Production of metal chelating compounds by white and brown-rot fungi and their comparative abilities for pulp bleaching

Wolfiporia cocos, Gloeophyllum trabeum, Trametes versicolor and Poria medula-panis basidiomycetes, were cultivated in wheat bran liquid medium and their supernatants were concentrated and separated by ultrafiltration into low and high molecular mass fractions, The molecular mass fraction below 5 kDa promoted a quick change in the color of chrome-azurol S (CAS)-blue reagent to purple, The compounds responsible for the positive CAS reaction in the fungal extracts were identified by specific assays. Catecholate and hydroxamate derivatives and oxalic acid were detected in extracts of low molecular mass from the fungi. However, higher amounts of these compounds were produced by the brown-rot fungi. Treatment of eucalyptus kraft pulps by these low molecular mass compounds (<5 kDa) evidenced the effectiveness of compounds from white-rot P. medula-panis and T. versicolor (13.6% and 14.4% delignification. respectively). These same compounds intensively decolorized the polyanthraquinone ligninolytic indicator dye, Poly R-478. The low molecular mass fraction of brown-rot G. trabeum degraded the pulps with 10.8% of viscosity loss and the kappa reduction and Poly-R decolorization was less effective. This results indicate that the mechanisms of lignin degradation by white-rotters are in part oxidative and non-enzymatic and did not cause the depolymerization of cellulose observed with brown-rot fungi. (C) 2002 Elsevier Science Inc. All rights reserved.