Viscosity‐enhancing bleaching of hardwood kraft pulp with xylanase from a cloned gene

Abstract

The idea of removing lignin from kraft pulp with ligninolytic microorganisms has been around for some time.' More recently, a peroxidase has been described2 which, in the presence of low concentrations of peroxide, might depolymerize lignin and, in combination with an extraction, thereby bleach kraft pulp. An alternative approach is based on reports that residual lignin in unbleached kraft pulp is linked to hemi~ellulose~ and that cleavage of this linkage will allow lignin to be r e l e a ~ e d . ~ Hydrolysis of the lignin-hemicellulose linkage would have to be specific, in order to prevent deterioration of pulp qualities, e.g. viscosity, due to cellulose hydrolysis . 5 Ideally, the ligninhemicellulose linkage itself should be the only one cleaved. In hardwoods, the linkage is mainly betwen lignin and xylan, possibly through arabinose side chain^.^ We recently produced clones of E . coli capable of endoxylanase or P-xylosidase production in the absence of cellulase,8,' which could find potential application in lignincarbohydrate cleavage, especially for hardwood kraft pulps where xylan is the predominant hemicellulose, and can even be redeposited on the pulp during cooking. We now describe experiments which demonstrate that lignin removal can, indeed, be aided by treatment with xylanase from a cloned system, and that the resulting pulp retains viscosity as well as the required strength properties.