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 s ystem and phenolics were characterized and quantified by means of HPLC. Ste aming mainly resulted in a darkening of wood tissues. Colour changes increa sed as temperature increased from 75 to 125 degrees C. Lightness L* gradual ly decreased from 0 to 24 h at 75 degrees C, whereas with higher temperatur es, most of the darkening occurred within the first 4 hours of exposure. Co ntrast in colour, between sapwood and heartwood in particular, could be red uced 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 deriv atives (E1, E2 and G respectively) were gradually degraded by steaming, whe reas a new flavonol (F) and oxidation products (OP) increased. Relationship s between wood colour and phenolic content are discussed.