Kraft pulp bleaching by redox enzymes

In the last five years, kraft pulp manufacturers have moved towards ECF and to some extent TCF bleaching. In Canada, several mills have installed oxygen delignification, and a growing number have added xylanase prebleaching. This scenario presents an opportunity for a second generation of enzyme bleaching technologies, which target lignin directly with redox enzymes. Laccase in the presence of oxygen and mediators was first found to delignify kraft pulp in 1992. The enzyme is now available commercially, and it is evident that a number of laccase substrates ate able to perform a mediator function. In 1993, we found that manganese peroxidase could also decrease the kappa number of kraft pulp. Furthermore, the bleachability of the resulting pulp by alkaline peroxide was improved more than would be predicted from kappa number alone. Other redox enzymes such as cellobiose dehydrogenase may be required for lignin biodegradation, and may also play a role in future commercial applications.