wine terroirResearchers have found that grape varieties in different regions carry distinctive patterns of fungi and bacteria, which may play a role in making a wine unique. Read Story in NYTimes  

Terroir is a concept at the heart of French winemaking, but one so mysterious that the word has no English counterpart. It denotes the holistic combination of soil, geology, climate and local grape-growing practices that make each region’s wine unique.

There must be something to terroir, given that expert wine tasters can often identify the region from which a wine comes. But American wine growers have long expressed varying degrees of skepticism about this ineffable concept, some dismissing it as unfathomable mysticism and others regarding it as a shrewd marketing ploy to protect the cachet of French wines.

Now American researchers may have penetrated the veil that hides the landscape of terroir from clear view, at least in part. They have seized on a plausible aspect of terroir that can be scientifically measured — the fungi and bacteria that grow on the surface of the wine grape.

These microbes certainly affect the health of grapes as they grow — several of them adversely — and they are also incorporated into the must, the mashed grapes that are the starting material of winemaking. Several of the natural fungi that live on grapes have yeastlike properties, and they and other microbes could affect the metabolism of the ensuing fermentation. (Several species of microbes are available commercially for inoculation along with yeast into wine fermentations.)

But are the microbial communities that grow on the grapes of a given region stable enough to contribute consistently to wine quality, and hence able to explain or contribute to its terroir?

Such a question would have been hard or impossible to address until the development of two techniques that allow the mass identification of species. One is DNA bar coding, based on the finding that most species can be identified by analyzing a short stretch of their genome, some 250 DNA units in length. The other is the availability of machines that can analyze prodigious amounts of DNA data at a reasonable cost.

Armed with these new tools for studying microbial ecology, a research team led by David A. Mills and Nicholas A. Bokulich of the University of California, Davis, has sampled grape musts from vineyards across California. Grape varieties from various wine-growing regions carry distinctive patterns of fungi and bacteria, they reported Monday in The Proceedings of the National Academy of Sciences.

They found, for instance, that one set of microbes is associated with chardonnay musts from the Napa Valley, another set with those of a must in Central Valley and a third grouping with musts from Sonoma. They noticed a similarly distinctive pattern of microbes in cabernet sauvignon musts from the north San Joaquin Valley, the Central Coast, Sonoma and Napa.

The discovery of stable but differing patterns of microbial communities from one region’s vineyards to another means that microbes could explain, at least in part, why one region’s zinfandel, say, tastes different from another’s. The links between microbes and wine-growing regions “provide compelling support for the role of grape-surface microbial communities in regional wine characteristics,” the researchers conclude.

“The reason I love this study is that it starts to walk down a path to something we could actually measure,” Dr. Mills said. “There are high-end courses on terroir, which I think are bunk. Someone has to prove that something about terroir makes it to the bottle, and no one has done that yet.”

Microbes are deposited on the grape surface by wind, insects and people, and may fail or flourish because of specific local conditions such as the way the grape vines are trained. And there may be genetic affinities between particular microbial species and each variety of grape, the researchers say.

Even if Napa’s chardonnay grapes, say, carry a distinctive pattern of fungal and bacterial species, the Davis scientists need still to prove that these microbes affect the quality of the wine. Microbes could exert an influence both during the lifetime of the grape and during fermentation, when they may add particular ingredients to the wine. “We will look at how overall microbial communities correlate with quality traits in the wine, and whether you can predict quality from the microbes present,” Mr. Bokulich said.

Thomas Henick-Kling, a professor of oenology at Washington State University, said it was plausible that microbes are a component of terroir. “Unripe grapes taste the same the world over,” he said. It is known that single strains of yeast can have a strong effect on a varietal’s flavor, he continued, “so it’s likely that microbes play a larger role than presently known and are probably a part of the regional differences that we recognize.”

While Dr. Mills said that “I make fun of terroir all the time,” he believes that regional distinctions between vineyards do exist and that microbes have a role in creating them. If the specific links between microbes and the sensory properties of wine can be identified, growers will be able to take a savoir-faire attitude to terroir instead of a je ne sais quoi shrug.

On the other hand, he added, pinning the qualities of wine on bacteria and fungi may spoil that frisson of enchantment for some connoisseurs. “Many people don’t want this figured out,” he said, “because it demystifies the wonderful mystery of wine.”

 

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