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Improving the brightness and bleachability of douglas-fir mechanical pulps using white-rot fungi and laccase enzymes Chandra, Richard P.

Abstract

Douglas-fir is a common west coast tree species, possessing long and strong fibers, however, the low brightness and poor bleachability of mechanical pulps from this tree have thus far limited its use to low yield Kraft pulps and lumber. The heartwood of Douglas-fir has been shown to contain a high amount of polyphenolic compounds such as dihydroquercetin and/or polymerized forms of these compounds. These compounds have been shown to undergo autooxidation, condensation, polymerization and metal chelation during either the aging of the tree or mechanical pulping of the wood, resulting in the formation of dark coloured complexes in the pulp. Although chemical treatments to improve the bleaching response of Douglas-fir mechanical pulps have been unsuccessful, biological treatments have yet to be applied. Douglas-fir mechanical pulps were treated either with wood degrading fungi or laccase enzymes to try to improve the brightness of mechanical pulps originating from the polyphenolic chromophoric compounds present in the heartwood portion of this tree. Results of fungal screening showed that the white-rot fungus Phanerochaete chrysosporium increased brightness of Douglas-fir refiner mechanical pulps by 1.5 pts ISO. Unfortunately P.chrysosporium treatment was unable to increase the brightness o f pure Douglas-fir heartwood thermomechanical pulps. Although, earlier work had shown that the production of laccase was induced in the fungus Trametes versicolor in the presence of dihydroquercetin, Trametes versicolor treatment of heartwood mechanical pulps combined with chemical extraction did not result in any brightness increases greater than what could be obtained using chemical extraction alone. Laccase treatments without enzyme mediators decreased the unbleached brightness of Douglas-fir heartwood mechanical pulps by 4-5 pts ISO, however, after bleaching, the pulp brightness surpassed the control pulps by 2-3 pts ISO. With the addition of oxygen, an enzyme mediator, or increasing the temperature, laccase treatments decreased both the unbleached and the bleached brightness values compared to the controls. However, the addition of oxygen alone was able to increase the brightness of all pulps by 1-2 pts ISO. Preliminary absorption measurements of laccase treated handsheets indicated that the production of coloured quinone type structures in the mechanical pulps may be associated with the positive effects of laccase treatments.

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