Influence of operation conditions on laccase-mediator removal of sterols from eucalypt pulp.

*Textile and Paper Engineering Department, ETSEIAT, Universitat Politècnica de Catalunya, Colom 11, E-08222 Terrassa, Spain. **Instituto de Recursos Naturales y Agrobiología, CSIC, PO Box 1052, E-41080 Seville, Spain ***Centro de Investigaciones Biológicas, CSIC, Ramiro de Maeztu 9, E-28040 Madrid, Spain; E-mail address: roncero@etp.upc.edu (M.B. Roncero)

The way how sterols, the main lipophilic compounds present in eucalypt kraft pulp, are eliminated by an enzymatic stage using the laccase-mediator system was evaluated. With this purpose laccase-mediator stage (L) was applied on an Eucalyptus globulus pulp under different operation conditions following a three-variable (laccase dose, mediator dose and reaction time) sequential statistical plan, to optimise the removal of sterols. The decrease in pulp sterol content during the enzymatic treatment was related to the decrease in kappa number and to brightness increase, as well as with the increase in some oxidation products of sitosterol (namely 7-oxositosterol and stigmasta-3,5-dien-7-one). The increase in reaction time from 1 to 5 h strongly reduced the sterol content, while no more sterols were eliminated during the 5-7 h period. Increasing the laccase dose from 1 to 20 U g-1 of pulp produced a high reduction in pulp sterols, whereas the increase in mediator (1-hydroxybenzotriazole) dose (from 0.5 to 2.5 % of pulp weight) had only a slight influence in removing sterols. Therefore, at 16 U g-1 laccase dose, 0.5% mediator dose, 4 h of reaction, practically all the sterols were removed. Finally, it was demonstrated that sterols were more sensitive to a L stage (practically 100% of sterols were eliminated) than to a chlorine dioxide stage (54% of sterols eliminated).

This work was funded by the Spanish MEC (BIOFIBER project, CTQ2007-68003-C02-01; and project BIO2007-28719-E), MICINN (ELLE project, AGL2008-00709) and the European Commission (project BIORENEW, NMP2-CT-2006-026456). Torraspapel S.A. (Zaragoza, Spain) and Novozymes (Bagsvaerd, Denmark) are gratefully acknowledged for supplying the pulp and enzymes used, respectively.

Peer reviewed