Bleaching of hardwood kraft pulp with manganese peroxidase from Phanerochaete sordida YK-624 without addition of MnSO4

In vitro bleaching of an unbleached hardwood kraft pulp was performed with partially purified manganese peroxidase (MnP) from the fungus Phanerochaete sordida YK-624 without the addition of MnSO4 in the presence of oxalate, malonate, or gluconate as manganese chelator. When the pulp was treated without the addition of MnSO4 the pulp brightness increased by about 10 points in the presence of 2 mM oxalate, but the brightness did not significantly increase in the presence of 50 mM malonate, a good manganese chelator. Residual MnP activity decreased faster during the bleaching with MnP without MnSO4 in the presence of malonate than in the presence of oxalate. Oxalate reduced MnO2 which already existed in the pulp or was produced from Mn2+ by oxidation with MnP and thus supplied Mn2+ to the MRP system. The presence of gluconate, produced by the H2O2-generating enzyme glucose oxidase, also improved the pulp brightness without the addition of MnSO4, although treatment with gluconate was inferior to that with oxalate with regard to increase of brightness. It can be concluded that bleaching of hardwood kraft pulp with MnP, using manganese originally existing in the pulp, is possible in the presence of oxalate, a good manganese chelator and reducing reagent.