The practice of barrel aging of spirits has been used for centuries. It began as an alternative storage and transportation method, but aging in an oak cask is now exclusively used as a means to impart flavor to the spirits. Oak wood is the wood of choice for barrel making, not only for its physical characteristics that lend itself to manufacturing a barrel, but also for its unique chemical properties that impart key flavors to aged spirits. Oak aging of spirits develops flavor in a number of different ways, all which contribute to a wide range of odor descriptions, creating the complex flavor with which we are familiar. Extensive research has been performed on oak wood and oak aged spirits; however, the identity of the component(s) responsible for the “woody/incense” flavor attribute of age spirits was, prior to this investigation, unknown. Experiments were conducted in order to unambiguously identify a compound responsible for a “woody/incense” odor note in oak aged spirits. The target compound was isolated from oak wood chips followed by several purification steps, as well as the use of a custom built GC-MS/olfactometry system equipped with a heart-cutting system/internal CryoTrap which enabled the acquisition of an interpretable electron-impact mass spectrum (EI-MS) for the compound. The EI-MS revealed that the unknown target compound possessed a molecular weight of 218. A thorough investigation of naturally occurring organic compounds having a molecular weight of 218, along with deducing the nature of the functional groups on the molecule, indicated numerous compounds as possible candidates. Most of these compounds were found to occur naturally in a number of roots, spices, oils, and herbs, which were subsequently analyzed. Results of the analyses revealed that the compound was most likely the sesquiterpene ketone 5-isopropenyl-3,8-dimethyl-3,4,5,6,7,8-hexahydro-1(2H)-azulenone, or rotundone. This identification was confirmed by comparison of the compound’s EI-MS and GC retention indices against those of authentic rotundone obtained by chemical synthesis. The next question addressed was whether this compound is present in oak aged spirits. Accurate quantification of this trace level target compound was done by stable isotope dilution analysis (SIDA). The presence of rotundone in different aged spirits including bourbons, rye, scotch, whiskey, rum, and tequila was demonstrated. Trends in aging were established, showing that rotundone increases with aging time; however, its quantity may also be influenced by other factors as there was a clear brand to brand variation. Interestingly, rotundone was also found in un-aged (silver) tequila, which suggests that the compound may also be present in the agave plant. Results of quantification of all potent odorant in bourbons, aged 4, 8, and 12 years and calculation of their odor activity values (OAVs) demonstrated rotundone’s importance to the overall flavor of bourbon. From the quantification data some interesting trends were established that demonstrate some effects of barrel aging. OAVs are used as a gauge for potency and, generally, any compound with an OAV above 1, provides evidence of whether a compound is important. With an OAV of 42.8 to 56.6, rotundone lies well above this requirement and is among the top 10 odorants quantified in these samples. Thus, it is concluded that rotundone is an important contributor to the flavor of these aged spirits
