Wine and spirits are complex mixtures of flavor and aroma compounds, some of which arise during aging in wooden barrels. Among other compounds, oak wood releases coumarins, but how they affect wine’s sensory properties is unclear. Now, researchers reporting in American Chemical Society’s Journal of Agricultural and Food Chemistry have detected and measured six coumarins in oak wood, wine and spirits, showing that a combination of these compounds can produce a bitter taste.

Oak barrels are often used during the aging of wine and some spirits, including cognac, rum and whiskey. Prolonged contact of the beverages with wood alters their sensory properties, and many oak compounds that contribute to color, aroma, mouthfeel and taste have been identified. Oak wood also contains coumarins –– compounds produced by plants as a defense against predators. Many edible plants make coumarins, which at high doses are used as blood thinners, but the tiny amounts in most foods are not large enough to have anti-coagulant effects.

Spirits showed higher levels of coumarins than wines, possibly because of longer aging and a greater alcohol content. The tiny amounts of most of the compounds were below levels that humans can taste, but when the researchers added a mixture of all six to non-oak-aged wines or spirits, sensory panelists detected a significant increase in bitter flavor. Future studies could examine ways to reduce the coumarin content of wooden barrels to produce better-tasting wines and spirits, the researchers say.