MECHANISM OF HEMICELLULOSE-DIRECTED PREBLEACHING OF KRAFT PULPS

The enzyme xylanase is known to facilitate chemical bleaching of both hardwood and softwood kraft pulps. For example, exposure of unbleached pulp to xylanase allows lower dosages of chlorine or chlorine dioxide to be used to achieve a required pulp brightness. We found that the prebleaching effect in black spruce pulp was associated with a marked drop in xylan degree of polymerization (DP) in the pulp. On the other hand, the xylan (pentosan) content and pulp yield decreased only slightly. A smaller bleaching effect was observed with mannanase in spite of a similar drop in mannan DP. These observations are consistent with the hypothesis that the prebleaching effect results primarily from depolymerization, but not necessarily solubilization, of xylan-derived hemicelluloses. Some commercial xylanase preparations contain cellulase; when pulps were treated for prolonged periods with a commercial xylanase, we observed large reductions in zero-span breaking length, even though the viscosities of the treated pulps were acceptable. However, purified xylanase treatments resulted in no losses in fiber strength.