ENZYMATIC SOLUBILIZATION OF FIBER-BOUND AND ISOLATED BIRCH XYLANS

Fibre-bound and isolated birch xylans from wood and from kraft pulp we re characterized according to their chemical composition, macromolecul ar properties and solubilization in enzymatic hydrolysis. During kraft pulping, xylans are extensively modified. Xylan isolated from kraft p ulp contained less substituents and had a lower molecular mass than na tive xylan isolated from wood. Reprecipitated xylan, which was a model of the xylan adsorbed on the fibre surfaces in the kraft process, was almost pure polyxylose. All isolated xylans were hydrolyzed efficient ly by xylanolytic enzymes, although the solubilization decreased sligh ly as a function of chemical processing. The fibre-bound xylans were c onsiderably less accessible to enzymes than the isolated xylans. Despi te high enzyme loadings, the degree of hydrolysis of fibre-bound subst rates did not exceed 20% of the theoretical value. Due to the low side -group content of pulp xylans, the side-group cleaving enzymes had onl y a moderate role in increasing their total solubilization. The result s obtained strongly suggested that the observed low solubilization of fibre-bound xylans is mainly due to poor accessibility of enzymes to t he fibrous material. The results are discussed in the light of new enz ymatic applications in the pulp and paper industry.