The last installment of this blog (Part One) talked about minerality in wine – how it tastes and which classic examples to look for. We also mentioned the traditional idea or ‘myth’ of minerality, in which rocky minerals beneath the ground are supposed to convey flavours into the finished wine.
It is worth starting our discussion with a reflection on the appeal of this time-worn idea. It is essentially a pastoral fantasy that can be consumed in any city by the pop of a cork. Many wine lovers are very satisfied in their belief that beloved landscapes are reflected in their wine glass. But more scientific minds may detect, if you’ll excuse the term, a somewhat pathetic fallacy.
Students of the WSET wine courses are allowed to describe wines as ‘chalky’. But they may get a black mark from their teachers if they use the term ‘mineral’. The M-word is viewed by some in the establishment as either too vague or too faddish, or both. So despite a vast popular uptake, the term occupies a fascinating no-person’s-land between meme and tradition, culture and science.
At first glance, soil-derived minerality seems quite ‘scientific’. It appears supported by what we know about the conveying function of microbes in the soil. It also seems to explain, superficially, why the most complex wines are usually grown in arid, dry and free-draining soils: the less moisture and nutrients, the more vine roots need to delve deep into the subsoil geology.
But does that prove that minerality comes from below?
Contemporary science is currently descending on this question, spoiling it for those who like a heavy dash of landscape in their wine. Traditional readers may wish to stop now – your cherished ideas are about to be torn down. But for those more hardy or inquisitive, the current science won’t confirm what minerality is – but it can certainly tell you what it’s not.
If science offers a headline about minerality it may be to do with the radical ‘disconnect’ between rocks and roots, between soil and fruit. It turns out ‘geological minerals’ in the soil (rocks, basically) are totally different from ‘nutrient minerals’ appearing in wine – they are not the same minerals. One key difference is that geological minerals are almost never soluble. Rocks cannot dissolve, so they cannot enter the plant’s roots, let alone the wine.
While ‘nutrient minerals’ like Potassium and Nitrogen do occur in both soil and wine, no one holds them directly responsible for flavours – either mineral or otherwise. Minerals like those in the ground are usually absent in wine, or if they are present (by coincidence through the ferment) they appear in minute concentrations well below the perception threshold. So we might be smelling something, but it just isn’t them.
Iron may be held up as an exception – a soil nutrient also perceptible in wine. The iron-rich red clays of Western Australia and Coonawarra may impart a rusty minerality to their Cabernets. This effect is debated by some, however, and it also undercuts the strict, classical claim of minerality as a conveyed taste of rocks (not rust). If a mineral we absorb daily in food might also enter wine, that seems like no kind of magic. But the point underlines how perverse it is to suggests that insoluble rocks – with no historic place in our diet – might do the same thing.
The real kicker, however, is that rock-like compounds have no flavour at all – no so-called ‘vapour pressure’ in science-speak. Even if they could enter wine, rocks would be completely flavour-neutral and simply couldn’t convey taste impressions. So what about those startled wine-tasting students, forced to lick rocks as if they were ice creams? Yes, they are learning ‘stoney minerality’ – but the taste doesn’t come from either stones or minerals.
So what is this taste then? Another study attributes the flavour of stones to an invisible substance called petrichor. Derived from organic matter like oils from plants, petrichor floats through the air and comes to rest as a thin film over everything on the ground, including rocks. This coating then releases its flavours (vapours) only when a rain hits the ground – giving it the epithet ‘the smell of rain’. Equally, of course, these flavours are released when the warm, wet tongue of a wine student happens to make contact with a rock.
So if rocks have a distinctive taste it may be, ironically, because they are the only things in our environment that have no flavour at all – uniquely allowing us to taste only the petrichor. Believe it or not, the supposed ‘taste’ of rocks is more like an effect of the mouth tasting them.
But fans of wine folklore have been known to fight back. One enthusiastic winemaker tried to test or prove minerality by soaking various rocks in vats of wine. The results were quite positive it seemed: a genuine physical exchange was implied by a palpable difference in the wine, depending on the rocks used. But the reported effects were largely textural. Perhaps this was because, as we know, rocks have no actual flavour. (Unless you count the thin film of organic petrichor, released by liquids…hmmm). And the physics of this experiment – already so different from those of the vineyard – could not clarify whether the rocks were adding something, or rather absorbing and removing elements from the wine.
One of the main ‘rocks’ prized by vineyard-owners is flint or silica – aka silex in French. Silica is often assumed to influence flinty whites from such places such as Chablis and the Loire. One calm analysis of the Chablis landscape, however, points out that the soils have chalky lime, yes, but hardly any silica. So how could the wines taste flinty? But even if the soils did have silica, this mineral – like most of the others – is completely insoluble, and therefore unavailable to infusion into wine.
On this note, scientist Alex Maltman makes an embarrassingly obvious point about silica. While the silica trapped in soils may have no ability to ‘touch’ wines, there is something touching every wine that is absolutely full of silica – i.e. glass. This simple idea seems to disprove the silica myth in reverse. A flinty note may be present in your wine, yes, but if so then it’s a fantasy to think of some distant subsoil, rather than the glass you’re holding in your hand. Or put another way, if silica compounds could dissolve into wine then every wine served from an ordinary glass bottle would be very, very flinty. (Needless to say, they aren’t.)
It seems that flint-lovers have no story to tell us about soil. So the spotlight turns to ‘reductive’ winemaking. Reductive wines are made with too little oxygen exposure, from very airtight vessels and winemaking methods, which results in the production of sulfides (with a ‘d’). The presence of sulfides create certain salty, struck-match notes which are, in effect, the same characters that terroir-lovers call ‘flinty’. Though considered a wine fault, these reductive effects can be very slight or balanced and thus be experienced as ‘good’ or ‘positive’ sulfides – i.e. additional character and complexity. Such positive cases would be the flinty nuances of some white Burgundy. Increasingly Burgundy-styled Chardonnay from the new world employs this style as well. Local examples are Vidal’s Legacy Chardonnay – New Zealand’s most controversial wine due to its heavy sulfide ‘funk’ – or, more subtly, the excellent Escarpment Chardonnays.
Since sulfides are a result of the ferment not the soil, the science of them has sent some terroir myths teetering, or crashing. A recent loss was the supposed sulfuric minerality of wines from Mount Etna’s famous ‘volcanic’ terroir. The Wine Advocate’s Monica Larner lately dispatched this effect as nothing but a local fad for reductive winemaking – an overly zealous ‘clean up’ a region formerly known for wines that were too oxidative. Oh well, there goes another one.
It may come as no surprise that the other common form of sulfur is also involved. Sulphur dioxide or sulfites (with a ‘t’) have been shown to affect taste perceptions of minerality. Sulfites are mainly used as a preservative in wine, but studies suggest their presence may ‘tweak’ the flavour profiles of wine in a more salty or bitter direction, supporting or augmenting mineral effects. This may especially apply to mineral effects in younger wines, where sulfites have been very recently added – as in many New Zealand wines that are locally consumed. Likewise, any wine with relatively high levels of active or ‘free’ sulfites may taste yet more mineral still (see our earlier blog on wine preservatives).
Even the UK-based wine establishment may favour chemistry over terroir. Master of Wine Sam Harrop attributes all flint, wet-stone and shellfish characters to the presence of complex sulphides from the ferment. In fact, according to Harrop there are other similar earthy characters that likewise result from the ferment, not the soil.
While we’re side-stepping into earthiness, lovers of earthy reds may be disheartened to hear their beloved flavours arent the result of some sort wonderful, natural osmosis with the landscape. Rather, wines can simply end up containing the same compound that is released from tilled soil. This terpene called geosmin – the smell of earth, all too literally– is often released under the same conditions as petrichor (mentioned above), and can be perceived at incredibly low concentrations. If geosmin content is above ‘taint’ levels you may feel like taking a shower after your first glass. But if not, even a tiny presence of it gives wine a distinct whiff of earthiness. So no mystery there either.
The textural qualities of acidity are closely linked to the impression of minerality in wine. Our common perception of ‘racy, mineral whites’ has been supported by one study, which identified minerality with high acids overall – measured in threefold manner by malic acid, TA (total acidity, basically) and tartrate levels.
Dr. Damian Martin (former viticulturalist at Ara wines) agrees with the focus on acid, but claims that the minerality effect isn’t about harsher acids – as some might assume. Rather wines feel more mineral when the acid is sort of ‘dampened’ by a rounded texture. This quality of ‘roundness’ is derived from phenolic solids – skins and stalk (from whole bunch pressing) – as well as from various kinds of softness that come with lees stirring and bottle aging. This liquid ‘weight’ means the acid is delivered more slowly to the palate, with a bulwark of texture softening the bright acids and preventing their green aggression from being perceived all at once. Instead flavours come gently ‘over time’ – meaning milliseconds, presumably. Thus whites from cool climates can made more palatable and can be justly called ‘mineral’ on the label.
Critics of harsh Kiwi Sauvignon Blanc may disagree, but in Dr Martin’s eyes a correct texture can make our wines taste rather like fine French Sancerre – focussed, integrated and ‘steely’ – without, crucially, being too harsh. Winemakers like Marlborough’s Clos Marguerite use methods very similar to what Martin describes: their beautifully sophisticated, mineral Sauvignons make superb evidence for his claims.
Another promising study looks at something called succinic acid. The effect and esters of succinic acid have been shown to make a wine feel more ‘vinous, sapid, salty and sometimes even bitter’. (This may also chime with the earlier claim that sulphuric flavours enhance minerality). Succinic acid, however, is produced solely within the chemical vagaries of fermentation, in which nitrogen levels are crucial – low ones, to be precise.
On reflection, the connection of succinic acid with minerality seems dangerously convincing. Because it allows us to draw a link backwards from the fermentation chemistry to – yes indeed – the minerals in the soil. Because nitrogen levels in fruit are actually affected by soils. To have low nitrogen levels in your ferment (and end up with plenty of minerally succinic acid) you must first grow your grapes on arid, low-nitrogen soils.
So there you have it, Chablis. Your poor soils may be the cause of minerality, just not in the way you thought.
There are friendlier and more ‘socially’ orientated studies that affirm the existence of minerality – as a human experience, at least. Tasting experiments suggest minerality is a palpable effect and a meaningful term, denoting definite characters to groups of people. With a fairly consistent linguistic and cultural usage, the gustatory effects that trigger it are also consistent in certain wines – like ‘stony’ or ‘steely’ notes in Loire and Kiwi Sauvignon Blanc. In the words of one researcher, the cultural and subjective experience of minerality is a ‘“cognitive” irrefutable reality’.
Indeed one such study opened a whole new angle on minerality. From an impressive blind tasting including public and sixty two wine professionals, it was found that two examples of the Swiss grape Chasselas were overwhelmingly identified as “mineral”. These two wines were found to have the highest levels of a compound called H2S. Rather like the sulfides we have already mentioned, H2S (and HSSH) are ‘polysulfides’ that occur naturally during ferment and likewise produce pungent aromas of flint, matches or fireworks. Confusingly, these polysulfides are also common in soils. H2S can live ‘bound’ up with the iron molecules common in clay soils, to be released and smelt though soil friction or movement (hmmm, rather like petrichor and geosmin). Polysulfides are aromatic compounds that smell like rocks but, unlike rocks, they actually have their own flavour – one which, tantalisingly, is also tasted in wine.
Though present in both soil and wine, it is a big leap – and yet to be proved – that polysulfides start below your feet and end up in your glass. This might just be the ‘Chablis effect’ – in which geology and wine have a similar smell, for unconnected reasons. However the mineral here is identical in both, so the Swiss may claim this as an increment in the inevitable advance of wine science. Still in the end, all we really know is that a bunch of people agreed their Chasselas tasted “mineral”.
Consensus is all very nice, you see, but it can help to institutionalise our mistakes. One happy myth recently brought low was the ‘well-known’ minerality of Spain’s Priorat reds – debunked, of all things, by the science of etymology. The Catalan name for the slate soils, ‘llicorella’, has long been connected to a licorice note in the Grenache-led wines. But it turns out the pronunciation of the word in Catalan is quite unlike ‘licorice’ in English. In fact the Catalans knew the taste of licorice or aniseed, they just had a completely different word for it. So if you detect a licorice note in a Priorat red then it is less about geology and more about, well, certain gains in translation made by decades of wishful wine writing.
Finally, one of the more interesting takes on minerality is that it does not consist in the presence of certain flavours at all, but rather the opposite – an effect of flavour’s absence. Some studies suggest that the lack of strong flavours generally – both sweet and sour, whether from fruit or winemaking – allows tasters to identify the remaining, more neutral flavours and textures as ‘mineral’.
Is minerality, then, a kind of pure essence of wine? Is it simply the fresh, citric, vinous ‘platform’ on which all other fruit, phenolic and oak characters are built? If so, this would explain the perverse elusiveness of the term’s meaning, as well as our abiding thirst for its effects.
Perhaps, in the end, what we call ‘minerality’ is nothing but chance in action. The patchwork of the world’s wine regions, so wonderfully over-coded by wine culture, may in fact be nothing but a random chessboard of likely and unlikely fermentation outcomes. Not every terroir makes a claim to minerality, after all. So when a little place called Chablis finds a whiff of chalkiness in its wine, this may not be a classical standard – a rule of minerality – but rather one of the precious exceptions. Because these arid, dry-farmed soils happen to have lime-smelling soils, yes, but also a fairly consistent chemical balance – with low nitrogen. And therefore high succinic acids in the finished wine.
The vignerons of Chablis long ago noticed, in their goblet, a smell that reminded them of the soil they were digging that day. And thus the folklore began – from an absence of molecular chemistry and an earlier tendency toward simple, literal, magical thinking. C’est tout…?
Grape juice has been fermented almost longer than any other product. Yet, marvelously, the process is still too complex for science to fully understand.
So in the meantime, while the axe is raised over this historic and charming wine ideal, feel free to follow the heroic example of our Caro’s staff. Let’s bite the metal and taste minerality in every second wine. Lets imagine our glass half-full with traces of place, with rocks and flint and stones of alpine-rivers. Let’s get seaside in our Manzanilla and volcano in our Etna Rosso, lime in our Chablis and yes, dammit, licorice in our Priorat.
After all, what is tried is true. And on the question of minerality, centuries of wine discourse has certainly tried.