Scientia Vitis: Decanting the Chemistry of Wine Flavor

Pasteur studying the diseases of wine in 1863.

Pasteur studying the diseases of wine in 1863.

Fermentation and Tannin Control

Winemakers have been tinkering with wine flavor for millennia by varying the grape varieties, growing conditions, and fermentation processes, but the modern practices associated with the sciences of viticulture and enology date to only the late 19th century. Around 1860 the French physiologist Louis Pasteur firmly established that alcoholic fermentation is caused by yeast. The realization that fermentation was a biological process that might be controlled and might yield predictable results opened up an entirely new way of thinking about beermaking and winemaking.

Since then fermentation scientists have made profound contributions to a wide range of other scientific disciplines. Many of the early studies that broke ground for the fields of molecular biology and biochemistry relied on yeast as a model organism, and many were motivated by questions about the fermentation of wine, says Boulton. From mating assays that test genetic theories to cell-signaling experiments, simple brewer’s yeast (Saccharomyces cerevisiae) remains one of the most commonly used organisms in the field of molecular biology.

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.”