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  • Manny Crisostomo / Sacramento Bee file

    Vineyards at the Castello di Amorosa, a winery in Calistoga, have, like all vineyards, their own unique “terroir,” says wine lore.

  • UC Davis professor of viticulture and enology David Mills.

Wine terroir goes under the microscope at UC Davis

Published: Sunday, Jun. 1, 2014 - 12:00 am

UC Davis professor of viticulture and enology David Mills knew that he was firing a shot across the bow when he recently presented a scientific paper addressing one of wine-making’s most beloved mysteries.

That mystery is the somewhat ineffable concept known as “terroir” – a French word with no English corollary – defined as a wine’s unique growing environment that contributes to its distinct aroma and flavor. For many wine experts, terroir is the elusive force that gives a wine its personality. It’s why a cabernet sauvignon from Bordeaux tastes different from one produced in the foothills.

“Questioning this subject has sommeliers questioning their expertise,” Mills said. “In a sense, you are getting at the heart of their job.”

Traditionally, the explanation of terroir’s influence primarily has focused on weather patterns, geography and cultivation techniques, and soil composition. For example, attributes such as “chalkiness” or “minerality” in wine are often attributed to soil, despite a lack of scientific evidence.

Mills, however, reported that unique colonies of yeast, fungus and bacteria on the surface of wine grapes also could be significant in determining a wine’s regional quality.

Terroir gets top billing when it comes to French wine, whether it’s a bottle from the regions of Bordeaux, Burgundy or Champagne. In the United States, wines are marketed by specific grapes. Most people don’t say at a wine bar, “I’d like a Sierra foothills red and a Russian River white.” They ask for a zinfandel and chardonnay. Nevertheless, a wine’s designation (e.g., Napa) matters a great deal.

When it comes to terroir, microbes haven’t always been part of the conversation. However, the field of microbial research has been exploding, thanks to scientific advances, and microbes are being newly credited for everything from influencing human metabolism to the quality of wine.

Mills’ study used a technique that allows for the massive amplification of short sequences of DNA, which enabled him to conduct sensitive statistical analysis on grape surfaces.

He and Nicholas Bokulich, a UCD graduate student, analyzed 273 samples of zinfandel, cabernet and chardonnay musts (the skins, seeds and stems from mashed grapes) from Napa, the Central Coast and Sonoma. The results presented evidence that there were non-random patterns to the composition of yeast and bacteria on the surface of the grapes, depending on where they were grown.

For instance, grapes grown in Sonoma had a similar “microbiome” as other grapes grown in Sonoma, and grapes grown in that area in 2010 were similar to those grown in 2012. That connection between specific regions and consistent patterns provides “compelling support for the role of grape-surface microbial communities in regional wine characteristics,” the report stated.

Released in November, Mills’ and Bokulich’s research made international headlines with coverage from outlets such as the BBC and The New York Times. Mills was quoted in T he Times article as saying: “There are high-end courses on terroir, which I think are bunk. ... I make fun of terroir all the time.”

The coverage created something of a stir within segments of the wine-making community, which can be protective of the tradition of terroir. Some in scientific circles also took issue, arguing that mainstream outlets oversimplified the study and overstated the connection between microbes and their potential affect on wine’s sensory properties.

Reached by phone in Taipei, Taiwan, where he had been invited to give a talk on his research, Mills said the “make fun” remark was taken out of context.

“It’s not that I don’t believe in terroir,” he said. “It’s that you have to measure it. ... Winemakers speak of it as a given, but it doesn’t have a scientific basis and we need to do a better job to prove it.”

Joe Vaccaro, chief operating officer and wine director for the Selland Restaurant Group, said he has no qualms about scientific research into terroir, which “encompasses so much. Soil, climate drainage, sun, and even … native yeasts and bugs and things that fly around and land on the grape skins. I don’t see how those could be removed from the terroir equation.”

Dana Stemmler, an associate winemaker at Clarksburg’s Bogle Vineyards who heard Mills speak on the topic at an industry event last year, also said she’s interested in knowing more about the microbes.

“I think potentially if they could map out very specific compounds these microbes are producing, and whether they have a positive or negative effect on the wine, we could use it to survey the growers we have contracts with and find the more favorable fruit to purchase,” said Stemmler, whose employer also owns estate vineyards.

Mills is working to correlate the microorganisms in the wine must with the wine’s chemical composition. After that, he will move on to testing how microbes influence the flavor profile of the wine.

The lack of the latter was a frequent criticism of the study, but Mills said that each component must be investigated one careful step at a time. What’s clear is that many await his findings.

“At this point, asking me whether I believe in terroir is like asking me if I believe in God – it’s very difficult to prove or disprove,” he said. “But people believe in it fervently.”

Read more articles by Becky Grunewald

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