Many wine lovers consider terroir—a sense of place you can taste in the glass—to be what makes great wine special. Whether that’s determined by soil, climate or other factors, however, is understood vaguely at best. Can we prove that individual wine regions—indeed, individual vineyards—convey site-specific signatures into finished wines?
This was the question taken up recently by Prof. David Mills and his lab at the University of California at Davis. Culling 273 samples of grape must from four different regions of California in two different vintages, the researchers observed microbiota—microorganisms such as fungi, bacteria and yeasts in a given ecosystem—in lots that represented individual vineyard blocks. They analyzed these samples to characterize their composition of fungal and bacterial communities. By sequencing the genes of these microbiota, they discovered that different regions, different grape varieties and different climates each displayed distinctive microbial patterns.
“The microbiota content of the grape at harvest, in a sense, is a record of the life history of that grape,” Mills told Wine Spectator. In other words, the various microbes that live on a grape are reflections of its site, its genetic characteristics and the environmental conditions that it weathered.
These results could inform winemaking decisions. For example, Mills’ team found that Zinfandel grapes were associated with higher concentrations of Gluconobacter, an aerobic, non-fermentative acetic acid bacterium that is likely to produce excessive volatile acidity if untreated with sulfur dioxide. A winemaker concerned that this microbe could make its way into her Zinfandel might adjust her SO2 levels.
Additionally, these microbiota compositions could serve as a barometer on the vine for potential wine quality. “If you knew which patterns result in better wines and which are the danger [patterns],” said Mills, “then you could predict [quality] before harvest and change things accordingly.”
He also emphasized that winemakers don’t need to be making “natural wine” in order for these distinctive organisms to live on their grapes, in their winery and, potentially, in their wines. Mills does not believe that commercial yeast inoculations or SO2 additions, for instance, completely eliminate the microbiota that could affect how a finished wine tastes. “I think people overestimate the amount of influence that humans can actually have to reduce a microbiota coming in,” he said.
The fact that these patterns exist is merely a first step toward proving that humans can taste terroir in a glass of wine. Whether microbiota directly influence the character of a wine, or whether they merely function as a microcosm of other quality factors like climate, vintage, variety and soil, remains to be seen. Researchers would need to investigate whether the distinctive microbiota sequences of unfermented grapes have any bearing on the chemicals that define wine’s flavors. Ultimately, sensory analysis must be performed by experts, to prove that there is a link between what’s in the soil and what we taste in a glass.
Mills cautions that much more work needs to be done to truly understand how site impacts wine. “[Terroir] is always spoken of as a given, proven subject,” Mills said. “It is not. In Galileo's time, everyone was convinced—by their own visceral experience—that the sun revolved around the earth. Obviously, that turned out to be wrong.”
“I am not saying the folks who believe climate and geography impacts wine flavor are wrong,” Mills wrote. “I think they are right. [But] we need to get away from the anecdotal nature of this and into robust, mechanistic hypotheses we hopefully can test.”