Chemical analysis of industrial-scale hydrothermal wood degraded by wood-rotting basidiomycetes and its action mechanisms

The hydrothermal treatment has been developed as an industrial method to enhance the biological durability and dimensional stability of wood. The resistance against wood-rotting fungi of Pinus radiata and Fraxinus excelsior industrially-heat treated at 210 °C was evaluated inoculating Trametes versicolor and Gloeophyllum trabeum according to standards and comparing with untreated samples after 8-and 24-weeks incubation. Furthermore, wet and instrumental chemical analyses were carried out to quantify changes in chemical composition and to analyze fungal degradation mechanisms. The results showed enhanced durability against wood-rotting fungi in industrial hydrothermal wood at 8 weeks of exposition (Durability Class = 1; MC = ≤6%). However, the moisture content increased over time (MC = 9-10%) favoring a gradual oxidative degradation and allowing an enzymatic action with higher mass losses (up to 10% after 24 weeks). Moreover, it was confirmed that industrial hydrothermal wood interacts as a crosslinking network where polysaccharides are less available to rotting fungi, but low molecular weight fragments released such as the phenolic compounds could be merged into the lignin fraction, producing a relative amount of nutrients vulnerable to degradation as shown in the decrease of the lignin content and in the elevated depolymerization degree after fungal exposure.