The potential of biosulfite pulping in dissolving pulp production

Biological alternatives which could aid the hemicellulose and lignin removal from dissolving-grade pulps imply the use of microorganisms to pretreat wood chips prior to pulping (biopulping) and/or use of enzymes to pretreat pulp prior to bleaching (biobleaching). Five stains of the white-rot fungus Ceriporiopsis subvermispora (SS-1; SS-3; SS-4; SS-5; SS-10) have been screened for their abilities to facilitate the acid sulfite pulping and bleaching processes of Eucalyptus grandis wood to dissolving pulp. Pulp properties such as brightness, kappa number, α-cellulose, alkali solubilities S10 (degraded cellulose content) and S18 (hemicellulose content), and yield have been measured and compared. After sulfite pulping, kappa numbers decreased by up to 10% and did not correlate with the brightness values. Yieldwise, only C. subvermispora SS-3 strain produced pulp comparable to the control of 47.4%. Biosulfite pulp was fully bleached in three bleaching combinations: OD1E(o)D2H, OD1E(o)D2P, and XOD1E(o)D2P. In most instances, a brightness gain over the controls (up to three brightness points) was observed. Interestingly, xylanase pretreatment of pulp when used in conjunction with biopulping did not always improve brightness and at the same time reduced viscosity of dissolving pulp. Although the yield of the SS-5 pretreated pulp was lower than the control after biosulfite pulping (by 0.5%), it was higher after bleaching in all three combination (by up to 1%). Results suggested that pretreatment of wood with selected strains of white-rot fungi may be used as a means of improving the selectivity of both pulping and bleaching, thereby increasing either the final pulp yield or brightness.