# Identification of Rotundone as an Important Contributor to the Flavor of Oak-Aged Spirits
## 1 Introduction  
The practice of barrel aging in the production of distilled beverages has been used for centuries. Oak wood is the wood of choice in barrel making for its physical characteristics that lend itself to manufacturing a barrel, and its unique chemical properties that impart desirable flavors, such _smoky_, _clove-_, and _vanilla_-like notes. Volatiles from oak wood are either naturally present in the wood or formed during the post-harvest treatment. Seasoning, the first step of post-treatment, is performed to dry the wood, which equilibrates the moisture content to prevent further shrinkage or swelling, and to prepare the wood for toasting/charring. Due to loss of moisture, seasoning also creates a higher concentration of volatile constituents in the wood. The subsequent toasting/charring step affects the volatile composition of oak through hydrothermolysis, during which lignin pyrolysis produces the more familiar flavors of oak aging including guaiaicol, 4-ethylguaiaicol, and 4-vinylguaiaicol (_smoky_), eugenol and isoequengenol (_spicy_, _clove-like_), syringol and syringaldehyde (_sweet_, _smoky_), _p_-cresol (_barnyard_, _bandual-like_), and vanillin (_vanilla_). Carotenoids, which are unique to oak wood, include both \(\beta\)-carotene and lutein, which break down to form volatile compounds such as \(\beta\)-ionone, \(\beta\)-damascenoene, dihydroactinolide, and megastigmatrienones [1, 2]. Lipids and carbohydrate go through reactions to produce volatile aldehydes, alcohols, esters, furans, lactones, and, most importantly, (\(Z\))- and (\(E\))-\(\beta\)-methyl-\(\gamma\)-octalactone, i.e., \"oak lactones\" or \"whiskey lactones\" [3, 4]. The starting grain and subsequent treatment post-distillation, i.e., oak aging, are the two main contributors to the flavor of whiskey and other oak-aged spirits. With whiskey, the starting grain, typically barley, corn, wheat, or rye, goes through a malting step in which the grain partially germinates. Moreover, during the malting process, volatiles areformed via Maillard reactions, resulting in a product very similar to un-hopped beer [5]. The aroma impact compounds from fermentation consist of fused alcohols, acetates, and esters, which impart fruity or solvent-like characteristics, and include 2-methyl-1-propanol, 2- and 3-methyl-1-butanol, 2- and 3-methylbutyl acetate, acetaldehyde, 2-methylpropyl acetate. Post-distillation oak aging is the most important step in developing the flavor of whiskey. The flavor compounds from the oak, as discussed previously, are directly extracted into the spirit as well as developed from the ethanolysis of the acids present. The latter process results in the formation of fruity esters, including ethyl propionate, ethyl butanoate, ethyl hexanoate, and ethyl octanoate along with several branched-chain ethyl esters. Rotundone was at one time a fairly unassuming compound, first noted as a very potent aroma reminiscent of incense or black pepper, where it was first isolated from the root of _Cyperus rotundus_[6] and not mentioned again in the flavor science field until 2008, where it was identified as a potent odorant in Syrah grapes and wine [7]. It was again identified in agarwood oil, a dark fragrant resinous material from the heartwood of trees in the genus _Aquilaria_, a very prized and expensive material used in perfumery, described as warm, sandalwood, rich, woody, and ambergris [8]. The discovery of rotundone in grapes prompted its identification and quantitation in other food matrices [9]. Recently, it was reported as a character-impact odorant in chicory coffee [10]. Rotundone has an extremely low odor detection threshold of 8 parts-per-trillion (pptr) in water or 22 pptr in wine [7]. Therefore, even when present at low concentrations, rotundone may serve as an extremely potent odorant. Despite extensive research on aroma active compounds, the identity of a component responsible for the \"woodiness\" in oak-aged spirits remains unknown [11; 12].