# Identification of Rotundone as an Important Contributor to the Flavor of Oak-Aged Spirits
## 2 Results and Discussion
### Relative Potency of Rotundone in Bourbon Whiskey
The most potent odorants were vanillin, (\(Z\))-whiskey lactone, syringol, 2-phenethylethanol, 2-/3-methyl-1-butanol, guaiacol, (\(E\))-isoeugenol, eugenol, and (\(E\))-\(\beta\)-damascenone. These were followed by mainly ethyl esters. The order of potency of these can vary between whiskkeys as ethyl esters are formed during the aging process when the ethanol reacts with the wood acids. As these samples are of different ages, the amount of ethyl esters formed would be expected to differ and, thus, should not be consistent among the whiskkeys. The current study was the first to identify the presence of rotundone in bourbon whiskkey. It ranked among the moderately potent odorants, being detected at log\({}_{3}\)FD factors between 2 and 4, thus indicating that it has some impact on the overall flavor. The remaining compounds identified are well-known constituents of oak-aged spirits. 3.2 Concentrations and OAVs of Selected Potent Odorants in Bourbon Whiskeys of Increasing Age (4, 8, and 12 Years)  
A total of 25 odorants identified by AEDA in bourbon whiskkeys were quantitated by stable isotope dilution analysis (SIDA) (Table 4). Some interesting observations can be made with respect to aging based on the analysis of these 4-, 8-, and 12-year-old bourbons. Statistical analysis revealed that all 25 compounds varied significantly among bourbons. Sixteen compounds showed definite linear increases in concentrations as a function of age. Previously, the effect of aging on the concentration of volatiles in whiskkey showed that ethyl esters (ethyl octanoate, ethyl hexanoate, ethyl butyrate, ethyl vanillate, and ethyl 3-methylbutanoate) increased over time [33], which illustrated that ethanolysis was not selective as to the acid backbone structure with which it reacts. In the present study, only ethyl butanoate and ethyl vanillate showed a linear increase in concentration from 4 to 8 to 12 years of aging. One might expect that a plateau would eventually be met by 12 years as the starting material for ethanolysis becomes depleted. However, the concentrations could also increase as a result of evaporation of both water and ethanol during aging, thus decreasing the total volume and subsequently increasing the concentrations of some odorants. This could explain the increase in concentration of the fused alcohols (2-methyl-1-propanol and 2-/3-methyl-1-butanol). A linear increase with time was also observed for the important oak wood extractives vanillin, guaiacol, syringaldehyde, (\(E\))-isoeugenol, (\(Z\))-whiskey lactone, and (\(E\))-whiskey lactone. These compounds are derived directly from the oak wood, so a valid assumption may be that the longer the spirit is in contact with the wood, the greater the amount would be extracted. The whiskey lactones, in particular, are considered to be among the most important components of the oak influence on whiskkey, and correlate with a positive assessment of whiskkey flavor [4]. In the present study, vanillin had a high coefficient of correlation (1.0) with respect to its increase in concentration with aging. This is in agreement with previous reports in which vanillin was observed to form not only during the charring step of cask manufacturing, but also during aging by a hydrolytic mechanism. Whiskey, being slightly acidic, causes acid-catalyzed hydrolysis of the lignin during aging, resulting in the formation of vanillin [37]. The relationship between eugenol and isoeugenol is also of interest. Both display a linear trend during aging; however, they are inverse to one another, as eugenol is isomerized to isoeugenol by migration of the double bond. This conversion was previously observed in model studies involving the artificial aging of apple cider using oak chips [38]. Meanwhile, our compound of interest, rotundone, also increased linearly with whiskey age. Although rotundone was measured at the lowest concentration of all the compounds quantitated, this is not a direct reflection of its potency owing to its very low odor detection threshold.