Modifications of hybrid walnut (Juglans nigra 23 x Juglans regia) wood colour and phenolic composition under various steaming conditions

The effects of steaming were studied on Walnut wood from a hybrid (Juglans nigra 23 x J. regia) tree in terms of wood colour and phenolic composition. Wood samples were subjected to treatments at 75, 100 and 125 degrees C for 4, 8, 16 and 24 hours. Colour changes were measured in the CIELCh colour system and phenolics were characterized and quantified by means of HPLC. Steaming mainly resulted in a darkening of wood tissues. Colour changes increased as temperature increased from 75 to 125 degrees C. Lightness L* gradually decreased from 0 to 24 h at 75 degrees C, whereas with higher temperatures, most of the darkening occurred within the first 4 hours of exposure. Contrast in colour, between sapwood and heartwood in particular, could be reduced by steaming at 125 degrees C. The best steaming treatment, leading to a colour of steamed sapwood close to that of natural heartwood, was at 100 degrees C/16 h. Hydrojuglone glucoside (HJG), ellagic and gallic acid derivatives (E1, E2 and G respectively) were gradually degraded by steaming, whereas a new flavonol (F) and oxidation products (OP) increased. Relationships between wood colour and phenolic content are discussed.