Changes in Chemical Structure of Thermally Modified Spruce Wood Due to Decaying Fungi

Fungi play a critical role in the decomposition of wood and wood-based products in use. The ability of decaying fungi to cause degradation of polysaccharides and lignin in the thermally modified Norway spruce (Picea abies L. Karst.) wood was examined with pure culture decomposition tests in laboratory conditions using the brown-rot fungus Serpula lacrymans (Schumacher ex Fries) S.F. Gray and white-rot fungus Trametes versicolor (Linnaeus ex Fries) Pilat. Spruce wood samples were primary thermally treated under atmospheric pressure at the temperatures of 100, 150, 200, 220, 240 and 260 °C during 1, 3 and 5 h, whereby larger losses in their mass, holocellulose, mannose and xylose were achieved at harder thermal regimes. Meanwhile, the holocellulose percent content reduced considerably, and the percent content of lignin increased sharply. Spruce wood thermally modified at and above 200 °C better resisted to brown-rot fungus S. lacrymans than the white-rot fungus T. versicolor. Due to the decay processes, the mass fractions of holocellulose, cellulose and hemicelluloses were lower in those spruce wood samples in which thermal degradation was more intensive, with achieving the highest mass loss values after thermal treatments, after which the decay attacks were poorer or even none with the minimal mass loss values due to action by the brown-rot fungus S. lacrymans and the white-rot fungus T. versicolor. The mannose and glucose percent content in thermally–fungally attacked spruce wood was intensive reduced, e.g., by 17% to 98% in wood after thermal treatments at temperature equal and above 200 °C.