Tannins from oak barrels that flavor wine are ‘fingerprinted’ by researchers

UNIVERSITY PARK, Pa. — For centuries, wine has been aged in oak barrels with the wood imparting flavors described as coconut, vanilla, spice, caramel and smoke, to name a few, that affect the taste and mouthfeel of the wine. But scientists have been unable to identify which tannins — water-soluble compounds found in both the wood and grape skins — are responsible for which flavors. Now, a team led by Penn State scientists has developed a way to chemically characterize and identify individual tannins in wine that come from oak barrels and contribute to its flavor profile.

“Up to now, we have put all these tannins that affect taste and mouthfeel — bitterness, dryness/astringency — and color stability that influence how wine evolves during aging under one umbrella, but this study gets at which compounds are responsible for the really enjoyable sensations provided by the tannins in wine,” said research team leader and senior author Misha Kwasniewski, associate research professor of food science in the College of Agricultural Sciences.

The researchers — who described their novel method as “fingerprinting” tannins — published the study in Food Chemistry. Using a combination of chemical analyzes, the approach separates complex mixtures, identifies molecular components and quantifies substances, even at low concentrations.

Previously, constituent identification techniques involved chemically breaking tannins apart with acid, which often damaged them before they could be accurately quantified. The new method — that the researchers described as in-source fragmentation — involves an analytical tool called a mass spectrometer separating tannins into their molecules, or fragments, and detecting the chemical signature of each fragment. Together, the signatures comprise a kind of fingerprint of the tannin.

“In this work, we are able to identify tannins from the grapes and tannins from oak,” Kwasniewski said. “These tannins unite in billions of combinations, so we have to sort it out with machine learning — a kind of artificial intelligence (AI) — to detect these complex fingerprints of what is flavoring the wine. My lab has developed — and is continuing to add to — a machine learning model that has the ability to integrate a new, important class of tannins. It then will be applied to help understand flavor, bioactivity, plant biochemistry and more.”

In this study, the researchers focused on “hydrolysable” tannins — those with chemical bonds capable of being broken down into smaller molecules by reacting with water — found in oak wood and transferred into wine during barrel aging. They include ellagitannins — common in oak, which strongly influence wine aging and structure; and gallotannins — found in both wood and grape skins, that contribute to bitterness and astringency.

This research involved a collaboration with Gallo, a California-based winery that provided some of the 22 red and 20 white wines analyzed in the study. The team conducted experiments using 15 commercial oak chip products representing three types of oak used for wine-aging barrels: French oak, Hungarian oak and American oak. After adding the various types of oak chips to wine samples, the researchers could then characterize the differences in hydrolysable tannins in those treated wines.

Study results revealed that French oak contained the highest concentrations of both ellagitannins and gallotannins, followed by Hungarian oak and American oak. The differences among oak types, the researchers pointed out, likely are influenced by botanical origin, native tannin content and cooperage practices — techniques including selecting and seasoning oak and bending staves for barrels using fire or steam — that affect the quantity and stability of extractable tannins.

In addition, the researchers evaluated chemical transformations that occur in the wood during toasting, which is the process of applying heat to the interior of oak barrels to influence aging.

“Toasting significantly changes the tannins in wood barrels, breaking them down to reduce harshness and converting them into softer, more complex compounds,” Kwasniewski said. “This heat treatment caramelizes wood sugars and alters chemical structures, reducing ‘raw’ oak tannins and releasing compounds, markedly influencing the sensory properties imparted by the wood into the wine.”

Study first author Yanxin Lin graduated with a doctoral degree in food science and is now a postdoctoral scholar at the University of California Davis. Other co-authors were Bruce Pan, Robert “Qiang” Sui and Ping Yu, all with Gallo.

This research was supported by the Crouch Endowment for Viticulture, Enology, and Pomology Research in the College of Agricultural Science at Penn State, the American Vineyard Foundation and the U.S. Department of Agriculture’s National Institute of Food and Agriculture.