Colour extraction and concentration
Oak heartwood has a natural yellow/brown colour, with a different intensity of lightness according to age and position in the trunk [14]. This property is not limited to oak wood [22]. Although colour is in relation to polyphenol extraction, especially ellagitannins [13], other parameters such as wood structure, anatomy and genetics may also affect it [10].
Solvents have a strong effect on the efficiency of colour extraction. Oak heartwood is rich in water-soluble extracts [13], but these extracts present heterogeneous composition with different groups of polymers: polysaccharides, ellagitannins and part of the low weight lignans [23]. Because the major origin of colour is from ellagitannins [14], solvent range and mixture on those that can be extracted quickly with a focus on ellagitannins, mainly polymeric ellagitannins and the corresponding part of soluble colour are specifically investigated. Ethanol and methanol with 10% vol. water were used to quickly obtain a good dry extract that was rich in polyphenols with dark yellow colour components. Acetone with 30% vol. water presented the best compromise with a higher dry extract, polyphenols index and colourful matter in accordance with previous investigations [11]. For polymeric ellagitannins and the corresponding colour fraction, water seems to be necessary to improve the extraction, probably due to wood fibres hydration [24, 25] and the improvement of polarity of the solvent mixture. Diethyl ether and MeOH–chloroform, both low polarity solvents, produced limited and lightly coloured dry extract containing low weight polyphenol.
Maintaining the temperature to 80 °C, as well as raising the pH to 8–10, improved the rate of extraction but also by intense degradation, yielded additional colouring matter not corresponding to those naturally present in heartwood (data not shown).
The reliability between the species of Q. robur and Q. petraea concerning colour parameters are uncertain, even if the species affected the ellagitannins concentration [10, 26] and the ellagitannins represent the main precursor of heartwood colour. At least a part of the difference can be attributed to the sampling procedure; the other part could be determined by genetics [10].
Characteristics and evolution of oak heartwood colour
During the heartwood ageing colour change, the colour becomes darker, redder and yellower, which increases with tree age [9]. Alongside the natural colour evolution, some fungal infections promote red–orange [27] and brown [28] discoloration, as do climatic accidents such as plain frost [29]. This type of wood must be excluded from investigations into the natural colour of oak heartwood.
By liquid extraction, such as with efficient solvents or a mixture of solvents, all heartwood colours are not completely removed from the wood [13]. The repartition of soluble colour from heartwood is not constant; the quantity increases during the ageing of the heartwood and explains why colour is more abundant in the inner heartwood than in the outer heartwood [14]. Colour progressively changes along the heartwood in relation to its age from light and a weak yellow to darker and brown for older wood in the middle of the tree. The water solubility of the extracts and the colour follow the same tendency [13]. A large part of the soluble heartwood colour, but not all, is due to the polymeric form of ellagitannins. Further sources of colour exist that are redder than polymerised ellagitannins, which are derived from specific lignans [30].
Similar reactions occurred in Castanea sativa heartwood and probably explain its colour [12]. Although both hydrolysable tannins in woods such as Quercus or Castanea and condensed tannins in woods such as Juglans, Larix, Pinus or Pseudotsuga have a different chemical structure of polyphenols, similar relationships have been presented to illustrate the wood colouring process [17, 31, 32]. In Quebracho heartwood, the increase in the molecular weight of condensed tannins from the outer heartwood to the inner heartwood is clearly demonstrated [33]. Furthermore, these types of reactions concern not only tannins but also the general polymerisation of simple phenolic compounds in polymers of undefined structure that are highlighted in various conifer woods [34].
As expected, wood colour is correlated with the dry extract materials and polyphenols content [22, 31] as well as the insoluble ellagitannins [11]. Polymerisation during ageing is a frequent reaction to promote the colour of the wood. At the beginning of a reaction, the majority of formed polymers remain soluble because of their limited molecular mass. For older heartwood, since polymers continuously increase in size, the water-soluble fraction and the soluble colour decrease.
Origin of colour
The soluble colour of the heartwood extract is a result of the complex reactions of oxidation and the copolymerisation with cell-wall polymers as polysaccharides. This reaction is probably non-enzymatic. The pre-heated treatment of wood samples (60 °C, 10 h) before extraction, solubilisation and oxidation in a solution by air oxygen did not affect the oxidation and polymerisation of the ellagitannins and the formation of corresponding colour compounds (data not shown). These reactions, which are spontaneous in heartwood during the natural duraminisation process, can be mimicked in an in vitro solution [14]. The oxidation promoted robust structural alterations of ellagitannin structure [14]. Using IR spectroscopy, we observed a decrease in vibration bands 3500–3200 cm−1 due to the association or linkage of the OH functions of ellagitannins, the appearance of a signal at 1715 cm−1 was attributed to the C=C bonds, and a preponderance of signals occurred in the epoxy structures (1250 cm−1, 830–890 cm−1). This unsaturated system allowed some complicated reactions with several non-tannin compounds as polysaccharides [35]. Additional research studies were conducted for investigating the nature of soluble coloured polyphenolics polymers found in old heartwood. 1H NMR spectra of polymers presented specific signals of hexahydroxydiphenol moiety of ellagitannins, as well as specific signals of polysaccharides. This was then confirmed by the main products identified in THM/GC/MS and pyrolysis/GC/MS experiments, respectively for ellagitannins and polysaccharides. Despite in lower quantity, the presence of substantial pyrolytic derivatives obtained from lignins highlighted the complexity of the heteropolymers structures. These different sources of structural information confirmed the nature of polymers formed by the heterocondensation of various compounds, with a major part of ellagitannins and polysaccharides. This kind of products was well-reported in previous works [13, 33, 34].
Moreover, the ellagitannins are well-known highly oxidised compounds with consequential biological activity [36, 37]. Most chemical oxidation and UV light also impact wood colour through a photodegradation process [38]. However, this reaction is limited to the exposed surface and any global influences in heartwood colour appear weak.