FAMILY-10 AND FAMILY-11 XYLANASES DIFFER IN THEIR CAPACITY TO ENHANCETHE BLEACHABILITY OF HARDWOOD AND SOFTWOOD PAPER PULPS

Enzyme-aided bleaching of softwood and hardwood kraft pulps by glycosy l hydrolase family-10 and -11 xylanases and a family-26 mannanase was investigated. The ability to release reducing sugar from pulp xylan an d to enhance bleachability is not a characteristic shared by all xylan ases. Of the six enzymes tested, two xylanases belonging to family 11 were most effective at increasing bleachability and improving final pa per brightness. None of the enzymes had a deleterious effect on pulp f ibre integrity. The efficiency of individual xylanases as bleach enhan cers was not dependent on the source microorganism, and could not be p redicted solely on the basis of the quantity or nature of products rel eased from pulp xylan. Cooperative interactions between xylanase/xylan ase and xylanase/mannanase combinations, during the pretreatment of so ftwood and hardwood pulps, were investigated. Synergistic effects on r educing-sugar release and kappa number reduction were elicited by a co mbination of two family-10 xylanases. Pretreatment of kraft pulp with mannanase A from Pseudomonas flourescens subsp. cellulosa and any one of a number of xylanases resulted in increased release of reducing sug ar and a larger reduction in kappa number than obtained with the xylan ases alone, confirming the beneficial effects of family-26 mannanases on enzyme-aided bleaching of paper pulp.