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Old stones, old wines, new theories

By |  October 28 2014

As a tasting term, minerality has come in for a bit of stick of late. A descriptor that wasn’t really used for wine until the 1980s, it has since become very popular in tasting notes, but there’s recently been a minerality backlash.

There are two main criticisms. The first is that the term is used without any real definition. Different people describe different wine characteristics as "mineral," and this has been shown in experiments by Dijon-based researcher Jordi Ballester, who has examined the use of this term by professionals. The second is that the idea that minerals are taken up by the vine and can then alter wine flavor has been criticized by scientists who say this is biologically implausible.

But I am very attached to minerality as a tasting term, and here I want to make a heretical suggestion. Could there be a correlation between wine minerality and longevity? And – even more heretically – could there be an element of causation here? Before I expand on this heresy, first some background.

The current scientific consensus is that the way that soils have their influence on wine quality is through their effects on water availability. According to this idea – made popular by the pioneering terroir studies of Seguin in Bordeaux – an ideal vineyard gives vines enough to drink, but not too much, and gives the vine a bit of water stress just at the right time. According to this view, soil chemistry is not important.

But the anecdotal experience of many winegrowers would be that in addition to regulating access to water, there’s also a contribution from soil chemistry. The type of soil matters in ways that extend beyond its water-holding capacity. Plant science would suggest that the mineral composition of soils could be having both a direct effect and also an indirect effect on wine flavor. The direct effect is through the minerals being taken up by the vine roots, ending up in the grapes, and thus ending up in the wine. It’s not likely to be a strong effect, since most mineral ions have no smell and relatively little taste, but it could be important.

Indeed, there is some evidence that soil type will influence the mineral composition of wine. In 2000, a plant researcher from Germany, Andreas Peuke, grew Riesling vines in pots containing three different soils from Franconian vineyards: loess, muschelkalk (seashell lime), and keuper. He collected sap from the vines and analyzed its chemical composition, and found differences among the different soil types. It’s not the world’s greatest ever study, but it shows proof of principle.

The indirect effect is through minerals (via their presence or absence) altering plant gene expression, thus leading to differential production of flavor compounds or their precursors in the grapes. The chemical composition of grape must can also have an important effect on the way that yeasts and bacteria carry out fermentation, altering fermentation dynamics and causing microbes to produce different levels of flavor compounds.

Where do these soil minerals come from? The bulk of soil mineral content comes from decaying organic material, not decomposed rock, and it is microbial activity in the soil that affects the ability of soil to break down organic matter into mineral ions that can be used by the plant. This is one reason that soil microlife has become such a hot topic of late.

Tasters tend to refer to three different forms of minerality in their wine descriptions. The first is minerality as an aroma. Wines that smell "mineral" tend to be white wines, and the source of this matchstick/mineral character is most likely a volatile sulfur compound produced during fermentation by yeasts. The classic matchstick/struck flint character on the nose of some white Burgundies is really attractive, and it’s now being deliberately sought by Chardonnnay makers the world over. The other two uses of the term involve minerality as a taste. The first, and least justifiable of these, is the minerality associated with high acidity in white wines. It’s very easy to reach for "mineral" as a descriptor when we come across bright, acidic whites, when really we should just say "high acid." A related condition is when a wine tastes "stony," again likely due to the acids. The second of these mineral tastes is, I think, the best use of the term, and it’s a sort of salty minerality as you might find in some mineral waters, albeit usually at a low level. It’s a textural sort of mineral quality.

So, to the heretical question: Is there a connection between a wine’s minerality and its ageability? I can’t think of a scientific mechanism to explain this, but some wines that I’d describe as more mineral seem to age much better than their less mineral peers.
This is particularly relevant to white wines. A number of times I have come across whites that have aged much better than you’d expect from the analysis. The scientific view would be that white wines are protected from oxidation by free sulfur dioxide, and that this protection is enhanced by low pH (high acidity) because more of the sulfur dioxide is in the active molecular fraction at low pH (acid itself is not protective against oxidation). As a rule of thumb, when free sulfur dioxide drops below 10 ppm, then the wine will begin to show signs of oxidation. Yet some wines don’t seem to follow this rule, and live a lot longer than you’d expect. In particular, some natural wines which have little if any sulfur dioxide present don’t oxidize as fast as you’d expect, and stay in good condition for some time after opening.

These protected whites seem to have more mineral characters in them. I visited a grower in Lake Balaton, Hungary, who showed me some of his whites. In particular, there were certain terroirs he worked in that produced wines that lived longer than wines from neighboring terroirs, even though practically everything else was the same about them. It’s observations like these that make me wonder about soil-derived mineral content conferring some sort of protective effect. The problem is that I can’t provide a plausible scientific mechanism, but this could simply be a matter of my understanding of wine chemistry being naïve and incomplete. There’s a lot of mystery to wine that has so far eluded scientific explanation.

Red wines are protected from oxidation both by the sulfur dioxide that’s added to them and also the polyphenols they contain. If minerality confers protection on whites, then it’s also likely to do it for reds. It is just less obvious in reds, because of their reduced fragility.

This association between mineral-tasting whites and longevity, should it exist at all, could be correlation, or it could actually represent a causal relationship. The correlation might be between wines with this sort of palate and higher levels of glutathione, which occurs at varying levels in white wines and confers protection against oxidation. Or it could be a mouthfeel correlation: wines with this texture may have low levels of polyphenols, and in white wines polyphenols can, rather confusingly, actually cause oxidation (in reds, there are so many they are actually protective). Some winemakers allow deliberate juice oxidation in white winemaking to get rid of the polyphenols, which if left intact, can cause the wine to oxidize faster later in the winemaking process.

This heresy aside, minerality in wines remains a very interesting topic, albeit an under-researched and complicated one.

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