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.