Heat Treatments of High Temperature Dried Norway Spruce Boards: Saccharides and Furfurals in Sapwood Surfaces

Carbohydrates that migrate to wood surfaces in sapwood during drying might influence properties such as mould susceptibility and colour. Sugars on the surface of Norway spruce boards during various heat treatments were studied. Samples (350mmx125mmx25mm) were double-stacked, facing sapwood-side outwards, and dried at 110C to a target moisture content (MC) of 40%. Dried sub-samples (80 mm x 125 mm x 25 mm) were stacked in a similar way and further heated at 110C and at 130C for 12, 24, and 36 hours, respectively. Glucose, fructose, and sucrose as well as 5-hydroxymethylfurfural (HMF) and furfural in the sapwood surface layer of treated wood were analysed using HPLC (RIand UV-detectors). Carbohydrates degraded to a lower extent at 110C than at 130C. Furfural and to a larger extent HMF increased with treatment period and temperature. Heat treatment led to a decrease in lightness and hue of the sapwood surface of sub-samples, while chroma increased somewhat. Furthermore, considerably faster degradation (within a few minutes) of the carbohydrates on the surface of the dried spruce boards was observed when single sub-samples were conductively hot pressed at 200C. Treatment period and initial MC influenced the presence of the carbohydrates in wood surface as well as colour change (∆Eab) of the hot pressed sub-samples.