Biobleaching of softwood kraft pulp by white-rot fungi and its related enzymes

The roles of oxidizing enzymes produced during the biobleaching of unbleached softwood kraft pulps (SWKPs) by Phanerochaete chryosporium and Trametes versicolor were investigated. Both fungi produced laccase and manganese peroxidase (MnP), hut lignin peroxidase production was not observed. While no appreciable delignification of SWKP was obtained by T. versicolor, laccase was produced extensively with this fungus compared with P. chrysosporium which showed greater delignification. The MnP production during the delignification of SWKP by P. chrysosporium was much less than that of unbleached hardwood kraft pulp (HWKP). Furthermore, in vitro, isolated MnP cannot degrade residual lignin in SWKP without the addition of Mn(II), although MnP can degrade that in HWKP. The addition of Mn(II) enhanced the MnP production and delignification during the biobleaching of SWKP by P. chrysosporium. The delignification of SWKP by an in vitro MnP treatment also was obtained with Mn(II)-addition. Therefore, MnP may be involved mostly in the delignification of SWKP under Mn(II)-sufficient conditions, although other enzymes, except MnP, may contribute to the delignification of SWKP under Mn(II) -insufficient conditions.