One of the fascinating characteristics of wine is its capacity to transform over time. It’s why age-worthy wines like great Bordeaux and Barolo are so prized—as the basic, fresh fruit from early days goes through gentle oxidation and other slow transformations, and gets supplemented with secondary and tertiary aromas like forest floor, mushrooms, hazelnuts, and dried fruit or balsamic notes.
One researcher who’s taken a closer look at the chemistry behind the evolution of aromas in wines is Stéphanie Marchand, a professor at the Université Victor Segalen in Bordeaux. She has been working for years on the “aging bouquet” of her home region’s wines. As she has been discovering new components and understanding their roles better, she’s remained aligned on a basic premise: “Any aroma that goes against the fruity character of the wine is a fault. Old wine is quality wine only if it retains some of the fruit of its youth. Everything that adds to that primary fruit without masking it is a sign of quality, and anything that hides or destroys it is a fault.”
One could wonder if this idea of preserving fruit is in line with current tastes for younger, fruitier wines, or if it is something more timeless. Marchand thinks the latter. “It’s true that a few decades ago, we were more tolerant of certain things like some phenolic elements that are no longer quite as acceptable,” she says. “But even in the early Twentieth Century, descriptions of older wines pointed to fruity aromas.”
Would you like a bit of truffle with that?
One of the key predictors of a good, complex aging bouquet, Marchand explains, is a somewhat unexpected component: dimethyl sulfide, or DMS. If you look up this compound, you’ll find a lot of unflattering words about its aromatic character: rotting cabbage, canned corn, cooked cauliflower, etc. It’s a contributor to the smell of a ripe Camembert cheese, too.
But lo and behold, DMS is also the key component of a rather prized gastronomical treat: truffles. So, clearly, depending on context—and the way it blends with other chemical compounds—DMS can be a positive. In red wines, Marchand points to work by researcher Georgia Lytra, who showed that DMS acts as a booster for aromas of red and black fruits. As the presence of DMS grows over time, its effect on the preservation of fruity aromas in older wines is very much in line with the researcher’s basic premise about aging bouquet.
Although dimethyl sulfite is generally seen as a reductive component, Stéphanie Marchand indicates that, “It’s not a reductive fault like H2S or mercaptans. It doesn’t have an SH function, so it’s not very dependent on the oxidative regime in the wine, and it essentially reveals itself through time in bottle.”
How does that happen? “Well, the mechanisms remain unknown. The whole thing isn’t super clear,” says the Bordeaux researcher. What is known at this point is that the potential for DMS expression starts in the berry itself, and that factors like a relatively quick alcoholic fermentation seem to be favorable to its appearance, and to that of a good aging bouquet in general, since it also preserves other components that protect wines from oxygen.
Since DMS and other aromatic precursors originate in the berry, researchers have also been looking at correlations between growing conditions and the emergence of a complex, pleasant bouquet over time. “A certain level of water stress is good—but not too much, and the amount of nitrogen in the berries has to be non-limiting for the alcoholic fermentation,” says Marchand.
That does tie in with generally accepted principles about producing quality grapes—and thus, quality wines. The key is achieving a good balance, something that varies from wine to wine, as Marchand indicates. “The matrix of each type of wine will mean different equilibriums between different factors, and it also means that it will react a bit differently to the same levels of oxygen and other compounds.”
Patience is Key
As Marchand and others try to delve further into the complex mechanics that produce a complex aging bouquet, one thing is clear: it takes time. “We once tried to add precursors of certain components to wine, to see how they would be impacted by oxygen, but the result was too simple, like a caricature,” she says. Instead, various wines are being tested over years to see how their chemical signatures change. Sometimes, researchers are just like wine collectors: They need a good cellar and years of patience.
Marchand adds that this patience is constantly rewarded, since there are always new things to look at when trying to define what makes old wine tick. She points out that a postdoctoral researcher who has been working with her is soon to publish an article on a newly-identified component that also plays a specific role in developing the prized “aging bouquet.” What is that, exactly? You’ll have to stay tuned. And there’ll be plenty more to come after that for those who want to find out even more. Marchand concludes: “It’s endless. We’ll never get to the bottom of that well.”