Effects of purified endo-beta-1,4-xylanases of family 10 and 11 and acetylxylan esterases on eucalypt sulfite dissolving pulp

Sulfite dissolving pulp from Eucalyptus grandis contained approximately 3.8 % O-acetyl-4-O-methylglucuronoxylan with a molar ratio of xylose:4-O-methyl glucuronic acid:acetyl group close to 13.6:1:6.2. The effects produced by p urified endo-xylanases from two different glycosyl hydrolase families (fami ly 10 and 11) as well as acetyl xylan esterases were examined and assessed on pulp in relation to their bleaching abilities. The purified endo-xylanas es hpdrolyzed only a limited portion (less than 30%) of the acetylglucurono xylan present in the pulp. The enzymes of family 10 produced acetylated xyl obiose and xylotriose whereas acetylated xylobiose was not observed among t he products released from the pulp by the family 11 xylanases. The esterase s however were not capable of deacetylating the acetylated aldouronic acids generated by the xylanases. Regardless of the different mode of action of the endo-xylanases on dissolving pulp, their effect on pulp bleaching was n ot related to the amount and nature of sugars generated or the glycosyl hyd rolase family. No additional brightness gain was obtained when endo-xylanas es were used in conjunction with acetyl xylan esterases, suggesting that th e latter do not play an important role in biobleaching of eucalypt sulfite dissolving pulps. (C) 2000 Elsevier Science B.V. All rights reserved.