Arguing “microbial terroir” from microbe to metabolite

Short: New microbial terroir research provides even more evidence that local differences in yeast and bacteria associated with a vineyard make a difference to wine quality.

Longer:

Bokulich and Mills of UC Davis* have published a series of papers about the communities of yeast and bacteria that live in various winemaking-associated places and arguing for why differences in those communities – usually differences in place; sometimes differences in time – matter to wine as a finished product. Some of their past work outlines distinctive microbial communities around California and connects those communities to climate at a regional level. Their newest publication, out this week, tries to show microbial distinctiveness at the level of near-neighbor wineries and to connect those microbial profiles to wine composition.**

The article is open-access on MBio but, to be honest, it’s a challenging read if you’re interested in following the methods. The study at first seems to involve a two-way comparison between Far Niente and Nickel and Nickel, both in Oakville, California in the Napa Valley. But both of those wineries pulls grapes from multiple vineyards distributed around Napa and Sonoma, and so the comparison is actually among 13 chardonnay and 27 cabernet sauvignon wines, from different vineyard locations, fermented individually in one of those two wineries.

The first part of the study is about showing that grape musts from each of these vineyard sites have unique microbiomes (including bacteria and yeast populations), though their sample size is too small in this case to convincingly argue for a microbial basis to Napa-Sonoma distinctiveness at the AVA level. Unsurprisingly, the diversity of the microbial population and its distinctiveness decreased as fermentation progressed.

The second part of the study is about connecting initial microbial distinctiveness observed in grape musts to wine composition. The authors – and a bunch of statistics – drew some specific connections between the presence of specific metabolites (i.e. chemical compounds created during fermentation by microbial metabolism) with important sensory implications and the presence of specific microbes. On that basis, they argue (they = the authors and the statistics) that they “demonstrate that the microbial composition of grapes accurately predicts the chemical composition of wines made from these grapes and are therefore biomarkers for predicting wine metabolite composition.” That’s true in the sense that they have – in these wines, in this place – created data that mean that identifying X microbe in a must predicts finding Y chemical in a wine, for a limited number of Xs and Ys. It remains a pretty strong way of making the statement.

To be fair, the authors initial frame this as a proof of concept study. Do geographically neighboring vineyards have different microbiologies that matter to wine chemistry? Yes, in these cases, they do. More can be done to substantiate that point, and to follow up on any number of the other questions this paper raises about what some of the microbes linked to specific wine metabolites but with unknown roles in fermentation are actually doing (if they are, in fact, doing anything at all rather than serving as a marker for something else) to make that link happen.

An important note: the framing of this paper, and some others dealing with microbial terroir, can suggest the idea that terroir is quantifiable, reducible to measurable differences in straight-forward wine chemistry. That’s balderdash. Terroir is about regional character. Quantifiable chemical differences are absolutely part of that character, but so is the human history of a place, the character of the people who live there and who make the wine, and the stories that come along with it. Some of those more nebulous influences surely do translate into chemical differences, but not all of them, or at least not all of them in ways contemporary science has an easy time handling. I have no trouble believing that the stories we tell about the wine we’re drinking produce neurochemical changes that affect the sensorineural mechanics of taste perception and that effectually alter the flavor of the wine. Someday soon our sciences may be sophisticated enough to measure those changes. Someday further away, maybe our sciences will be sophisticated enough not to imagine that those measurements explain away why stories are important, too.

 

*Bokulich has newly moved to Northern Arizona University per the UC Davis press release, though that move is so new that he doesn’t yet seem to have a web presence at his new institutional home.

**Bokulich and Mills’ work is an interesting complement, along different lines, to the microbial terroir work Dr. Matthew Goddard’s group is doing to understand connections amongst regional populations of Saccharomyces cerevisiae, something he presented at the recent International Cool Climate Wine Symposium in Brighton and about which I’ll be writing elsewhere.

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