Oxidative treatment and nanofibrillation softwood kraft fibres with lytic polysaccharide monooxygenases from Trichoderma reesei and Podospora anserina

Cellulose oxidation and enzymatic treatments can be employed to reduce the energy consumption in production of cellulose nanofibrils (CNFs). Lytic polysaccharide monooxygenases (LPMOs) offer new biocatalytic tools for oxidative pretreatment of cellulosic fibres in this process. In this work the capability of LPMO enzymes to enhance fibrillation of bleached softwood kraft fibres was studied using two LPMO enzymes, i.e. C1/C4-oxidising AA9A from Trichoderma reesei (Tr AA9A) and C1-oxidising AA9E from Podospora anserina (Pa AA9E). The enzymatic treatments were carried out after mechanical pre-refining (grinding) of the pulp, which resulted in clear reduction of the intrinsic viscosity of the refined fibres compared to the previously published work, presumably due to better access of the enzymes on the fibre polysaccharides. The Tr AA9A treatments were carried out using different enzyme dosages (0.25-10 mg/g dry fibre) and fixed reaction time (24 h), resulting in fibres characterized by increased aldehyde content, which is in line with the C1/C4 oxidising activity of this enzyme. Comparison of the Tr AA9A to Pa AA9E with fixed enzyme dosage (2 mg/g dry fibre) and reaction time (24 h), showed that no remarkable increase in total charge of the cellulosic fibres could be obtained with either of the LPMO enzymes in these conditions. All the LPMO pretreatments improved fibrillation of mechanically refined fibres in microfluidization, seen as clearly lower residual fibre content and higher proportion nanosized material. In comparison of the Pa AA9E to Tr AA9A, clearly faster fibrillation was achieved with the Pa AA9E pretreatment. The mechanical and oxygen barrier properties of CNF films prepared from LPMO pretreated fibres were very similar to the reference CNF films while water vapour transmission rate was somewhat higher.