Piano Guidance
Piano Guidance
Photo: ROMAN ODINTSOV
Thymoquinone as a naturally occurring, pencil-like smelling substance is a new find in pine wood. However, in our recent study15 on incense cedar wood, thymoquinone was found to have an even higher odour potency, possibly being one of its key odorants.
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Read More »In total 39 odorants of the 44 detected compounds were identified using GC analyses. The number of odorants found in Pinus sylvestris L. can be arbitrarily increased by injecting higher concentrations of the aroma extract but experience has shown that in most cases the 20 most potent odorants of an aroma extract are sufficient to reconstitute the overall odour impression of the sample17. Most of the substances found in the Pinus sylvestris L. samples were present in all three samples. Fourteen substances occurred in two samples only, whereas each of the odour-active substances was present in at least two samples. Generally, non-detection by the sniffing experiments does not rule out the presence of the respective odorants in the wood sample, it means that it is present in concentrations below the perceptual level with regards to FD 1 (the most concentrated sample). Moreover, there were mostly no large variations in FD factors between the samples. Experience shows that in natural samples FD-factors can vary within up to three FD stages18,19. Larger differences in FD factors may be due to age differences between the three trees or different external conditions such as light exposure and humidity. The results of the sensory evaluation and the results obtained by gas chromatography showed close agreement: In the course of the sensory testing the highest intensity rating by the panellists was for the attribute resin-like. This odour impression is likely to be correlated with α-pinene, which smells woody/ resinous. It is interesting to note that α-pinene is one of the largest peaks in the chromatograms of each of the Pinus sylvestris L. samples. This, together with its relatively high odour potency, suggests that this terpene has a main impact on the overall odour impression of the wood samples. This finding is not surprising because α-pinene is long known to be an odorous constituent of pine wood20. Among the identified substances were many fatty smelling mono- and di-alkenals such as (E)- and (Z)-non-2-enal, (E,E)-nona-2,4-dienal and (E,E)-deca-2,4-dienal. These are reported here for the first time as odorants in Pinus sylvestris L. wood. Their smell impressions are likely to be represented by the fatty/cardboard-like note perceived during the sensory evaluation. The citrusy note can be assumed to result from octanal, linalool and nonanal, whereas the green, grassy smelling pentanal and hexanal as well as the savoury-like smelling sotolone are likely candidates for being responsible for the herb-like note. Furthermore, (E,Z)-nona-2,6-dienal (cucumber-like) and butanoic acid as well as 3-methylbutanoic acid (cheesy) can be associated with the fresh-cut cucumber-like and cheesy/sweaty odour impressions. The pencil-like smell can be traced back to thymoquinone and the peppery note to α-bisabolol. Rubber-like/plastic-like/waxy notes may stem from a rubber-like smelling unknown substance, and hexanoic acid or heptanoic acid (both smelling, inter alia, plastic-like). In contrast to these correlations between the sensory evaluations and GC-O results, some odour impressions from the sensory evaluation (frankincense-like, carpenter’s shop-like/sawdust-like, wood glue-like) could not be directly aligned with specific substances. These odour impressions may be a result of mixed odour effects and may be further influenced by additive, synergistic and suppressive effects of all odorous constituents present in the wood. Previous studies showed that unexpected odorous impressions can arise from mixtures of odorants that can be barely predicted, such as the grapefruit-like note induced by a mix of mainly citrusy and black current-like odour notes as described for grapefruit aroma18,19. Future studies involving quantification and reconstitution experiments would be required to answer this question. The identified odorants in the Pinus sylvestris L. samples belong to a variety of substance classes that exhibit great diversity of odour character. A large number of these odour-active substances stem from fatty acid degradation. Fatty acids and waxes are part of the extractable fraction in wood21. Since wood is naturally exposed to sun and air, degradation processes of fatty acids are promoted, resulting in various alkenals, ketones, alkylic acids and intramolecular esters. The odour impressions of these fatty acid degradation products range from green, grassy (pentanal, hexanal), citrus-like (octanal, nonanal, linalool) and fatty (e.g. (E)-non-2-enal, (E,E)-nona-2,4-dienal) to cheesy (butanoic acid) and coconut-like/peach-like (γ-octalactone, γ-nonalactone). Degradation products of fatty acids represent the largest group of odour-active constituents in wood with 69% (see Fig. 2).
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Read More »Figure 2 Percentage composition of the odour-active wood constituents [%] in Pinus sylvestris L. and potential source of origin as determined in the present study. Note: the percentages do not represent absolute quantitative data. Full size image Apart from the various fatty acid degradation products, a series of odorous molecules of terpenoic structure was found to be present in the Pinus sylvestris L. samples. Terpenes are part of the resin located in the resin channels in the heartwood and sapwood of conifers22. Representatives of this group are the monoterpenes α-pinene (woody, resinous), linalool (citrusy), borneol and fenchol (both mouldy) as well as the sesquiterpene α-bisabolol (balsamic, peppery). Phenyl compounds are another prominent group of odour-active constituents in wood. They occur in wood due to the degradation of lignin. Lignin is a macromolecule which consists of phenolic units and is located in the cell walls of wood23. Odour-active phenyl derivatives identified in the samples are vanillin (vanilla-like), 3-phenylpropanoic acid (vomit-like, fruity), phenylacetic acid (honey-like) and p-cresol (horse-like). Furthermore, thymoquinone, which smells pencil-like, can be included in this group as the formation of quinones results from the oxidation of phenols. Thymoquinone as a naturally occurring, pencil-like smelling substance is a new find in pine wood. However, in our recent study15 on incense cedar wood, thymoquinone was found to have an even higher odour potency, possibly being one of its key odorants. In pine wood the attribute pencil-like was rated only with an intensity of 1.0 during sensory evaluation whereas in incense cedar wood it was the most dominant odour impression15. This reveals that even in those samples where thymoquinone was detected by olfactometry, it did not impact the overall smell of the pine samples as other compounds were present with much more sensory impact, thereby obviously covering the smell impression of this compound. Two substances had an androstenone-like, perfume-like smell. Their chemical structures could not be unequivocally identified. Mass spectral matching, however, with the NIST library proposed the substances (5β)-androst-2-en-17-one and androst-2,16-diene. However, their identity could not be satisfactorily confirmed due to a lack of respective reference compounds. At first sight, the appearance of such substances in plant material might be surprising. Nevertheless, sterols are known constituents in the extractable fraction of wood23. Accordingly, the formation of compounds with a steroid structure is possible, meaning that the substances that were found might indeed be at least structurally related to androstenone. Three substances remained unknown. They were characterised by rubber-like, leather-like and mint-like smell impressions respectively. A mass spectrum could not be obtained as no relevant peak was observable. Accordingly, their concentration in the samples is below the instrumental detection limit and they are, moreover, superimposed by other odourless substances, despite having low odour thresholds and being detectable by the nose. It is interesting to note that the leather-like smelling substance (RI = 2300) had an odour quality similar to 4-methylphenol and other phenolic compounds such as 4-ethylphenol (horse stable-like, faecal, phenolic; RI2168) and 2,3-dimethylphenol (phenolic, ink-like, leather-like; RI 2105); therefore the prospected substance is likely to have a phenolic core moiety. Nevertheless, the RI values did not match these odorous substances.
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Read More »A number of substances identified in the present study have previously been reported as odorants in toasted wood from diverse wood species, namely acacia, chestnut, cherry, ash and oak. Such wood is commonly used for manufacturing barrels for wine ageing13. In another study, Diaz-Maroto et al.14 performed investigations on aroma-active compounds in different oak woods. Some of the identified odour-active substances also correlate with those compounds that we detected in Pinus sylvestris L. wood. A series of odorants stemming from fat degradation such as hexanal, (E)-non-2-enal and butanoic acid as well as phenolic derivatives such as p-cresol and vanillin have been reported as odorants in woods. However, as previous investigations on wood odorants were mainly based on hardwood used for barrel-making, this study provides the first targeted characterisation of odorous substances in pine wood. Pine wood contains a higher amount of resin. Thus, more terpenoic substances and higher amounts of terpenes are present in pine wood24. This substance class is already known as a source of volatiles in pine wood, contributing to its odour: For example, several terpenoids have been detected and identified as important odorants in incense cedar wood (e.g. thymoquinone, fenchol, α-bisabolol and γ-octalactone), which is also a softwood15. Others have generally been reported as volatiles in wood without further specification of their smell properties and their overall impact on the respective wood aroma (e.g. α-pinene, borneol)9. Nevertheless, in the present study several odour-active compounds have been identified for the first time. A total of 11 substances were identified for the first time as odorants not only in pine wood but in wood in general: linalool (citrusy), (E)-dec-2-enal (fatty), borneol (moldy), pentanoic acid (cheesy), pentanal (grassy), dodecanoic acid (perfume-like, soapy), (E,Z;Z)-trideca-2,4,7-trienal (fruity, blood-like, metallic), γ-octalactone, δ-nonalactone (coconut-like), γ-decalactone and γ-dodecalactone (both peach-like). Hitherto, lactones such as these were only described as aroma compounds in relation to wine ageing in oak barrels and were only recovered from the aged wine and not from the wood itself, meaning there was no confirmation of their origin in the wood material25.
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