Evaluation of cell-wall modifying enzymes for improved refining of pulp from two Eucalyptus species

[摘要] English: Refining of pulps requires a large amount of energy, capital input and maintenance costs. Wood fibres can be treated enzymatically prior to refining to reduce the refining energy requirement of the pulps and/or to improve drainage and strength properties. Cell-wall modifying enzymes such as the cellulases and hemicellulases can assist refining by opening up the crystalline and amorphous cellulose and hemicellulose in the fibre walls. This application requires cell-wall modifying enzymes to be matched to the specific pulp.Characterisation of commercially-available cellulases showed that the activities of the enzymes were suitable for paper mills. The protein content of the enzyme formulations were determined to establish levels for comparative dosing. Using this information the effects of over-dosing and the extended incubation with the enzymes were also determined. The pulp yield and fibre strength appeared to be at risk of being decreased with elevated enzyme dosages, since Ecopulp® Energy, Novozyme 613 and Celluclast 1.5L increased the release of reducing sugars. The critical period for fibre modification by enzyme treatment was the first 30 min of incubation. The most suitable enzyme for application to a paper mill appeared to be Novozyme 476 due to its ability to improve bonding strength with minimal loss in fibre strength.According to published reports, dried and never-dried Eucalyptus globulus pulps responded differently to enzyme treatments and it was, therefore, likely that dried and never-dried E. grandis pulps would also respond differently. Ecopulp® Energy, Novozyme 476 and Celluclast 1.5L were applied to E. grandis pulps and refined on pilot scale. Overall, the dried E. grandis fibres responded differently to the enzyme treatments than never-dried fibres and enzymes should, therefore, be matched to the specific fibre used. It was proved that the enzymes can be used to save on refining energy usage as well as to improve strength properties of paper made from dried or never-dried E. grandis pulps.Ecopulp® Energy and Novozyme 476 were subsequently applied to the E. nitens pulps and refined on pilot scale. All of the enzyme applications had a similar effect on the never-dried pulp, where the properties were not changed as much as on the dried pulp. Novozyme 476 was the most suitable enzyme for application on the dried E. nitens pulps, because it saved on refining and drying energy due to the decreased freeness and water retention values. The Novozyme 476 also improved the physical properties of the pulps where tear and tensile indices and porosity were increased and bulk was reduced. This work demonstrated that enzymes must be selected to match the pulps being used and to suit product requirements.Novozyme 476 was selected for application on a fine-paper machine in order to improve the refining efficiency of E. nitens pulp. At the high dosage (100 ml/t) the refining current of the mixed stock refiner was reduced and the porosity and tear strength was decreased and tensile strength was increased. The reduction in tear strength was probably due to too much refining energy applied to enzyme treated pulp, thereby damaging the fibres. The damage to the fibres was confirmed by electron microscopy, showing excessive fibrillation of the fibres treated with the endoglucanase. The endoglucanase successfully improved the refining efficiency of the paper machine and should be considered for application on a permanent basis.

[发布机构] University of the Free State