Structural changes of residual lignin in softwood kraft pulp treated with manganese peroxidase

Structural changes of residual lignin in unbleached softwood kraft pulp (SW KP) during manganese peroxidase (MnP) treatment were investigated to obtain some understanding of the biobleaching action of SWKP with MnP treatment. Alkaline-extracted lignin from darkened SWKP by MnP showed more intense col or and contained more o-quinone than that from control SWKP. However, no di fference in the conjugated alpha-carbonyl was observed between the lignins from MnP-treated and control SWKP. The nitrobenzene oxidation analysis reve aled that oxidative condensation of non-condensed lignin in SWKP occurs dur ing an early stage of MnP treatment. These observations were supported by t he model experiment in which the lignin prepared from control SWKP was subj ected to MnP treatments three times, and the changes of color and functiona l groups in the lignin were determined after each treatment. These results suggested that an increase in o-quinone and the condensation reaction of no n-condensed lignin in SWKP are responsible for the characteristic darkening of SWKP during MnP treatment. It was also ascertained that darkened lignin was degraded and brightened by repeated MnP treatments.