Scientia Vitis: Decanting the Chemistry of Wine Flavor

An illustration of white wine grapes from Jean-Antoine-Claude Chaptal’s Traité théorique et pratique sur la culture de la vigne. Roy G. Neville Historical Chemical Library, CHF. Photo by Douglas A. Lockard.

By Amy Coombs

The University of California, Davis (UC Davis) wine cellar is lined with rows of wooden barrels and old-fashioned wine bottles—some dating back to the end of Prohibition, when America’s wine industry had to start from scratch. Look inside the newest bottles, however, and you’ll see and taste the results of four decades’ worth of modern research on what makes a great wine. Continuing a century-old tradition, researchers at the Department of Viticulture and Enology at UC Davis are investigating the complex dance between science, art, and nature that creates flavor and aroma in wine.

The Tasting Booth

In Hildegarde Heymann’s lab tasting booths are illuminated by the deep glow of a dark red lamp. “We give people wine in dark glasses and change the room lighting to make the color of the samples harder to discern,” says the UC Davis professor of enology. “We want them to focus on what they taste and smell, not what they see.” A world-renowned expert on the molecular basis of aroma, Heymann says that wine flavor is a subjective, blurry experience that results from complex interactions between many different classes of compounds. Molecules intermingle and coalesce, just like the lingering notes of raspberry and blackberry that remain on the palate after sampling a characteristic young Shiraz.

Acids, sugars, and tannins are the most obvious contributors to wine flavor, but these three classes of molecules are accompanied by a remarkably varied cast of organic molecules—often aromatics—that, in combination, can produce an astonishing variety of flavors. Generally speaking, fruity flavors are attributed to interactions among esters, alcohols, and acids. Tannins, or phenol compounds, give wine an astringent mouthfeel, and sugars determine the sweetness of the wine. Yet to make things even more complicated, the interaction of these chemicals seems to depend on growing conditions and fermentation practices.

“The chemistry that sets a merlot apart from a pinot noir is nearly impossible to identify,” says Heymann. “There may be over 500 different flavor compounds unique to each variety.” Nevertheless, Heymann and her colleagues are attempting to connect specific combinations of molecules with familiar flavors. Since two different people can taste two very different things when sampling the same bottle of wine, Heymann passes cups of apple peel and soy sauce through tasting booth windows—definitions of flavor that everyone can identify. Instead of chemical reagents and noxious gases, the cabinets in Heymann’s lab contain bottles of soy sauce and parcels of chocolate—all used as a basis for comparison. This approach has allowed scientists to trace some of the more notable wine characteristics back to their chemical roots.

Molecules with a Single Flavor

One level below Heymann’s tasting room, Roger Boulton, a professor of viticulture, runs experiments on the sulfides produced during fermentation, surrounded by a laboratory full of spectrometers, chromatographs, and other traditional analytical equipment. “After 2,000 years of winemaking, only a few molecules have been correlated with a specific flavor,” says Boulton. One instance where a direct link has been established, he explains, involves methoxypyrazines, a family of molecules that makes wine taste like bell peppers.

Methoxypyrazines were initially found to play a role in wine flavor in 1975, says Boulton. They are now understood to be particularly prominent in cabernet grapes. While trace amounts of the molecule are considered acceptable, too much can overwhelm wine, producing a strong vegetable flavor. Heymann and her colleagues have since shown that the molecule breaks down under light, and viticulturists are now experimenting with growing practices that expose grapes to more sunshine in an attempt to minimize the chemical’s presence. Leaves are pulled off of the plants, which are then compared to control groups that grow with leaf cover. So far taste tests have shown that pepper-juice flavor can be altered by modifying growing conditions. “People can tell the difference,” says Heymann. “The way you grow the grapes absolutely matters.”

Yet, according to Boulton, the connection of taste to a particular molecule is rare. To show that the methoxypyrazine was involved in flavor, it first had to be isolated, then a receptor in the nose had to be identified. Finally, panels of tasters had to demonstrate that wines with higher levels of the methoxypyrazine smelled differently from those without. Scientists in search of flavor molecules in wine are rarely able to pass all these tests.

One of the most difficult problems in identifying the molecular source of flavor is that many of the suspected compounds have astonishingly low detection thresholds. In the case of methoxypyrazine, for example, the flavor is noticeable at 2 parts per trillion. As Heymann puts it, just a few drops of methoxypyrazine in a swimming pool would be enough to make you think you were swimming in bell pepper juice. “All of the flavors we are dealing with are very small in concentration, and analytically we don’t always even know their identity,” says Boulton. “This is especially true with red wines.”

As many of the flavor molecules in wine are quite potent, the nose can detect very small amounts. Unfortunately the chemistry equipment used in the lab isn’t as sensitive as the human nose.

Only a handful of other molecules have been tied to a distinct flavor. Short-chain volatile aldehydes like hexanal, pentanal, and nonanal have been shown to contribute grassy, nut-like, and orange-rose flavors, respectively. Specific turpines have been shown to give Riesling its unique aroma. Glycosides from cabernet sauvignon and merlot grapes are thought to smell like fig, tobacco, and chocolate, but the flavors haven’t been correlated with a specific compound. Sometimes a molecule is associated with a specific place, as is the case with 3-mercapto-hexan-1-ol, a thiol that produces a rich citrus flavor in Sonya Blanc wines from New Zealand: “You can make this style in other countries using the same grapes,” Heymann explains, “but it’s much more difficult and doesn’t have the same flavor.”