Scientia Vitis: Decanting the Chemistry of Wine Flavor

Pasteur studying the diseases of wine in 1863.

Pasteur studying the diseases of wine in 1863.

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 terpenes 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.”

An important early scientist was Eugene Hilgard (1833–1916) who would eventually found the viticulture department at UC Davis. Bavarian-born and raised in the United States, Hilgard studied in Germany with such leading chemical thinkers as Carl Friedrich Plattner, Johann Joseph Scherer, and Robert Bunsen. He returned to the United States where his deteriorating health motivated a career change: he became an advocate for state-sponsored sciences that brought him to work outdoors in fresh air, particularly geological and agricultural surveys. With Hilgard, UC Davis found an outspoken advocate for practical, applied scientific research that would benefit the state’s growing wine industry.

Like most grape producers of his day, Hilgard believed that color was a marker of the fermentation process. In an ingenious but little-known experiment, Hilgard used a stereoscope—a popular Victorian device that creates the illusion of depth in a photograph by presenting a slightly different image to each eye—to track wine’s aging process. Hilgard’s stereoscope was designed by Michel Eugène Chevreul, a chemist whose work with dyes and pigments influenced the Impressionist and Neo-Impressionist movements. Chevreul observed that the eye naturally fused colors of slightly different shades, allowing contrasting hues to lend depth and intensity to an image. Whereas Chevreul used the stereoscope to observe distinctions between objects in a painting or textile, Hilgard used it to study the change in a wine’s color during fermentation. Spots of wine were applied to paper at increments during the aging process. “You could compare the paper to a fabric of a known color,” explains Boulton. As the green grape juice fermented, it changed to pink, red, and then finally to purple—a process that had been observed for thousands of years. “[Hilgard’s experiments] let people know when the color transitions had peaked, and the wine could be transitioned into aging barrels,” adds Boulton. As later work would show, tannins leach out of grape skins early in the vatting process. Allowing the juice to sit beyond this peak might result in too many tannins, with no additional color. Boulton credits Hilgard as “the first to quantify this process.”

Hilgard’s methods are now being automated. In 2001 Boulton and other UC Davis colleagues launched the Hilgard Project—a network of pressure transducers that monitor fermentation in vats around the world. “As only one crop of grapes can be grown each year, it can take decades for a vineyard to collect enough data to make any real conclusions,” says Boulton, “[but] with the Hilgard Project we are compiling enough data for real analysis to be performed.” The data are made available for public use, and Boulton says the scope will soon be expanded to include other sampling methods. Plans are in the works to install colorimeters and other sensors that can be used to monitor tannin levels and alcohol concentration directly. Identifying the threshold at which tannins stop contributing to color but continue to affect mouthfeel is a future goal.