Modification of bleached kraft pulp with Trichoderma reesei cellulases and hemicellulases: Licenciate thesis

Carbohydrate-degrading enzymes are useful tools for modification of wood pulps. Higher enzyme dosages may be used for analytical purposes, whereas limited dosages can be used for controlled modification of pulp carbohydrates in order to obtain desired changes in fibre properties. The aim of this work was to obtain better understanding of the effects of purified cellulases and hemicellulases of Trichoderma reesei on the properties of bleached kraft pulps. Both extensive and limited treatments were used to elucidate the effects on fibre properties. The enzymes used were endoglucanases (EG I, EG II), cellobiohydrolases (CBH I, CBH II), xylanase (XYL) and mannanase (MAN). The work was focused on i) the comparison of individual enzymes as well as their combinations, ii) the comparison of pulps from varying origins and produced by different methods, iii) the effects on never-dried and recycled pulps and iv) the effects on development of properties during refining. Extensive enzymatic removal of pulp hemicelluloses did not cause deterioration of the properties of never-dried pulps produced by different pulping and bleaching methods. The removal of xylan decreased the total pore volume of the fibres, indicating the significance of fibre charge on porosity. The enzyme-treated pulps were dried and the effects on pulp properties were evaluated. The detrimental effects of drying, i.e. hornification, were significantly amplified when hemicelluloses were removed prior to drying. The removal of xylan or glucomannan equally increased the relative changes in the sheet properties caused by drying. The presence of hemicelluloses in the fibre pores and in the interfibrillar spaces appears to hinder hornification of fibres by preventing the cellulose microfibrils from coalescing and forming irreversible bonds during drying. Recycling of kraft pulp was simulated by repeated drying, rewetting and refining stages. When recycled fibres were treated with cellulases or hemicellulases, the water retention capacity of the hornified fibres could not be recovered by any of the enzymatic treatments. However, the impaired pulp drainage was clearly improved by the endoglucanase treatments, especially EG II. Hemicellulase treatments produced a slight improvement in drainage, whereas CBH I treatment had practically no effect. Combining hemicellulases with the endoglucanase treatments increased the effects of the endoglucanases on pulp drainage. Apparently endoglucanases enhance dewatering by hydrolysing the amorphous hydrophilic cellulose which is the main constituent of fines formed in refining. Both endoglucanases decreased the strength properties of recycled and virgin pulps even at rather low dosage levels, the effect of EG II being more pronounced. Combining MAN with the endoglucanases also appeared to have an additional negative effect on the strength properties. The cellobiohydrolases had no significant impact on the properties of bleached kraft pulps. The combination of XYL or cellobiohydrolases with endoglucanases did not enhance the effects on the pulp properties, although the hydrolysis levels were increased. The results demonstrated that carbohydrate-degrading enzymes can be used to controlledly modify the pulp properties of virgin or recycled kraft pulps. However, the exploitation of cellulase enzymes for fibre treatments requires careful optimisation of both enzyme composition and dosage. The endoglucanases appeared to be the key enzymes in improving pulp drainage or beatability. The presence of hemicellulases may enhance the effects of the endoglucanases while the effects obtained by cellobiohydrolases are insignificant.