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).

Experiments at Chateau Margaux: biodynamic, organic, and conventional viticulture

Far from the stuffy reputation for sticking to tradition, Chateau Margaux has one of the most active experimental programs in the world of wine. Paul Pontallier presented the results of some of these experiments in a seminar in New York this week. “I believe in doubt,” he says, explaining that he thinks viticulture and vinification should be based on knowledge gained from testing situations rather than on unsubstantiated beliefs.

The first experiment was a comparison between wines made in the 2012 and 2011 vintages from vines that had been cultivated conventionally, organically, and by biodynamics. This experiment started 5-6 years ago with a 2 ha plot—unfortunately not one of the best, says Paul—and is going to be extended to a slightly larger, more homogeneous, plot next year. The plot is divided into groups of rows that are cultivated with different methods, and every effort is made to stop treatments from spreading into the other rows. There’s more than one separate block of each type in order to minimize soil effects.

I have always been a skeptic about the effects of different types of viticulture. It seems obvious that organic viticulture is better for the environment than conventional treatments with herbicides and pesticides, but it does not seem axiomatic that it will necessarily produce fruit of better quality. Whether biodynamic treatments add anything to organic cultivation has always seemed rather doubtful to me. One problem is that no one has tested the effects in any sort of controlled way, and you might well argue that many of the well known organic or biodynamic wines are better than conventional wines simply because the producers are more skilled at what they do. So this was a very rare opportunity to see whether wines made under exactly the same conditions, but from grapes cultivated in different ways, show any differences.

The wines were tasted blind: all we knew was that the first three were from 2012 and the second three were from 2011. The immediate surprise was that in each group two wines were closely similar and the third was distinctly different. The two similar wines shared brighter fruits and acidity, more sense of aromatic uplift, more presence on the finish: in each flight the other wine had a slightly flatter profile with less finesse. My assumption that the last wine must be the result of conventional viticulture turned out to be correct. I had not expected such a striking demonstration of the advantages of organic viticulture,  but I feel the results were completely convincing.

The differences between organic and biodynamic examples were much narrower: in 2012 I had a slight preference for the organic wine, whereas in 2011 I had a very slight preference for the biodynamic wine. The differences were slight enough that I would not have argued if I had been told they were different bottles from the same lot.

Paul Pontallier says that to date they have found no objective differences in grapes or wines from the different treatments; and soil measurements this year suggested that if anything the conventional soils have more diversity. One of the most stunning aspects of the comparison, it seems to me, is that a clear difference should be evident between conventional and organic/biodynamic in only five years, given that it takes at least three years for a vineyard to be converted. Many producers whom I’ve asked about the effects of conversion say that the most significant difference appeared after something closer to a decade, so it will be fascinating to see whether these differences are sustained and broaden in the future.