Minerality in wine: Where are we now?

It's a term that has long confounded and fascinated us, but are scientists any closer in understanding what causes us to perceive these characteristics in a wine’s aroma and flavour? Professor Alex Maltman gives his expert insight

Another article about minerality… It was more than 15 years ago that I first wrote a piece on this topic and I’ve been asked to do quite a number since then, as have countless other authors. There have been conferences, masterclasses and workshops devoted to minerality in wine, and still they keep on coming. So what is it about this subject that continues to fascinate?

Who knows, but maybe it’s to do with a melding of the pragmatic usefulness of the word – evocative of the much-loved link between wine and the land – with the continuing lack of consensus on what the term actually means.

That the word is attractive is clear, and not only with wine: minerality is now reported from beef, tea, watercress, maple sugar, milk, oysters, marijuana… So where are we now, with understanding the perception in some wines that we call minerality?

Recognising and reporting minerality

First, how do we sense it? It varies. In one recent study, 20% of wine professionals detected minerality in Chablis wines by taste and 16% by smell, and the remainder used both senses. Interestingly, all three groups varied markedly in their assessments of the intensity of the minerality, and they described it differently.

For instance, in the smell-only condition about two-thirds correlated minerality with gunflint and reductive aromas, and lack of fruit, while (when nose-clipped) about the same proportion related minerality to acidity and bitterness. Another investigation concluded that French tasters relied more on smell than those from New Zealand, who tended to use both nose and palate.

Other studies have noted that where both smell and taste are used the acidity of the wine is important, whereas with smell alone the particular varietal becomes more significant. So, which grapes tend to show minerality?

Investigations at Spain’s University of La Rioja of red wines made from Tempranillo, Syrah and Grenache found weak responses and inconsistent patterns; minerality is mostly associated with white wines, though there is disagreement on the particular varietals.

In a study at California’s UC Davis, Riesling, Pinot Gris and Sauvignon Blanc wines were judged to be more mineral than Chardonnay wines. (It’s worth noting that the latter included Chablis, for some the archetypical mineral wine.)

Of course, the majority of vines are grafted, and it’s actually the rootstock rather than the cultivar that interacts with the soil. I’m unaware of any study that assesses the role of rootstocks in minerality.

Sensing something you’re going to call minerality is just the start: how are you going to convey to others what you mean?

Several studies have grouped the words people use to express minerality into categories, for example acidity, tenseness and freshness; seashore-related things such as iodine, saltiness and shellfish; and stone-related sensations including wet or hot stone, flint and chalk.

In one report, winemakers distinctly preferred using terms that invoked place and soil, and they regarded minerality positively, unlike the vaguer and negative connotations indicated by some consumers.

Another illustration of the communication problem is the different word associations reported by research groups at Lincoln University in New Zealand and at California’s UC Davis. While both teams noted positive correlations between minerality and words like citrus, fresh, zingy, flinty, and smoky, the Lincoln researchers differed from those at Davis in finding no correspondence with acidity or reductive notes.

One study showed that minerality represented different things to Swiss and French wine consumers, and that the Swiss group used a markedly broader vocabulary. Another experiment showed that the vocabulary used to convey minerality depended on the level of knowledge – ranging from the most inexperienced respondents never having heard of minerality, through comparisons with bottled water (especially popular, for whatever reason, with female tasters), to the importance to experts of acidity, terroir and earthy flavours.

See also: Tasting notes decoded – Understanding wine aromas

What is minerality in wine due to?

In view of the inconsistencies outlined above, it is no surprise that science continues to have difficulty identifying what in a wine might be triggering the perception that we label ‘minerality’. Most studies have focussed on the possible roles of acidity, reductive sulphur compounds and the lack of fruit.

Regarding acidity, one of the earliest published studies, on Italian Rieslings and Grüner Veltliners, implicated the weak, slightly saline-tasting succinic acid but that suggestion has never been substantiated.

A later investigation, from UC Davis, reported that professional tasters found minerality in wines with greater malic and tartaric acidity and, to a lesser extent, free and total sulphur dioxide. However, a New Zealand study, while supporting a role for sulphur dioxide, found no correlation between acidity and perceived minerality, nor with reductive notes.

Several studies, though, have reported a relevance for sulphur compounds, such as various methanethiols and polysulphanes. In their selfless quest for a better understanding of the world around us, a Swiss team was studying toilet malodours – yes, that’s right: toilet malodours – when they accidentally isolated ‘a flint-like odour’, and showed it was due to hydrogen disulphane or HSSH. They then found that in blind tastings of Swiss Chasselas wines, the two that showed greater minerality contained significantly more HSSH than the others.

Otherwise, despite investigating a comprehensive list of candidate compounds, studies to date haven’t confirmed a clear relationship between minerality and reductive phenomena.

Similarly, although tasting statistics give some support to the perception of minerality arising from a lack of other wine flavours, it has not been substantiated by chemical analysis. For example, paucity of the known key aromatic ingredients of Sauvignon Blanc wines (eg thiols and isobutyl methoxypyrazine) is not matched by greater perceived minerality.


Chablis Limestone Soil

But what about the vineyard?

So maybe minerality comes chiefly not from organic compounds produced during vinification but from the soil – exactly as the name suggests. This would nicely tally with it being related to earthiness, stones, flint, slate, chalkiness, and the rest.

There is some work that relates minerality to place, for example a study of Chablis wines from the left and right banks of the Serein river. Those from the left bank, when assessed by smell alone, showed greater minerality. On analysis, they showed more sulphur-bearing methanethiol (which has a shellfish aroma) and a lower copper content, presumably some effect of the soil there.

The authors suggested that on the right bank the more abundant copper may be reacting with the methanethiol to produce an odourless compound, and hence decreasing perceived minerality. Intriguingly, this would suggest that the grand cru sites of Chablis (all on the right bank) give less minerally wines.

Most commentators appear to have accepted the scientific arguments that minerality is not simply due to vines somehow sucking up geological minerals from the soil and transmitting them through to the finished wine for us to taste. Nevertheless, for many the concept retains strong geological connotations.

However, the earthy words linked with minerality have to be metaphors, mental associations, recollections of some past encounter involving rocks, and not the direct taste of geological things that were originally in the vineyard ground. Here’s why.

Rocks have no taste. Any stone surface exposed to the air will soon be filmed with all manner of bacteria, algae, moulds, lipids and the like, which are all around us and which produce highly aromatic vapours when warmed on a sunny day or wetted in a shower of rain.

Similarly, tilled earth, damp cellars and struck pebbles produce familiar smells – but they are not of the rocks themselves. It’s easily tested if you have access to a rock saw and some different kinds of rock. Smooth, freshly sawn surfaces will give a cool, tactile sensation on your tongue but they will have no aroma or taste. If you lick and smell them blindfolded you won’t be able to tell the rocks apart. It seems to me that talking of a ‘taste of slate’ and the like has to be involving the imagination in a constructive way – conjuring what it would be like if slate did have a taste.

Grey Slate Soil

Grey Slate Soil

The science of soil and wine taste

Vines take up dissolved chemical elements (with a positive electrical charge and hence properly called cations) from the soil, often called nutrients or just ‘minerals’. These elements are slowly unlocked from geological minerals by weathering, but in practice are largely derived from the organic material – the humus – in the top metre or less of the vineyard soil.

At depth, the subsoils and unweathered bedrock have very little nutrient availability: vines develop deep roots in order to seek supplementary water. Deep roots are a good thing for the stability of water supply to the vine, but they are not accessing something magical down there.

Similarly, stony soils are often said to promote minerality, but the stones exist because they have resisted weathering – they are inert.

Wine commentators are fond of using phrases like ‘mineral-rich soils’, and implying that this somehow gives rise to greater minerality in the wine. It certainly sounds seductive, but all rocks and soils are made of (geological) minerals, not some more than others.

And if it means rich in nutrient minerals, that’s the same as saying fertile, and it’s pretty much axiomatic in viticulture that highly fertile soils are to be avoided, as they lead to high vigour, lower grape quality and poor wine.

Chablis Fossils in Bedrock

Chablis Fossils in Bedrock

Nutrient elements are essential for the vine to grow, but their source is irrelevant. The fossil oysters of Chablis, for instance, are well known and often said to engender minerality, but the creatures are preserved because they have been replaced by a durable geological mineral, in this case calcite. Any nutrient minerals that the vine roots obtain from these fossils are indistinguishable from those from the host soils, or, for that matter, from fertiliser.

A few of these mineral nutrients may survive into the finished wine, joining those introduced during vinification, and while almost certainly they themselves can’t be tasted, at least individually, their presence can indirectly influence our taste perceptions. But such effects are complex and circuitous, very different from the notion of directly tasting minerals from the vineyard soil.

Comparisons are sometimes made between wine minerality and the taste of some bottled waters. Putting aside the fact (from time to time gleefully reported in the tabloids) that in blind tastings most people can’t tell expensive bottled waters from tap water let alone distinguish between bottled waters, here are two observations.

Most bottled water is drawn from the ground, where it has remained for long periods (the average residence time in the UK is more than a century) in direct contact with the host aquifer. So anything that’s remotely soluble is taken in by the water, unlike the regulated uptake of cations by vine roots. Consequently, the mineral concentrations in bottled water are typically greater than those found in wine, and crucially – and in contrast with wine – they can include plentiful anions (negatively charged). And these are the main inorganic contributors to flavour and mouthfeel.

There is a saying in the brewing industry that while cations in the water govern the methods, it’s the anions that give the flavour. In fact, the effects of dissolved anions on taste are particularly evident in beer.

For instance, the classic ales from England’s Burton-on-Trent have whopping bicarbonate and sulphate concentrations (the reason for the sulphury ‘Burton snatch’ beloved of beer aficionados). Similarly at Tadcaster, England, when the Samuel Smith’s brewery recently considered bottling the tasty well-water used in its ales, they had to abort the plan because the level of sulphate was above the limits allowed in bottled water.

Yet tellingly, the beer style in which minerality is normally trumpeted is lager, which ‒ classically from the Czech town of Plzeň (Pilsen) ‒ has a remarkably low mineral content.

Minerality in wine

Finally, if wine minerality was due to mineral nutrients, then it should be easy to enhance it by simply adding some more. But for various reasons it doesn’t work.

As one example, water tasters report that as the presence of cations becomes increasingly detectable, the taste becomes more and more disagreeable. And because water lacks competing flavour compounds, in wine the detection thresholds must be vastly higher and hence, presumably, even more distasteful. This doesn’t sound like the desirable perception we label ‘minerality’!

So what, then, is minerality due to? The jury is still out. But maybe if I’m still writing about minerality in another 15 years’ time I’ll be able to say.