In parts 1 and 2 of this exploration of soils and wines, I looked at the issue of minerality. Do soils influence wines directly by means of root uptake of ‘minerals’ that find their way into the wine? Science suggests this is not the case, but there is a possibility that root mineral ion uptake could be having some effect, direct or indirect, on wine character.
What about a chemical explanation of minerality? That’s what we’ll explore here.
Some people use ‘mineral’ to describe aromas of wine; it’s something they get on the nose. In this case, it could be that tasters are ascribing minerality to what is in reality the presence of certain volatile sulfur compounds in the wine, also known by the term ‘reduction’. In its rawest state, reduction is caused by hydrogen sulfide, and smells of rotten eggs and sewers. This is rare in a finished wine, and wouldn’t be classed as mineral. Far more common is the presence of complex sulfides and mercaptans (also known as thiols). These sulfur compounds, like hydrogen sulfide, are produced largely by yeasts during fermentation. Their expression depends on their concentration and the context of the wine, but in some cases they can give a flinty or struck-match aroma that can be quite ‘mineral’.
There is good reason to suggest that flintiness in white wines is a result of some low level reduction. Great white Burgundies frequently show a little of this good reduction: a matchstick element to the nose is complexing. Some new world Chardonnay producers are now beginning to work out how to achieve this through winemaking practices. There’s also a link here with terroir: some sites naturally have nutrient deficiencies that can stress the yeasts a bit and cause them to produce more of these volatile sulfur compounds.
It’s tempting to assume, in the absence of good solid data, that reduction is responsible for quite a bit of minerality in wine. But it is unlikely to be the whole story, and many advocates of minerality would prefer this not to be referred to as such. ‘I believe most people confuse the “mineral” flavours like petrol, reductive notes, flint and stony aromas with minerality,’ says Olivier Humbrecht. ‘As such, it is impossible to smell minerals!’
But scientists tend to prefer this second definition of minerality because they dispute the first – the more literal definition in which minerals in the soil end up flavouring the wine. This tends to make some believers in minerality act a little defensively when discussing the subject. ‘I fully understand that when I use the term it may have no scientific validity,’ says Jasper Morris, wine merchant and Burgundy expert. ‘I have been told by enough geologists that you shouldn’t call Chablis flinty, because flint is not soluble in water and therefore it can’t have a taste. But is an image.’ Morris illustrates his point by comparing two Burgundy vintages. ‘I would use two vintages of white Burgundy to illustrate this point: 2007 and 2008. The acidity is higher in 2008, and when I taste those wines I feel that they are acid and not especially mineral. In 2007, the acidity is a little lower, but I find the wines distinctly mineral. By that, I mean that they have a fresh zingy zest to them that in my mind this puts up the single word mineral. In 2006 the wines are fat, rich and round, and some of them have enough acidity to provide balance but you don’t feel mineral when you taste the wines.’ He continues, ‘also in Burgundy, in terms of the hillsides you expect minerality in those soils which have more active limestone, and you expect less in those soils which are more clay based. For example, with any vineyard in any village which includes the words “charmes” in the name, you rarely get the concept of minerality.’ Morris is frustrated that the scientists seem to be criticising minerality, without offering an alternative explanation: ‘Scientific fundamentalists are denying that we get it right rather than offering any certainty in the other direction.’
Paul Draper of Ridge in California is a winegrower who believes in minerality from the soil, even in the face of questioning from scientists. ‘Though I am well aware of what soil scientists say about minerals or other elements in the soil and the impossibility of their traveling through the vine and into the wine, the roots deep enough into those minerals are effected and the wine shows that effect,’ he states. ‘I think of minerality as a wet stone quality in a wine. Our subsoils at Monte Bello are limestone and at times are at the surface or a meter below. In other places our backhoe pits find them several meters down. Perhaps 70% of our vine roots are deep in the limestone. I have seen minerality in some shales as well so I don’t think the effect is necessarily limited to limestone. We see the most marked minerality (crushed rock, perhaps flint are other descriptors) is in our more eroded blocks where the limestone is closer to the surface. In the youngest blocks where pits have
shown considerable limestone we don’t see the minerality as yet but expect to when the roots
Minerality remains enigmatic. As we begin to understand more about it, the picture seems multifactorial, with different mechanistic underpinnings for what wine tasters describe as mineral in their tasting notes. From a personal viewpoint, I used to favour the more established scientific viewpoint, assuming that much of minerality could be explained by volatile sulfur compounds. But I’m increasingly drawn to the idea that minerals in wine, derived from the soil, could be affecting wine flavour in interesting ways—and, in particular, helping to create long-lived compelling white wines. It’s a subject that deserves more attention.
In part 4, we’ll look at the interaction between plant roots and soils in more detail, and explore the strong evidence for ways in which roots might be affecting grape quality.