The Pseudo-Science of Investigating Terroir (and Why Some Reports May Prove the Opposite of What They Think)

I am fed up with reading reports in the press that scientists have shown a role for microorganisms in terroir. I don’t know which is worse: the lack of logical analysis in the original scientific papers, or the uncritical acceptance of the conclusions by the press.

The running defect in all these papers is a misunderstanding of the meaning of terroir. The concept of terroir is simple enough: fruit grown in one place will have consistently different characteristics from fruit grown in another place. In the case of grapes, this translates to differences in wines according to their origins. But a crucial feature is consistency: it does not prove a basis for terroir to show a difference between grapes from different sources, but that difference has to be persist over multiple vintages.

Whether local microbial populations – in particular yeast, which are responsible for creating most of the flavors of wine during fermentation – contribute to terroir is the subject of paper just published by a group led by Sarah Knight at the University of Auckland.[i] It goes so far as to put “evidence for a microbial aspect to terroir” in the title. But the work is fatally flawed.

The approach superficially seems controlled: yeast were collected from different vineyards and then used to ferment examples of the same batch of sterilized juice from Sauvignon Blanc grapes. Differences in the resulting wine must be due to the yeasts. The analysis rests upon previous work from the same group that showed genetic differences in S. cerevisiae populations in different locations in New Zealand.[ii] Ignoring the fact that only 4 yeast genotypes were found on grapes (potentially able to affect the properties of wine) while the remaining 21 genotypes were limited to soil, this makes the unsurprising point that there are variations in natural yeast populations.

Each isolated yeast population gave wine with a different profile of volatile compounds. Talking about reinventing the wheel! There have been too many demonstrations to count that different cultivars of yeast can have profound effects on the character of wine. Indeed, this is the basis for a sizeable industry in which yeasts selected to emphasize (or de-emphasize) particular aspects of wine character are available commercially. It’s almost trivial to choose yeast to bring out herbaceous character in Sauvignon Blanc, for example.

The logical error in the paper is to conclude that the observations demonstrate any role for microbes in terroir. Taking a single snapshot entirely misses the point, because the population of yeast may be quite different the next year. According to Ribéreau-Gayon’s authoritative book on viticulture and vinification: “In a given vineyard, spontaneous fermentation is not systematically carried out by the same strains each year; strain specificity does not exist and therefore does not participate in vineyard characteristics. Ecological observations do not confirm the notion of a vineyard-specific yeast.”[iii] Surely it behooves any new work to explain why this wrong?

A couple of years ago, a group led by Dr. David Mills at the University of California showed directly by DNA sequencing that different microbes were present on the skins of grapes in different vineyards.[iv] That work was fatally flawed because most of the microorganisms were what are known as spoilage organisms, and are probably not part of useful fermentation (see The Answer to Terroir Does Not Lie on the Skin).

After that, a group led by Régis D. Gougeon at the University of Dijon sampled two vineyards in Burgundy and claimed that they could find differences in both grapes and wine, although differences between vintages were more significant than differences between vineyards. I reckon their samples were too small to be significant because they analyzed only 100 berries from each vineyard, but anyway it’s interesting that vintage was a bigger effect than origin (see Will people please stop trying to prove terroir exists. It’s more useful to look for gold at the end of the rainbow).

These studies are naively touted in the press as showing the involvement of microbes in terroir, but to date there is really no evidence at all: in fact, if the microbes vary significantly from year to year, they may dilute the effect of terroir rather than contributing to it. One could make a career of debunking these studies, which could provide a really good exercise for students of science in how to misinterpret uncontrolled studies.

References

[i] Sarah Knight, Steffen Klaere, Bruno Fedrizzi & Matthew R. Goddard.Regional microbial signatures positively correlate with differential wine phenotypes: evidence for a microbial aspect to terroir. Nature Scientific Reports 5, 2015, doi:10.1038/srep14233.

[ii] Sarah Knight and Matthew R Goddard. Quantifying separation and similarity in a Saccharomyces cerevisiae metapopulation. (The ISME Journal (2015) 9, 361–370; doi:10.1038/ismej.2014.132; published online 25 July 2014).

[iii] Pascal Ribéreau-Gayon et al., The Handbook of Enology, Volume 1, The Microbiology of Wine and Vinifications, 2nd edition (John Wiley & Sons, New York, 2000), p. 46.

[iv] Nicholas A. Bokulich, John H. Thorngate, Paul M. Richardson, and David A. Mills. Microbial biogeography of wine grapes is conditioned by cultivar, vintage, and climate (Proc. Nat. Acad. Sci. USA doi/10.1073/pnas.1317377110).

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The Answer to Terroir Does Not Lie on the Skin

The most common descriptor associated with terroir is “undefinable”—it’s attributed to a mix of influences including soil, exposure, and climate that create distinctive character in the wine coming from different vineyard sites. At its simplest extreme, it is scarcely rocket science to accept that grapes (or for that matter any other crop) will be quite different if grown in a sunny, well drained spot at the top of a hill from those grown in a shady, waterlogged spot at the bottom of the same hill. That reductio ad absurdum disguises the fact that there can be vineyard sites which to all appearances are identical in all the parameters you might think distinguish them, but which nonetheless consistently produce different wines. No one to date has been able to explain this mystery in any rational way.

A large part of the mystique of wine would disappear if we could explain terroir on a scientific basis, but there is actually little threat from the latest attempt, a report in the Proceedings of the National Academy of Sciences from a group at the University of California, Davis headed by Dr. David Mills. The basic finding is that microorganisms on the skins of the grapes are different in various vineyards in California. These observations were made possible by the ability of rapid DNA sequencing to identify large numbers of microorganisms without the need actually to culture them.

So vineyards in different places have different sets of bacteria and fungi, very likely related to both region and climate. Wind, temperature, and humidity were identified as pertinent factors. This is scarcely a major surprise. But that’s a far cry from supporting the conclusion that “these differences may help explain regional patterns in wine chemicosensory properties,” which is quasi-scientific speak for arguing they might be involved in terroir.

For these microorganisms to be involved in determining wine character, they would most likely need to play some role during fermentation (their mass could scarcely be sufficient to provide any significant level of flavor components in themselves). Well, here is a problem. A significant proportion of the microorganisms that were detected are so-called spoilage organisms, which if allowed to act on the grapes in fact spoil the wine. Precautions are taken to prevent this (most typically the addition of sulfur to block bacterial or fungal action before fermentation).

In fact, wine producers divide into two groups on the issue of the role of yeasts in fermentation. Probably most producers in California are in the group who kill off all the indigenous yeasts before fermentation; cultured yeasts are then added. This provides control of the process and prevents spoilage organisms from acting, making it quite unlikely that any yeasts or bacteria on grape skins are involved in determining wine character.

Some producers allow fermentation to be catalyzed by indigenous yeasts. But here is an interesting point. In spite of a longstanding general belief that this is part of the properties of the vineyard (which makes the present article appear somewhat less than novel), it appears that the yeasts that actually catalyze fermentation are different every year. Eminent oenologist Ribereau-Gayon said, “In a given vineyard, spontaneous fermentation is not systematically carried out by the same strains each year; strain specificity does not exist and therefore does not participate in vineyard characteristics.” So it is hard for this to explain terroir. Curiously, the present paper finds that there is greater vintage variation between small vineyards than between wider areas (but this is based on a comparison of only two years). To make a case that microorganisms are involved in terroir, wouldn’t you need to show that there is some consistency in them over substantial periods of time?

The microorganisms also differ from regards to grape variety, again not a surprise, as grapes with thin skins (more easily damaged) are likely to attract different microorganisms from tougher grapes with thicker skins. This goes back to the point that many of the microorganisms are in the spoilage class, and the last thing you want is for them to affect wine character.

My bet would be that if microorganisms are involved in terroir, it would be more likely to be those in the soil that act on the roots of the grapevines, and which might therefore indirectly affect the properties of berries as they develop. Maybe the answer lies in the soil.

Reference: http://www.pnas.org/cgi/doi/10.1073/pnas.1317377110