Permeability improvement of Norway spruce wood with the white rot fungus Physisporinus vitreus

Abstract

As solid wood, Norway spruce (Picea abies (L.) Karst.) is used in a wide range of applications for interior and exterior situations. Due to its low natural durability against wood decay fungi, a treatment with wood preservatives is necessary for certain applications. In addition, there is a growing interest for treatment with waterborne surface modification substances that specifically improve selected wood properties of Norway spruce such as UV-resistance, hydrophobicity, hardness and fire resistance. The impregnation with such substances requires often a wood permeability that allows a sufficiently deep and homogenous distribution of the substances in the wood in order to obtain an efficient wood property improvement.However, Norway spruce wood is known to have a low permeability which is related mainly to the aspiration of bordered pits during wood drying. In the living tree, permeability is mainly determined by the bordered and half bordered pits that constitute the interconnecting voids between the tracheids and the xylem ray parenchyma. During wood drying, most of these pits get irreversibly closed due to the aspiration of the flexible pit membranes. The resulting low impregnability complicates treatment of Norway spruce with liquid preservatives or wood modification substances.Short term incubation of Norway spruce wood with the white rot fungus Physisporinus vitreus is a biotechnological approach for improving the permeability of this refractory species. The process, called "bioincising", is based on the growth of the fungal hyphae through the tracheids and xylem ray parenchyma. As discussed by Schwarze and Landmesser (2000) and Schwarze et al. (2006), fungal activity induces the degradation of pit membranes after 6 weeks of incubation and thus significantly improves the uptake of liquid substances, causing only negligible losses of impact bending strength.The scope of this thesis was to evaluate the effects of P. vitreus' activity on the properties of Norway spruce sapwood and heartwood during incubation times between 3 and 9 weeks. In the course the thesis, three major fields of research were defined: 1) description of fungal activity and of the resulting wood properties after bioincising; 2) uptake and penetration depth of modification substances into the bioincised wood was, and 3) combined effect of bioincising and treatment with modification substances on the wood properties.It could be shown that fungal activity of P. vitreus induces both a selective delignification and a simultaneous degradation in the tracheid cell walls. Degradation of pit membranes and damages of the tracheid cell wall occurred simultaneously. Mass losses were comparably low and surface hardness was moderately reduced, indicating that P. vitreus is a fungus with a comparably low lignolytic capacity. Substance uptake and penetration depth were significantly improved after 5 weeks of incubation in both sapwood and heartwood specimens. However, impregnation of the bioincised wood with modification substances for UV-protection, hydrophobation, hardness and fire resistance did not result in a pronounced property improvement.The bioincising technique is still in the process of optimization. Parameters such as water activity aw, temperature, pH and nutrient supply influence mycelia growth speed and homogeneity of fungal activity. These parameters must be adjusted for an optimal process control. The results of the presented research activities contribute to the improvement of this process control. A new pilot incubation system has recently been developed and promises further improvement of the bioincising process.However, large scale industrial application is still far away. From the current state of development, products with smaller dimensions, lower quantities and higher value creation appear to be a feasible pathway for the near future of bioincising.