Science

The Science of Esters in Wine

Wine experts discuss what differentiates esters from other aroma compounds

Illustration by Jeff Quinn
Photo illustration by Jeff Quinn.

Among wine aromas, esters are the most delightful—and fleeting. Typically associated with fruity flavors like banana and pineapple, this diverse group of aroma compounds can also be citrusy and even floral. More than 160 esters have been identified in wine, and many of them occur in concentrations below our sensory threshold. Esters also have the ability to influence each other. When found together, certain esters can create a totally different aroma from what they smell like individually—and the presence of one can also greatly affect the intensity of another.

In some fruits, such as apples, an abundance of esters is contained in the fruit itself. Grapes, however, have very few. The vast majority of esters that occur in wine are created during fermentation. This is another feature that makes esters unique in the world of wine aromas. The other main aroma compounds in wine—terpenes, thiols, pyrazines, norisoprenoids—do originate in the grape. Esters tend to be much less stable than these other aromas. They peak in intensity at the end of fermentation, when they dominate a wine’s character, but they degrade almost entirely over the first year of a wine’s life and leave no long-term imprint.

The Formation of Esters

The formation of esters is immensely complex, and as with many subjects involving wine, the story gets more complicated the more deeply you look into it. Ester development depends on numerous reactions among multiple compounds. And there are rules governing the formation of esters, as well as exceptions to those rules—all of which are dissertation-worthy topics. And again, as with much in the study of wine, we have yet to understand the entire picture. For the purposes of this article, let’s say that esters are formed from the reactions between alcohols (primarily higher alcohols) created by yeasts and components (usually fatty or organic acids) of the grapes.

The two most important factors affecting the formation of esters are yeast strain and fermentation temperature. Different yeast strains yield different quantities of esters. Yeast strains that produce lower quantities are called neutral yeasts. Those that give higher quantities are usually referred to as aromatic yeasts; they’re typically used to craft wines that are fruity in style and intended to be drunk young—before the esters degrade. The widespread use of aromatic yeasts in mass-produced commercial wines has been trending for a while now. It may be one of the reasons for the common misperception that all commercial yeasts impart aromas to wine, when in fact it’s the more aromatic yeasts that do this (and, moreover, their effects will be gone by the time most wines are consumed). In addition to using aromatic yeasts, winemakers can also employ carbonic maceration to achieve high levels of esters during fermentation.

Fermentation temperature is the other major factor in the quantity and type of esters produced, as well as the degree to which they’re retained in the wine. A winemaker who wants to create high-ester wines will usually use an aromatic yeast, clarify the must, and then ferment the wine at as cold a temperature as the yeast can tolerate. Though cooler temperatures lead to the production of fewer esters than do warmer fermentations, more esters are thought to be retained during and after fermentation at cooler temperatures.

Michael Jones, the fermentation specialist for the well-known yeast purveyor Scott Labs in Petaluma, California, has spent the last 40 years working in nearly every part of the wine industry, from grape nurseries to vineyards to many famous wineries. Recently, Jones and his team at Scott Labs have been getting some interesting results in studies of temperature’s effects on ester production. They’re seeing that warmer temperatures (in the 70s) indeed lead to greater production of esters; however, they’re finding that so many more are formed that even with loss from the volatilization of higher temperatures, warmer ferments yield wines that contain more esters than those fermented cold. Esters created at warmer temperatures are thought to last longer, but Jones believes this perception may just be a result of there being so many more of them.



Fermentation temperature also dictates the type of esters produced. The fruity aromas usually associated with esters are not produced at warmer temperatures. Jones explains that “lower temperatures, in the 50s, give you more tropical, fruity-smelling esters—banana, fruit cocktail. When you get into the 70s, the esters become much more floral. In the 60s, you get a mix of both.”

Could the notion that “more” esters are created at lower fermentation temperatures be attributed to those esters having a lower sensory threshold—and therefore greater intensity at lower quantities—than those created at higher temperatures? Hildegarde Heymann, Ph.D., a professor and sensory scientist in the department of Viticulture and Enology at U.C. Davis, says no. “The temperature of formation does not affect the sensory threshold [of esters],” says Heymann. With that being the case, it’s likely that the “cooler equals more” idea may result from the perception of esters’ association with fruitiness.

The Role Esters Play

Esters are a critical component in many wines, says Mike Roth, the cofounder and winemaker of Lo-Fi Wines, based in Los Alamos, California. “People usually enjoy wines, at least white wines, as fruity as possible,” he says. At Grgich Hills Estate in Napa Valley, California, where Roth cut his winemaking teeth, he says they used to push the envelope to make the wines as fruity as possible. “We’d settle the wine really well,” he says, “then ferment as cold as possible with aromatic yeasts.”

Esters also play an important role in Roth’s own Lo-Fi wines, though they’re not the dominating characteristic. “I want my wines to be highly drinkable,” Roth says. “Carbonic and semicarbonic wines are very drinkable wines. Ester-y, fruity wines are more drinkable, as smell dictates, in part, how something tastes.”

The recent rosé craze can also be thought of as an ester craze—people want the youngest rosés possible, bottled just months after fermentation, while the wines’ fermentative, ester-y freshness remains, even though many industry professionals agree that good rosés get better after the first year.

All of the other important wine aroma compounds—the terpenes, thiols, pyrazines, norisoprenoids—result from the interaction between particular grape varieties and the site in which they’re grown. For example, the presence of various thiols or terpenes can tip us to a wine’s variety or the region where it was cultivated. Non-ester aromas are enduring and will define a wine over its lifetime, whereas esters will be gone in a matter of months or years. Esters stand out as anomalies in the world of wine aromas—they tell us nothing of a wine’s origin, but boy, do they smell wonderful!

Alex Russan, based in Santa Barbara County, California, is the owner-winemaker of Metrick wines and Alexander Jules, his sherry label and the company through which he imports Spanish wine. He also writes about and teaches enology, viticulture, and tasting and has a background in specialty coffee, botany, and philosophy.

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