Enzymatic modification of Douglas-fir pulp (2009)

A cellulase and a xylanase enzyme preparation were assessed for their potential to enhance the fiber characteristics of both mechanical and kraft pulps derived from Douglas-fir wood chips. The effects of cellulase treatments on the pulp properties were dependent on enzyme dosage and resulted in improvements in handsheet density and smoothness, pulp freeness and fiber coarseness. However, this was achieved at the expense of both strength and yield loss. Enzymatic treatments of individual fiber length fractions indicated that, in general, all fiber length fractions demonstrated similar trends to those observed with the cellulase treated unfractionated pulp. In contrast, the individual fiber length fractions responded differently to xylanase treatments, as indicated by the solubilization of highmolecular- mass, UV-absorbing material released during enzyme treatments. Xylanase treatments enhanced the handsheet density and smoothness as well as some strength properties. However, the response of the different fiber length fractions to xylanase treatments was not uniform, indicating that fiber composition plays a role in determining the efficacy of the treatments. The application of hydrolytic enzymes to the pulp fibers resulted in changes in both fiber and paper properties. Monitoring pore volume, degree of polymerization, crystallinity, FT-IR spectra, and scanning electron microscopy helped elucidate changes in fiber composition and morphology. There was strong evidence that the reduction in paper strength resulted from the collective effects of decreased intrinsic fiber strength and the reduction in the degree of polymerization of a large portion of the hemicellulose component of the fibers. Other contributing factors included fiber defibrillation and fines hydrolysis. Since the traditional hydrolases (cellulases/xylanases) appeared to modify the fibers by changes to the fiber surface, cellobiose dehydrogenase. (CDH), purified from Phanerochaete chrysosporium was assayed to see if its oxidoreductase activity could further enhance access of the enzymes to a Douglas-fir kraft pulp. Although the addition of cellobiose dehydrogenase alone had little effect, supplementation with cellobiose and iron resulted in a substantial reduction in the degree of polymerization of the pulp cellulose. This indicated that cellobiose dehydrogenase generated hydroxyl radicals via Fenton's chemistry, which subsequently resulted in the depolymerization of the cellulose. In this way a substantial reduction in the degree of polymerization of the cellulose could be achieved without a significant release of sugar or yield loss. Having established that cellulase enzymes could cause the greatest alterations in fiber morphology and that their attack was most noticeable on smaller, thinner fibers, further studies were carried out on the selective treatment of the larger coarser fibers. These cellulase treated fibers were then recombined with the untreated fibers and refined or, alternatively, first refined and then recombined with the untreated fibers. Laboratory scale fractionation treatments resulted in significant improvements in both the tensile (17 %) and burst indexes (24 %) (at 100 PFI revolutions) with minimal enzyme addition and little yield loss. A subsequent study was carried out to determine if this combined approach, using industrial scale fractionation, could also provide such positive effects. Improvements in both tensile and burst indexes were observed, however, not to the same extent as was observed when the fibers were separated by laboratory fractionation.