Effects of fungal and enzymatic treatments on isolated lignins and on pulp bleachability

Modifications of wood powder lignin (WPL) and black liquor lignin (BLL) were studied with growing cultures and cell-free lignin peroxidases and laccase of Phlebia radiata. WPL was easily degraded by growing cultures of Phlebia radiata. The modifications achieved by using these enzymes were, however, less marked. Only laccase caused a slight change in the molecular mass distribution of WPL. BLL was verv resistant even to fungal attack, although some changes in the molecular mass distribution of the dissolved fraction were detected. The capability of the lignin-modifying enzymes of Phlebia radiata to improve bleachability of kraft and peroxyformic acid pulps was also tested. Bleachability of kraft pine pulps was improved only when laccase was used after hemicellulase treatment. Hemicellulases apparently increase both lignin extractability and the accessibility of lignin to lignin-modifying enzymes. In the peroxyformic acid pulps the more oxidized lignin was further oxidized by lignin peroxidases, as indicated by a decrease in brightness.