6 Wood Discoloration

4.3 Degradation of Hemicelluloses 93
galacturonic acid. Further enzymes are needed for the other pectins and for
side chains.
Various plant pathogenic fungi and bacteria and several soil (Bacillus spp.)
and water bacteria (Clostridium spp.)arecapableofdegradingpectin(Schlegel
1992). Wood-inhabiting bacteria lead to the “overuptake” of wood preservatives
and paints (Willeitner 1971) after the degradation of the bordered sapwood
pits (Chap. 5.2) during wet storage of wood.
4.3
Degradation of Hemicelluloses
Hemicelluloses of wood are a complex combination of relatively short polymers
made of xylose, arabinose, galactose, mannose, and glucose with acetyl and
uronic side-groups. The major hemicellulose (polyose) of hardwoods is the O-
Acetyl-(4-O-methylglucurono)-xylan, also named glucuronoxylan or briefly
xylan. The xylan content in hardwoods ranges from 15 to 35%. For example,
birch wood contains 22–30% xylan and 1–4% glucomannan, while pine
wood contains 5–11% xylan and 14–20% glucomannan (Viikari et al. 1998).
In monocotyledons, hemicelluloses may amount to 40% and exceed the cellulose
portion. Beech wood xylan consists of about 200 β-1-4-linked xylose
units (xylopyranose). About five to seven acetyl groups (linked to C-2 or
C-3) and one 4-O-methylglucuronic acid residue (α-1-2) occur per ten xylose
units (Timell 1967; Fengel and Wegener 1989; Eriksson et al. 1990; Puls 1992;
Fig.4.2. Diagram of enzymatic xylan degradation. x xylose residue, Ac acetic acid residue,
4-O-Me-GA 4-O-methylglucuronic acid residue
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