6 Wood Discoloration

238 9 Positive Effects of Wood-Inhabiting Microorganisms
ably also the wood-degrading bacteria only clear the hurdle of lignification,
exclusively the white-rot fungi and their ligninolytic system additionally use
the lignin as a carbon source and are therefore predestined for bioconversions
(Table 4.3). All other microorganisms as well as their isolated enzymes need
first a pretreatment of the substrate wood, which loosens the chemical/physical
association of carbohydrates and lignin or reduce the lignin content or improve
the physical accessibility of the degrading agents to the substrate. The various
possibilities of a pretreatment can be grouped into biological, chemical,
and physical methods (Dart and Betts 1991). Saddler and Gregg (1998) distinguished
four main pretreatment methods currently being researched and
commercialized to make lignocelluloses more easily digestible to hydrolytic
enzymes while preserving the yield of the original carbohydrates for bioconversions:
organosolv, steam explosion, dilute-acid prehydrolysis, and ammonia
fiber explosion. Some of the bioconversions described below like “myco-wood”
or “palo podrido” may occur a little strangely to some readers, but are examples
that wood bioconversion can work.
9.1
“Myco-Wood”
In Eberswalde, Germany, around 1930, J. Liese started to cultivate edible mushrooms
on wood like Flammulina velutipes, Kuehneromyces mutabilis, Lentinula
edodes (Fig. 2.17a) and Pleurotus ostreatus to improve the food situation
of the population (Liese 1934). Due to the import stop of wood from overseas
into the German Democratic Republic (GDR) at that time which was
needed for pencils etc., his student, W. Luthardt thought about a possible
use of the wood substrate remaining after mushroom production to produce
pencils and other form-stable products. In 1956, Luthardt got the patent for
“myco-wood” for the GDR and in 1957 under license for the Federal Republic
of Germany: “Myco-wood is a wood that is loosened through the controlled
action of certain wood-inhabiting fungi and which has changed its technological
characteristics to a large extent or may obtain defined technical qualities”
(Luthardt 1969). For myco-wood production, 50-cm-long stem sections of Fagus
sylvatica were inoculated on the crosscut surface with a mycelium paste of
Pleurotus ostreatus or Trametes versicolor, respectively, and were incubated in
the constant climate of former air-raid shelters for different periods. Through
the controlled white rot, a white and porous raw material free from tension
was obtained that showed improved carving and sharpening ability to be used
for form-constant products like pencils, rulers, and drawing boards. For example,
after 3 months of incubation, the wood showed 30% mass loss, was
completely colonized by mycelium, and was now suitable for rulers. One of
these rulers is still used in our laboratory and looks like newly manufactured.
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