Chemo-sensory characterization of odor-active constituents in native woods and their structure-odor relationship

The living tree and its obtained wood have played a prominent role in human everyday life in the form of building material and furniture, or in products of daily use. The aroma components emanating from wood-based products have a powerful influence on human Psychophysiological activity and have been used for mental, spiritual and physical healing. The wood aroma can be used in the indoor environment to foster relaxation and stress release effects. The current study was conducted to determine the individual wood odor and its constituents in two diverse types of woods, namely Oak wood and cembran pine wood.

Oak wood has been used since long during the maturation and aging process imparting aroma and flavor in alcoholic beverages and vinegar as by-products that have been either fermented or aged in oak barrels. Several investigations targeted the aroma composition of modified and heat-treated oak wood, but only little information about aroma-active compounds constituting the native and raw oak wood is available. The Cembran pine wood (Pinus cembra L.), also known as Arolla pine, Swiss pine, or Stone pine, has a prominent and long-lasting aroma that has been reported to relieve stress and that is widely used in aromatherapy. However, the odorants responsible for this distinctive aroma are not yet comprehensively revealed. Therefore, the main objective of this thesis was to characterize the impacting odorants and to deepen the knowledge about native oak wood and unique Cembran pine wood odor by applying a combinatory chemo-analytical approach based on human-sensory evaluations and multidimensional gas chromatographic and mass spectrometric techniques. Furthermore, it was tried in this thesis to gain deeper insights into formation pathways and odor sources in these two types of wood and their applications. Most of these odorants comprised aldehydes, acids, lactones, a series of terpenes, mostly mono- and sesquiterpenes, and several odor-active compounds containing a phenolic core moiety. The odor profiles of all wood samples were compared, the potent odorants were determined, and selected odorants of both types of wood samples (oak and Cembran wood samples) were quantified by application of stable isotope dilution assays (SIDA). Odor activity values (OAVs) were calculated based on the odor threshold (OTs) determined in water.

The second part of this thesis dealt with the investigation of the effects of different geographical regions and pedoclimatic conditions on the odor of different wood samples. This study focused on elucidating the smell profiles of several natural oak samples from Germany, Austria, and Hungary. Particular consideration is given to view the current challenges associated with climate change on the forestry sector, and thus concerning the respective regional conditions, and underwent statistical interpretation. In the case of Cembran wood, the influence of different geographical forest regions in Austria and Italy was determined and evaluated by means of principal component analysis (PCA) and statistical interpretation of obtained data. Recombination experiments by mixing the dominant odor-active compounds in their naturally occurring concentrations revealed a good agreement of the model mixtures with the overall smell of the respective native wood samples.

In addition, the structure odor relationships in representative substance classes of wood were elucidated. Specifically, the smell impressions of guaiacol and selected structurally related odorants such as alkylated, alkenylated, methoxylated and halogenated guaiacol derivatives were studied. All investigated compounds elicited characteristic smoky, vanilla-like, and /or clove-like smell impressions. Smoky smells, primarily comprising guaiacols, phenols and cresols, are enjoyed in diverse foods and beverages such as wine, whiskey, smoked ham, cheese, or baked goods such as pizza. Furthermore, the studies indicated that the odor properties of alkylated and halogenated guaiacols and phenols demonstrated the smoky smells are not the same for everyone and can be differently perceived in individual humans. Overall, the odor threshold investigation of guaiacol and its alkylated, alkenylated, and methoxylated derivatives showed that no specific substituent caused a significant and universal effect on OT. However, the positioning of the substituents to the hydroxyl function seems to have an impact on OT. Obtained data could be used to identification and analysis of the investigated compounds. Furthermore, the results can also be avail in understanding the structure and the odor correlation of compounds, which is still not fully resolved.

Finally, to investigate the wine off-odors, barrel-aged wine samples were analyzed by gas chromatography-olfactometry (GC-O), with regards to the presence of the typical earthy-smelling off-odor. The 2-methylisoborneol (2-MIB) was detected in these wines, although the wine samples were seven years old and all the previous studies showed that this substance is unstable in wine and must. Degradation products of 2-MIB were synthesised, and with regard to their odor, these dehydration products were found to be odorless. To investigate this paradoxical situation, 2-MIB stability was analyzed in wine and model solutions, starting with an elevated initial concentration. Therefore, it was concluded that although 2-MIB concentration greatly decreased with time it still can be detected in old wines and can induce the earthy off-odor.