Enzymatic solubilization of fibre-bound and isolated birch xylans

Fibre-bound and isolated birch xylans from wood and from kraft pulp were characterized according to their chemical composition, macromolecular properties and solubilization in enzymatic hydrolysis. During kraft pulping, xylans are extensively modified. Xylan isolated from kraft pulp contained less substituents and had a lower molecular mass than native 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 efficiently by xylanolytic enzymes, although the solubilization decreased slighly as a function of chemical processing. The fibre-bound xylans were considerably less accessible to enzymes than the isolated xylans. Despite high enzyme loadings, the degree of hydrolysis of fibre-bound substrates did not exceed 20% of the theoretical value. Due to the low side-group content of pulp xylans, the side-group cleaving enzymes had only a moderate role in increasing their total solubilization. The results obtained strongly suggested that the observed low solubilization of fibre-bound xylans is mainly due to poor accessibility of enzymes to the fibrous material. The results are discussed in the light of new enzymatic applications in the pulp and paper industry.