Xylanase molecular size, pulp pore-structure and pulp delignification

Bleaching kraft pulp with chlorine can result in toxic chlorinated species in bleach plant effluent. Removing lignin from kraft pulp with xylanase reduces chlorine required for bleaching, which, in turn, reduces toxic species in bleach plant effluent. Kraft pulp mills using xylanase pretreatment have reduced chlorine requirement for bleaching by 20 to 25% for hardwood pulp but only 10 to 15% for softwood pulp. The work reported here demonostrates that xylanase molecular size and pulp pore structure determine the amount of lignin that xylanase can remove from pulp. Using softwood pulp, xylanases with molecular weights of 20,000 Da, 39,000 Da, and 67,000 Da removed 48%, 39%, and 30% of lignin respectively. Mild cellulase pre-treatment of softwood pulp increased the apparent median pore size from 37 Angstrom to 67 Angstrom which enabled xylanase to reduce chlorine requirement for softwood pulp bleaching by 31%. The same process applied to hardwood pulp eliminated 41.8% of chlorine required for bleaching.