6 Wood Discoloration

248 9 Positive Effects of Wood-Inhabiting Microorganisms
tannophilus. Other fungi, e.g., molds, as well as aerobic and/or anaerobic bacteria
can produce amino acids, antibiotics, enzymes, organic acids, solvents,
and vitamins from glucose or glucose-containing wastes. Technical problems
of acid hydrolysis, such as corrosion of the reaction vessels and formation of
noxious by-products, led to research on enzymatic hydrolysis processes, which
promised, for example, higher sugar yields (Saddler and Gregg 1998).
Spent sulphite liquors contain at about 50% of the employed wood as lignin
sulphonic acids and simple sugars from the hemicelluloses. A number of applications
for lignosulphonates or the entire spent sulphite liquors have been
developed in the past (e.g., Faix 1992). Since the early past century, the hexoses
in spent softwood liquors were converted by yeasts to alcohol and the pentoses
in hardwood liquors to fodder or feeding yeast, respectively. For example, a mill
in Switzerland produced (in 1980) in two tanks (320 m 3 ) 82,000 hL alcohol and
7,000 t of yeast cells, respectively. In Sweden, 1.2 million hL of alcohol was produced
in 33 plants in 1945 (Herrick and Hergert 1977). In the 1980s, the sugars
in spent sulphite liquor were converted by means of the soft-rot deuteromycete
Paecilomyces variotii for use as animal feed in Finland (“Pekilo-process”; Forss
et al. 1986). Han et al. (1976) cultured Aureobasidium pullulans on straw hydrolysate
for production of single cell protein. Ek and Eriksson (1980) used
Sporotrichum pulverulentum for water purification and protein production.
Anaerobic treatment of pulp mill effluents by bacteria was reviewed by Guiot
and Frigon (1998).
9.6
Grinding and Steam Explosion
Among the physical pretreatment methods, grinding of lignocelluloses increases
the inner surfaces of the wood cell wall and thus improves the accessibility
for enzymes to the cell wall components. The particle size must be
reduced to 50µm to maximize the effect. The energy costs become prohibitive
at particle sizes of 200µm (Dart and Betts 1991).
Steam explosion methods saturate the lignocellulose with steam and then
allow it to undergo explosive decompression. The treatment releases acids
that contribute to the disruption of the cell wall (Dart and Betts 1991). In the
steaming-extraction process, chopped wood was treated in watery or alkaline
solution for a few minutes at 185–190 ◦ C (1,100–1,200 kPa). Subsequent washing
with water or thin sodium hydroxide solution separated the wood into
a solid component containing lignin and cellulose and a liquid phase of the
hemicelluloses (Dietrichs et al. 1978). In vivo digestibility of wood in a test
cow increased from about 5% of natural wood to 80% for steam-treated wood
(Puls et al. 1983). The hemicellulose fraction was used to produce Paecilomyces
variotii mycelium and enzymes (Schmidt et al. 1979).
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