Natural wood colouring process in Juglans sp. (J. nigra, J. regia and hybrid J. nigra 23´J. regia) depends on native phenolic compounds accumulated in the transition zone between sapwood and heartwood AbstractmRadial distribution of soluble phenolics was investigated at different heights in stems of Juglans nigra, J. regia and hybrids J. nigra 23´J. regia. Four major phenolic compounds were studied: hydrojuglone glucoside (HJG), quercitrin (QUER) and two unknown compounds characterized as two ellagic acid derivatives E1 and E2. HJG and E1 content increased gradually in the sapwood, peaked in the sapwood-heartwood transition zone, and decreased drastically in the heartwood. QUER was accumulated preferentially around the transition zone, and its content was relatively low in the outer part of the sapwood and in the inner part of the heartwood. E2 content was low in the sapwood and increased in the heartwood. The heartwood formation was marked by the accumulation of new soluble compounds. The relationship between wood extractives and wood colour were evaluated and discussed. HJG was considered to be a major precursor of heartwood colour providing chromophores through hydrolysis (deglucosylation), oxidation and polymerization processes.
Steaming conditionsSteaming was performed in a sterilizer providing a saturated steam atmosphere. Subsequently, 1cm 3 samples were collected from the middle sapwood (6 th ring from the bark), the sapwood -heartwood transition zone (13 th ring from the bark) and from heartwood (from 14 th to 16 th ring from the bark). Three temperatures (75, 100 and 125°C), and four times of exposure (4, 8, 16, 24 h) were applied, providing 12 different steaming conditions. Control samples, adja- SummaryThe 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°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°C. Lightness L* gradually decreased from 0 to 24 h at 75°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°C. The best steaming treatment, leading to a colour of steamed sapwood close to that of natural heartwood, was at 100°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.
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