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Mechanisms of terroir

As more new world producers start to take an interest in terroir, scientists are turning their attention to defining it and explaining how it affects a wine. Jamie Goode investigates. (This feature was originally published in Harpers Weekly, September 12th 2003)

‘Terroir’ is currently an in vogue concept. Once almost exclusively the preserve of the old world, it’s now a talking point in the new world, too. The traditional, old world definition of terroir is quite a tricky one to tie down, but it can probably best be summed-up as the possession by a wine of a sense of place, or ‘somewhereness’. That is, a wine from a particular patch of ground expresses characteristics related to the physical environment in which the grapes are grown. But why the sea-change in the new world, where for so longer the job of grape growing was seen merely as a mundane prelude to the work of the all powerful winemaker?

Even fairly recently, the new world response to ‘terroir’ was that it was a last ditch marketing ploy by European winegrowers who were panicking about their increasing loss of market share. This turn-around is because new world wine growers have realized that one of the keys to wine quality is starting with grapes that show homogeneous (even) levels of ripeness, and the recognition of the role that natural variation within and between their vineyards plays. With the increasing adoption of technique known as precision viticulture, vineyards are commonly broken up into sub-plots sharing similar characteristics (known as natural or basic terroir units) so that vineyard interventions can be precisely targeted to where they are needed. But you don’t have to look too far below the surface to see that there are subtle but important differences between old world and new world notions of terroir. Speaking generally, in the old world terroirists aim to make wines that express the typicity of the specific vineyard site, whereas in the more pragmatic new world, understanding terroir is seen as a route to improved quality.    

A taste of the soil?
The notion of terroir is fundamental to the wine industries of old world countries such as France, Italy and Germany. It’s a philosophical framework within which wine growers work. Local wine laws are built around the concepts of appellations, which lend official sanction to the idea that a combination of certain vineyard sites and grape varieties creates unique wines that faithfully express their geographical origins. 

Correspondingly, many old world growers feel they have a duty to make wines faithful to the vineyard sites they are working with. These growers will commonly make associations between properties of the wines and the soil types the grapes are grown on. In some cases these putative associations are quite specific: people will talk about mineral characters in wines and associate them with the minerals in the vineyard, taken up by the roots of the vines.

There’s even a French expression, goût de terroir, that is used to describe this. Do chalk, flint or slate soils impart chalky, flinty or slate-like characters to wine? As a scientist who has a working knowledge of plant physiology, I find this notion, which I call the ‘literalist’ theory of terroir, implausible. Yet I can’t get away from the fact that an overwhelming majority of the world’s most compelling and complex wines are made by people who hold the notion of terroir as being critical to wine quality. 

Thus the goal of this feature is to explore the mechanisms of terroir, focusing specifically on soils. Just how do soils affect wine quality? Is it a direct or indirect relationship? What are the scientific explanations for terroir effects?

Randall’s Rocks
In fact, the inspiration for this article came from a characteristically wacky experiment conducted by Randall Grahm, of California’s Bonny Doon. Even though he’s from California, Grahm is actually one of the most eloquent proponents of terroir, and he had the unusual idea of bypassing the vine and adding rocks directly to wine, to investigate their influence on flavour. 

‘Our experiments were incredibly simplistic and gross in comparison to the very subtle chemistry that occurs in mineral extraction in real soils,’ explains Grahm. ‘We simply took interesting rocks, washed them very well, smashed them up and immersed them in a barrel of wine for a certain period of time, until we felt that the wine had extracted some interesting flavours and we were able to discern significant differences between the various types.’ 

Surely, this is taking the literalist notion of terroir – that flavour compounds are translocated from the soil to the developing grapes through the vine roots – to its extreme. Grahm continues, ‘We initially screened a number of different rocks with bench trials and ultimate decided on a few for larger scale experimentation - rip rap (granite), Noyo cobble stone, black slate and Pami pebbles.  We certainly took the extraction way too far as we ended up seeing pH rises of 0.5–0.7 units, i.e. from 3.5 to 4.1 or 4.2, which is clearly beyond the pale for most wines.  Obviously the big pH shift and the lowering of the acidity caused major changes in the texture and mouthfeel of the wine, but we also observed dramatic differences in aromatics, length and persistence of flavour.’ 

Grahm is convinced of the importance of mineral flavours in wine. ‘In every case, low doses of minerals (before we really overdid it), added far more complexity and greater persistence on the palate.’ His view is that minerality in wines has important implications for wine quality. ‘It is my personal belief that wines that are richer in minerals just present way differently. I believe that in general mineral-rich wines there is a suppression of obvious fruit. What is most striking to me though about wines that have higher levels of minerals is that they seem to have a certain sort of nucleus or density around their centre, they are gathered, focused, cohered the way a laser coheres light. It is a different kind of density relative to tannic density, somehow deeper in the wine than the tannins.  In any event, I am utterly convinced that minerality is the one true key to ageability in wines and that everything else—tannin, acidity, sulphur dioxide—plays a far more secondary role.’ 

It is unlikely that adding rocks to wines will ever become a common winemaking manipulation, though. Grahm’s rock-infused wines ran into some trouble with the regulatory body in California who were finding elevated levels of nickel and antimony, among other things, so it looks like this experiment won’t be repeated.

Scientific views of terroir
While in some circles it is quite common to hear such literalist explanations of terroir, they are treated with a degree of incredulity by many new world viticulturalists. I asked viticultural guru Dr Richard Smart what he thought of popular notions of terroir which propose direct translocation of flavour molecules from the soil to the grapes, and hence the wine. ‘This is an absolute nonsense’, he replied. ‘I have never heard this, yet you say it is popular. Who on earth postulated this?’ 

Dawid Saayman, a South African viticultural expert known for his work on terroir, adds that, ‘I don’t believe that the minerals taken up by the vine can register as minerality in the wines. Minerality appears to me to be more the result of absence of fruitiness.’ But it’s pretty much a given that wines that made from grapes differing only in the soil in which they were grown taste different. 

So just what is the scientific explanation for these terroir effects? It is an important question, because providing a sound scientific footing for terroir is a worthy cause. Not only will it lend credibility to the concept in the eyes of sceptics, but it will also help the already converted understand and therefore better utlilize terroir effects.

If we are going to frame terroir in scientific language, then we’ll need to start with some plant physiology. The miracle of the plant kingdom is that these complex organisms build themselves from virtually nothing: all a plant needs to grow is some water, sunlight, air and a mix of trace elements and nutrients. All the complex structure and chemistry of an oak tree, a daffodil or a grapevine is fashioned from these very basic starting ingredients. What do vine roots take up from the soil? Primarily water, along with dissolved mineral ions. 

It seems implausible that such a complex structure as a vine is created from virtually nothing by photosynthesis – the capture of light by specialized organelles called chloroplasts, which turn light energy into chemical energy that the plant can use—but that’s the way it is. As Richard Smart emphasizes, ‘All flavour compounds are synthesized in the vine, made from organic molecules derived from photosynthesis ultimately, and inorganic ions taken up from the soil.’ 

Professor Jean-Claude Davidian of the Ecole Nationale Supérieure Agronomique in Montpellier, echoes these sentiments. ‘Nobody has been objectively able to show any links between the soil mineral composition and the flavour or fragrance of the wines’, he says. Davidian adds that ‘those who claim to have shown these links are not scientifically reliable’.

on to part two...

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