell
wall. As the different groups of wood decay fungi differ with regard to the
participating low molecular agents, these aspects are treated separately in the
chapters on cellulose and lignin degradation.
The complex ultrastructure of the woody cell wall (e.g., Booker and Sell
1998) affects its degradation (Liese 1970; Daniel 2003). A great part of the cellulose
is bundled up by hydrogen bonds to larger, crystalline units (“crystalline
cellulose”, Fig. 4.3), the elementary fibrils. The crystalline nature of the cellulose
prevents an attack of many microorganisms. Several elementary fibrils
result by linkage with hemicelluloses in the next larger unit, the microfibril. At
the surface of the microfibrils, hemicelluloses form a bridge to the incrusting
lignin, as chemical bonds exist between lignin and hemicelluloses (lignin carbohydrate
complex, Koshijima and Watanabe 2003; Fig. 4.1). Several models
depicting this molecular arrangement have developed (e.g., Kerr and Goring
1975; Fengel and Wegener 1989) although there is no accepted model (Daniel
2003).
Principally, the carbohydrates cellulose and hemicelluloses are rather easily
degradable, however, the lignin is resistant to most microorganisms due
to its structure of phenylpropane units and the recalcitrant linkages between
them. Thus, lignin incrustation of the carbohydrates inhibits the access to
the consumable holocellulose. The hydrophobic nature of lignin further prevents
a diffusion of the degrading enzymes inside the three-dimensional giant
molecule.
The composition of the microbial enzyme apparatus and its regulation affect
the type of rot. Lignin (Fig. 4.4) is effectively degraded only by white-rot fungi
and acts for other microorganisms as a barrier against wood decay. Table 4.3
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