French Researchers Unlock a Secret to Wine Bitterness: Oak Barrels

A study (and a very unpleasant blind tasting) reveals how one chemical compound in oak barrels may be the culprit for some less welcome flavors in wine

French Researchers Unlock a Secret to Wine Bitterness: Oak Barrels
Barrels, of course, also help make the magic of Bordeaux's greatest wines happen. (This cellar was not a source of wines in the study.) (Courtesy of Château Margaux)
Jul 30, 2020

Fans of full-bodied reds and spicy Chardonnays know they can, in part, thank oak barrels for the toasty, nutty vanilla flavors and smooth textures found in their favorite wines. But could wood be adding a bitter note to tipples? It makes sense: Oak imparts tannins, and tannins are astringent. In a recently published study, however, researchers from the University of Bordeaux focused on a different phenolic compound they believed to be the main culprit for barrel bitterness: coumarins. Where are they, how do they affect your wine—and can anything be done about them? With the help of taste-testers willing to try some very bitter potions, the scientists found some surprising answers.

Many plants, including oak trees, contain coumarins, compounds so caustic they can deter predators, explained Dr. Delphine Winstel, the postdoctoral researcher whose thesis formed the basis of the study, published last month in the Journal of Agricultural and Food Chemistry and titled "Role of Oak Coumarins in the Taste of Wines and Spirits: Identification, Quantitation, and Sensory Contribution through Perceptive Interactions."

Oak samples for coumarin testing
Bordeaux's cool new drink is … oak milk? Not quite: These jars contain barrel extracts for analysis of the presence and prevalence of coumarins. (Courtesy of Dr. Delphine Winstel)

But Winstel and her colleagues wanted to figure out which coumarins actually make it into oak barrels. They picked up some samples from the master coopers at Seguin-Moreau and successfully identified the five coumarins known to exist in oak—plus, one more, previously undetected. “It is always very satisfying to [find] a compound that had never been identified in wine,” Winstel told Unfiltered via email. (It's called fraxetin, and it tastes, well, bitter.)

How much of a pucker do coumarins really pack in the glass, and at what levels are they detectable? To find out, the team hosted a more-acrid-than-usual blind tasting of coumarin-laced wine and spirit samples for a group of 22 trained tasters. With noses clipped to block the coumarins’ noxious odor, the panel dutifully tasted through. “I'm not sure that tasting bitter molecules in a hydro-alcoholic solution in the morning is the best pleasure in life,” Winstel observed. “But every panelist was diligent!”

Barrels at Clos Floridène; researchers Delphine Winstel and Axel Marchal
Left: the barrel cellar at Clos Floridène; the winery was not part of the coumarins study specifically, but is a frequent participant in University of Bordeaux research. Right: Lead researchers Delphine Winstel and Axel Marchal in the lab (Courtesy of Dr. Delphine Winstel)

Winstel's and co. also analyzed 90 commercial wines for coumarin levels, plus some spirits: reds from Bordeaux and Burgundy, whites from the Loire and Alsace, Cognac vintages back to 1970, and more. They found higher coumarin levels in red wines than white, but beyond that, “there is no particular region or appellation that shows a higher level of all coumarins,” Winstel assured us (and, likely, winemakers around the world).

While the team determined how much was too much when it comes to coumarins, and is closer to knowing how coumarin levels can vary between different trees and perhaps even barrels, there's much work and unpleasant tasting yet to be done. But these new findings could still have a real effect on the wine industry. Vintners might one day work with coopers to limit the coumarin levels in their wines. And any discovery makes for a sweeter day in the world of wine sci.


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