6 Wood Discoloration

9.7 Recent Biotechnological Processes and Outlook 249
9.7
Recent Biotechnological Processes and Outlook
Several new applications of enzymes have reached, or are approaching, the
stage of commercial use in the pulp and paper industry. These include e.g.,
enzyme-aided bleaching with xylanases, direct delignification with oxidative
enzymes, energy-saving refining with cellulases, pitch removal with lipases,
slime control (Klahre et al. 1996) in the paper machine, removing contaminants
in the recycle stream, as well as deinking (Kenealy and Jeffries 2003; Messner
et al. 2003).
A colorless mutant of the blue-stain fungus Ophiostoma piliferum was used
to control pitch problems (Blanchette et al. 1992b; Farrell et al. 1993; Brush et al.
1994; also Fischer et al. 1994), and chip treatment with O. piliferum decreased
energy consumption and increased strength properties in mechanical pulps
(Forde Kohler et al. 1997).
Enzymes used in pulping can increase the yield of fiber, decrease further
refining energy requirements, or provide specific modifications to the fiber.
Cellulases, hemicellulases, and pectinases allowed for better delignification of
the pulp and savings in bleaching chemicals without altering the strength of
the paper (Kenealy and Jeffries 2003). Laccase and protease reduced energy
requirements in mechanical pulping. Cellulases and hemicellulases have been
used in the refining of virgin fibers. Agricultural residues like wheat and rice
straw have been mechanically pretreated followed by treatment with enzymatic
cocktails from Lentinula edodes for pulp production (Giovannozzi-Sermanni
et al. 1997).
The initial studies on the use of enzymes in bleaching were performed with
a goal of imitating the wood-decaying action of fungi in nature (Iimori et al.
1998; Viikari et al. 1998). However, different mixtures of lignin and manganese
peroxidases did not consistently delignify unbleached craft pulp. The use of
xylanases in bleaching can improve lignin extraction, alter carbohydrate and
lignin association, or cleave redeposited xylan. Recently, laccases or manganese
peroxidases, either alone or combined with low molecular weight mediators,
have been examined. In the laccase-mediator concept, laccase is combined with
a low molecular weight redox mediator resulting in generation of a strongly
oxidizing co-mediator, which then specifically degrades lignin (Jakob et al.
1999; Sealey et al. 1999). “Novel xylanases” deriving from thermophilic and
alkaline sources are of importance due to the prevailing conditions in pulp processing.
Progress in the knowledge of the xylanase-encoding DNA sequences
and the expression of xylanases in other microorganisms may lead to further
development in this area (Kenealy and Jeffries 2003).
Waste paper is the primary raw material of the European paper industry.
For Germany, the amount of waste paper for paper production has been
forecasted to about 14 million t in 2005. In 1995, the average composition of
www.taq.ir