6 Wood Discoloration

5.2 Bacteria 117
Woods from Tertiary fossil forests after 20–60 million years of burial showed
indications that nonbiological degradation was responsible for the changes in
the cell walls (Blanchette et al. 1991).
Bacteria are also involved in wood discoloration. The wood of the light
African Ilomba, Pycnanthus angolensis, is colonized after felling during storage
and shipment of the round timber. The bacteria spread in the stem interior
and cause reddish-brown discoloration. Further discoloration develops during
air-drying of the boards in the area of the stacked wood (sticker stain). As
causal bacteria e.g., Pseudomonas fragi (Eichholz) Gruber was isolated, which
remained active in the damp wood parts (contact with stacked woods) and
increased there the pH value from about 5.5 to 7.5–8.5 by producing ammonia
from the protein of the protein-rich wood species. This alkalinity results in
chemical reactions (phenol oxidation and polymerization) of accessory components
in the parenchyma cells, which cause the brown discoloration (Bauch
et al. 1985). The bacterium also discolored wood samples in vitro (Fig. 5.5).
Bacterial discoloration of Ilomba wood during air-drying could be almost
completely prevented by previous dipping the fresh boards in a solution of
each 5% formic acid and propionic acid.
Several bacteria were isolated from beech trees that possessed an irregular
stellar-shaped red heart (splash-heart). The bacteria caused also in vitro
brown discoloration of light beech wood samples and wood capillary liquids
by raising the pH value to over 7.3 (Schmidt and Mehringer 1989; also Mahler
et al. 1986; Walter 1993).
Pseudomonas aeruginosa (Schroeter) Migula discolored Obeche, Triplochiton
scleroxylon (Hansen 1988). In water-stored pine stems, bacteria produced
flavonoids from flavone glycosides, which diffused to the wood surface during
drying the sawn timber and caused there brown discolorations (Hedley and
Meder 1992).
Bacteria were inhibited by chromium-copper wood preservatives and further
preservation salts used against fungi. Concentrations used for fungi were
mostly sufficient to prevent bacterial activity (Schmidt and Liese 1974, 1976;
Liese and Schmidt 1975; Schmidt et al. 1975). Archaeological woods, like
the Bremen Cog of 1380, are stabilized against further deterioration using
polyethylene glycol (Hoffmann et al. 2004).
Fig.5.5.Bacterial discoloration of Ilomba
wood within 1 day by a pure culture of
Pseudomonas fragi inoculated as a line
on the light wood sample
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