Scientia Vitis: Decanting the Chemistry of Wine Flavor

Pasteur studying the diseases of wine in 1863.

Pasteur studying the diseases of wine in 1863.

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.

Since the molecules that account for specific flavors are elusive, much research on wine and flavor has focused on the role of acids, tannins, and sugars. Experts have long advised diners to consider tannin levels when pairing wine with food. At Pure Food and Wine, a raw-foods restaurant in New York City, sommelier Joey Repice seeks organic, handcrafted wines with a well-balanced tannin structure to accompany the delicate flavors of the fresh vegetarian entrees. “There are too many subtleties with our cuisine, and wines with lots of muscle and tannin structure go better with meat meals,” says Repice. For meat dishes he recommends Italian wines known for having a good tannin structure—perhaps a Barbaresco.

Tannins are polymers of phenols that seep from grape skins during fermentation. They are sometimes thought to provide clarity, but too many can cause a bitterness sometimes known as the “pucker factor.” While tannins have always played a key role in the vocabulary of connoisseurs, scientists are not convinced that the molecules are related to flavor. “Flavor can be traced back to a molecule interacting with a smell receptor, and tannins don’t do this,” says Boulton. Instead, tannins temporarily bind to generic proteins on the surface of cells in the mouth. They also change the texture and viscosity of proteins in the layer of saliva that coats the tongue, making it less fluid and slippery. Although tannins wash off when you rinse with water, there is an immediate impact on the entire mouth when you sip a glass of tannin-rich wine. With the exception of catechin and epicatechin—two tannins which happen to behave like flavor molecules—phenols generally don’t target a specific receptor. The mouth-puckering, dry sensation associated with tannins—sometimes compared to pomegranate or lemon pith—is more closely related to an astringent mouthfeel than a bitter flavor.

Similarly, acids are thought to contribute to tartness, but their role in creating flavor remains debatable. “When you change the acidity of a wine, the tartness often changes,” Boulton says, “but there are also acidic wines that are not tart.” Boulton suggests that the flavor molecules are likely separate from the acids. While he admits this hypothesis must still be tested, it’s likely that the acidity creates a climate in which tart molecules can work their magic, he says.

Like tannins, acids don’t directly add to flavor, but they influence how the wine feels in the mouth. Much like tannins contribute to the overall clarity of a wine, acids help balance out the sweetness and give wine a more rounded feel. The key—scientists and sommeliers agree—is to keep both tannins and acids in check. “While a subtle acidity is important, it has to be balanced,” Repice says.

Sugars, on the other hand, influence almost every aspect of a wine. At the most basic level fermentation is the process by which yeast converts the glucose and fructose in crushed grape juice into alcohol and carbon dioxide. Controlling the amount of sugar in wine has historically been difficult because yeast has a complex metabolic cycle and sugar levels vary depending on the ripeness of the grape. Early in the growing cycle berries expand because the cells inside are multiplying and dividing. But as the fruit matures, the individual cells begin to grow, as water and carbon dioxide are converted into glucose, fructose, and other sugars. Glucose levels start out much higher than fructose, but as the grape ripens the ratio begins to change. Eventually, if the grapes are allowed to overripen, fructose levels outpace glucose, and the grapes start to turn into raisins.

While raisining is generally a bad sign for winemakers, Sauternes from France, Trockenbeerenauslesen from Germany, and a few late-harvest table wines make use of shriveling to achieve sugar levels so high that the wines remain sweet even after fermentation. More typically, however, too much sugar makes wine taste like rotten candy, overpowering the subtle flavors that give it character and body.