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Return to Minerality: Acidity

In this little series on minerality – from which I have had an enforced break, partly down to my trip to the Loire to see the 2013 harvest in action and my Bordeaux 2013 reports – I have looked at many different explanations for the mysterious ‘minerality’ that now seems so commonplace in wine. Some explanations that have been offered – such as the presence of minerals, chemical or geological, in the wine, just don’t ring true. Others, however, have more promise; of these, the most enticing is related to what is commonly termed ‘reduction’, which is actually the presence of mercaptans in the wine, mercaptans commonly being produced during fermentation of nutrient-poor (especially nitrogen-poor) musts. Their presence is enhanced by protecting the embryonic wine from oxygen, hence they have become associated with ‘reductive’ winemaking techniques, and in a leap of faith have since been ascribed by many to reduction itself, which of course isn’t quite true. See my previous posts, starting with my examination of minerality and soil minerals, for more information.

Throughout all my previous posts, however, there has been an elephant in the room, and this particular elephant’s name is acidity. Acidity, and its relationship with minerality, has been there in the back of my mind all along, and it has eventually come to the fore. Now it’s time to take a look at acidity in a little more detail.

It seems to me that there is a strong correlation between the presence of minerality and acidity in wine. Draw up a short-list of wines that show minerality – Riesling from the Mosel and Rhine Valleys in Germany and Alsace in France, Chablis, Loire whites including both Sauvignon Blanc and Chenin Blanc and so on, and you clearly have a list of northern cool-climate wines that also tend towards higher levels of acidity. The correlation is far from solid, however, as there are many white wines rich in acidity that do not necessarily show minerally characters. This shouldn’t dissuade us from pursuing acidity as a potential part of the minerality ‘story’ though, as it may be that some other process in the winery influences whether or not acidic wines hold onto their ‘minerality’ (rather akin to how the presence of mercaptans can be preserved, or disrupted, by winery practices). Alternatively, it might be some aspect of the vineyard that is associated with minerality which also enhances the acidity. More on this in a minute.

Minerality and Acidity

The commonly proposed theory is that minerality in wine is not due to minerals at all (we know that much already), but is in fact down to the presence of acidity, perhaps accentuated by a lack of fruit ripeness which would otherwise disguise the chalky/stony/slatey minerality of the acids. This might explain why only more cool-climate northern regions tend to show minerality as although all wines have acidity it is only these regions that would produce wines with the lower levels of ripe-fruit character necessary to induce the ‘minerally’ sensation. Terroir may also be important; this is a vital consideration if we are to somehow link the apparent relationship between minerally wines, and minerally terroirs. It is plausible that some soils may influence acidity (and thus minerality) levels through their ability to radiate heat back to the vines and fruit, or by their propensity (or lack of it) to hold onto water. In addition it seems accepted that more alkaline soils – such as limestone, as found in Chablis, Sancerre and Vouvray, for example – tend to produce wines with higher acidity (regardless of how incongruous this birth of acidity from alkalinity may seem!). Variety must also play a role, as some varieties seem to express minerality much better than than others; Riesling in Alsace, for example, might be regarded as more minerally than Gewurztraminer. Riesling is also the more acidic of the two varieties, wines made from Gewurztraminer tending more towards a softer, low-acid style.

It all seems very tempting. Surely soil, acidity and minerality are intertwined? I see a couple of minor problems with the concept of low-fruit-ripeness plus acidity as the cause of minerality, and these are as follows;

First, the term ‘minerality’ has really only entered the wine tasting lexicon in the past 10 or 15 years. And yet the features that have been discussed here – soil, stones, climate, variety – all of which may have some effect on the eventual acidity level in the wine in question, seem to me to be long-standing constants. Sancerre has been as we know it today really since the immediate post-phylloxera era, when Sauvignon Blanc came to dominate the appellation (in place of Pinot Noir, which had been the mainstay until that time) during the replanting. The variety, the limestone soils, these are unchanged, so why weren’t tasters reporting this minerally sensation in the 1960s and 1970s?

Secondly, it seems very unlikely that we should suddenly begin detecting acidity as minerality, when acidity itself gives such a clear message to the palate. I’m not really prepared to accept that minerality and acidity are one and the same. Nevertheless it seems clear that minerality and higher acidity are related in some way; many observers – writers, critics, sommeliers, winemakers – have reported a correlation between the two. This has been confirmed in sensory studies, such as that by Wendy Parr of the University of Lincoln which was published in New Zealand Winegrower this year. In her study, Parr recorded the tasting opinions of groups of French and New Zealand tasters (all wine industry professionals), all of whom sat down to blind-taste 16 French and New Zealand Sauvignon Blancs. Looking at their assessments, it was clear that there was a correlation between perceived sourness/acidity and the perception of minerality in a wine. This still doesn’t tell us what is causing the sensation of minerality though, although again the combination of high acidity and low fruit-ripeness is put forward by Parr and colleagues. Of note though, other perceived aromas felt to be a description of minerality demonstrated a correlation with ‘reductive’ sulphide characteristics. Acidity seems to be part of the story, even if it is perhaps nothing more than a constant spectator, or perhaps a catalyst, but here again we have ‘reduction’ – as I discussed in Mineralty – A Reductive Phenomenon? – raising its head. I can’t help feeling – especially with changes in winemaking practises pushing many wines towards a more reductive character in recent years – that ‘reduction’, or rather mercaptans, play a somewhat more significant role than acidity.

Further reading: Parr W, Ballester J, Valentin D, Peyron D, Sherlock R, Robinson B, Breitmeyer J, Darriet P, and Grose C. 2013 The nature of perceived minerality in white wine: preliminary sensory data. New Zealand Winegrower, 78, 71-75. Link (forward to page 71)

Minerality. A Reductive Phenomenon?

Looking back over my previous posts on minerality, it seems as though this wine-tasting phenomenon isn’t related to the uptake of minerals (chemical or geological) from the soil, as described here and here. Nor, it seems, is it likely that it is due to petrichor, as described here. It seems increasingly likely, re-reading these posts, that what we call ‘minerality’ no more reflects the presence of minerals in the wine, than describing a wine as ‘floral’ implies there are flowers in it, or describing it as ‘rich’ implies it has somehow accrued some wealth.

‘Minerality’ seems to be a somewhat metaphorical term; what we sense in the wine is reminscent of minerals, but it is not really mineral. If we can accept this then we can call this hopeless search for minerals to a halt, and look for other more original explanations for what we call ‘minerality’ in wine. And on doing this up pops a rather strong candidate: reduction. To get to grips with this possible explanation, though, I think we first need to reconsider what we mean by reduction. As with minerality, ‘reduction’ is not what it at first seems to be.

When it comes to winemaking and oxygen, there are two extreme schools of thought. First, at the oxidative end, the wine is allowed increased contact with oxygen during the winemaking process; this might involve fermentation in an open environment, using more porous materials (i.e. wood), racking more frequently to allow contact with the air, and not adding an antioxidant such as sulphur dioxide. At the other end of the spectrum is reductive winemaking, avoiding contact between the must and the air, working in closed vessels, using carbon dioxide or other gases to prevent such contact, not racking and so on. [NB. As an aside, the processes of oxidation and reduction are more correctly described as transfer of electrons; oxidation is the loss of an electron often to oxygen (but also other molecules, often oxygen-derived, can take the electron) and reduction is the gain of an electron. In addition, the actions of sulphur dioxide are not accurately described when we refer to it simply as an 'anti-oxidant'. But I don't want to get sidetracked; for the moment let's just stick with the man-in-the-street definitions of oxidation, reduction and sulphur dioxide.]

Gravel in Bordeaux

During fermentation, yeasts produce many substances other than alcohol and carbon dioxide. In particular, when fermentations are carried out under nutrient-poor conditions, yeasts become stressed. Yeasts require nutrients – such as nitrogen – to thrive and multiply, and carry out the fermentation, and this nitrogen is usually obtained from ammonia or common nitrogen-containing amino acids (the building blocks from which proteins are made). In the absence of these yeasts look around for more esoteric nitrogen sources, including less common amino acids such as cysteine, which contains nitrogen but also sulphur. As a consequence, the yeast now starts pumping out sulphur-containing waste products, starting with hydrogen sulphide. This gives the wine a rotten egg smell, and it is fortunate that hydrogen sulphide doesn’t hang around for long. Other sulphur-containing waste products, including mercaptans (also called thiols) and other more complex sulphides, do hang around though. And the array of different aromas these complex molecules produce can be very broad; there’s everything from the less pleasant scents of cabbage, garlic and rubber, through to more (or less!) appealing aromas of passion fruit and cat’s urine (mercaptans/thiols are important determinants of the characteristic aromas of Sauvignon Blanc), and then there are also the very suggestive aromas of flint, struck match, smoke and gunflint. This latter group look like very good candidates for ‘minerality’, don’t they?

So what we call ‘reduction’ is really the presence of mercaptans and other aromas. Once present their existence can be assured by nursing the wine using a ‘reductive’ philosophy like that described above, but they originate from stressed nitrogen-poor fermentations. So from this comes a theory, and several questions.

1. Could it be that the most nutrient/nitrogen-poor fermentations originate from the most nutrient/nitrogen-poor soils? If the vines have struggled to find what they need from the soil, and produce nitrogen-poor fruit, is it not logical that the must will be nitrogen-poor?

2. If so, very stony, nutrient-poor sites might produce the wines most prone to mercaptan and complex sulphide production. Is it not possible that we could interpret some of these substances as minerality, and then in a typical wine-writing leap of faith ascribe this sensation to the stony vineyard soils? Could minerality be mercaptan-related and dependent upon a paucity of vineyard nutrients not the stones themselves?

3. If this is the case, how ‘minerally’ a wine is could depend on work in the vineyard, and how easy or difficult a life the vines have. This seems to fit with the view of some – including Olivier Humbrecht of Zind Humbrecht – that ‘minerality’ doesn’t come without careful viticulture.

4. Likewise, it seems that work in the cellars could enhance or inhibit the generation of ‘minerally’ characteristics, and there is some evidence that this is also true, for example the new wave of more minerally white wines coming out of Australia (I’m on shaky ground talking about Australian wines, but I hope you get the point).

5. Personally, this would explain why I find the characteristics of ‘minerality’ in some wines – especially the flinty-smoky-gunflinty character of Pouilly-Fumé – to be remarkably similar to ‘reduction’ (in other words mercaptans and other sulphides) that I find in other wines.

There are problems with this theory, admittedly, not least the fact that although there are correlations between minerality and reductive winemaking, and the lack of minerality and oxidative winemaking, it isn’t a pure or strong correlation. At the extreme end it seems clear (i.e. I don’t recall finding a minerally but oxidised wine) but in the middle ground wines can be made in a manner more oxidative (oak fermentation vats and racking for example) and still be minerally, whereas more reductive winemaking can still produce wines stubbornly free of minerally character. Of course, it might well depend on the soils (as indicated above) or other factors (such as variety, or climate) and maybe the reductive character is merely part of the story. Nevertheless, I feel it is probably a large part of the story. I would be fascinated to hear comments on this, especially from readers with more knowledge of winemaking than I have.

Minerality is . . . . Petrichor?

In the most recent instalment of this mini-series examining minerality I took a look at what we actually mean when we (you and me) use the term minerality. I’m not sure if I drew that post to a close with a definitive statement of how I use the term minerality, so here are the salient points with regard to my use of the word.

1. ‘Minerality’ best describes a ‘taste sensation’ that is perhaps best likened to sucking pebbles, or licking a stone. It may relate to the ‘structure’ of the wine as much as flavour.

2. However, because it seems ‘minerality’ is not really due to the presence of geological or chemical minerals in the wine, the term does not need to be bound by the physical properties of such minerals. Minerality may also therefore be used to describe a sensed aroma or flavour, in other words it may be that the wine smells or tastes like a stone, rock or other mineral.

One point I did stress, however, is that minerals themselves cannot be smelt, or tasted, because the molecules are not volatile and not water soluble. But this is at odds with my experiences of minerals; pebbles freshly plucked from a stream do smell of something, as do other rocks. And it is smells and tastes in the wine resembling these aromas that cause us to describe a wine as ‘minerally’. But if minerals can’t be smelt, what are the aromas we sense on pebbles, stones and rocks? It has to be something else. What is it on, or in, the rocks that smells?

This is a difficult question to answer, although there are clues in a parallel question on the smell of soil. Anybody who has had the occasion to pick up a garden fork and turn over a little soil knows that soil has a particular damp, earthy (sorry, I know that is a tautology) smell. Soil is a complex substance though, rich in minerals, organic matter, bacteria, yeasts, fungi and more, and the characteristic smell of soil is down to just one component called geosmin, a substance synthesised by bacteria in the soil. It also contributes to the earthy smell of beetroots, and it crops up in wine when rot-tainted fruit has been included in the fermentation.

Thus the smell that is ascribed to soil is in fact down to one specific volatile compound associated with soil. Likewise, it has been postulated that the small of minerals – stones, rocks, etc. – might also be down to associated volatile organic compounds present on the surface of the material. A fine example of this was the first description of an argillaceous (meaning from clay) odour by Bear and Thomas, two mineral geologists who published their findings in Nature in 1964. Bear and Thomas identified that this odour could be yielded by many different minerals, not just clay, provided they were treated in the right manner, namely left to bake dry in warm heat, and then exposed to water. They were obviously classicists as they christened the odour petrichor, a synthesis of petros, Greek for stone, and ichor, the fluid that flows in the veins of the gods in Greek mythology.

Unlike geosmin, petrichor is not a specific chemical compound, but an odour that occurs when the rocks dried in warm weather for several months are exposed to rain; when these rocks are subjected to steam distillation a golden-yellow oil is obtained. Although in their initial studies Bear and Thomas did not suggest a biological origin for the substance, further work suggested it was indeed produced by plants, and was a germination retardant released during warm dry weather. The advent of rains washes the retardant away (as well as aerosolising it so we can smell it) thus encouraging germination. Petrichor, then, is the oil responsible for that “just rained” smell that lingers in the air when rains arrive after a dry spell. It may also be a mechanism for how animals sense a coming storm, if petrichor is blown towards them from an advancing weather front.

Even so, the idea that volatile oils are associated with rocks, and have an appealing smell, has naturally led some to postulate that these oils are responsible for the wine characteristic(s) we call ‘minerality’. I see some problems with this though:

1. Petrichor is an aroma which is irrespective of source, in other words the same smell comes whether the geological source material is clay, slate, or otherwise. Does this fit with our impression that different minerals (slate, flint, etc.) can impart different characteristics to the wine?

2. If petrichor is released by plants stressed by warm growing seasons, to discourage germination by seeds until more beneficial rains arrive, does this fit in with our impression of when ‘minerality’ is sensed? If so, should we not see high levels in warmer vintages, like 2003 Bordeaux, the most un-minerally vintage ever? And levels would be highest in warm zones, such as the Languedoc, Italy, certain Australian regions and so on. Does this fit with the experience that it is cool-climate acidic wines that tend to show most minerality?

3. Petrichor is said to have a delightful, sweet, fresh, (“just-rained”) smell. Is this what we experience when we talk of minerality?

None of the answers to these questions see to suggest petrichor is the answer to minerality. Attractive as the idea might be, not least because of that funky name, petrichor just doesn’t tick enough boxes for me. I think I need to continue my search for likely causes of minerality.

Sources:
1. Bear IJ, Thomas RG. Nature of Argillaceous Odour. Nature, 201, 993 (1964)
2. Bear IJ, Thomas RG. Petrichor and Plant Growth. Nature 207, 1415 (1965)

Minerality Is . . . . Confusing

So far in this personal exploration of minerality (by personal, I mean I’m trying to understand this wine-tasting phenomenon as much as you are) I have discovered that there are no easy answers to the issue of minerality in wine. It doesn’t reflect the magical absorption of rocky minerals (feldspar, quartz and the like) from the soils. Nor does it reflect the absorption of chemical minerals (ions of calcium, manganese and so on) from those same soils. Whatever the answer to the minerality question, it is clearly going to need some protracted thought and head-scratching. With that in mind, perhaps we had better decide exactly what we are talking about when we refer to ‘minerality’ within a wine.

Unfortunately, not even this is without controversy.

First, let’s turn to the palate. Is minerality a flavour – the essence of wet stone, rain-splattered slate, hot schist and so on drifting across the palate – or is it more of a “taste sensation”, an extra structural component of the wine to be listed alongside tannin, acidity and residual sugar? The truth is, the term seems to be used in both ways.

First, I have seen the term ‘minerality’ used where the author is clearly communicating a well-known flavour of the wine of the wine in question. Long before I first read the word minerality within in a tasting note, I was aware that Mosel Riesling often had a slatey flavour to it. Likewise, Chablis was also recognised to have stony edge, said by many to reflect the classic Kimmeridgian limestone of the premier and grand cru vineyards.

Second, and this is perhaps a more precise meaning, when the term crops up in a tasting note the author is I think often referring to what I call a “taste sensation”; not a flavour as such, but a structural element of the palate. It is difficult to describe, but the best analogy (and I accept that this stretching the definition of an analogy a little) is that it is like the dry, tingling-puckering sensation that you might get from sucking on a pebble. It is not so much the flavour of the pebble (this is a flawed statement by the way – I’ll come back to why this is so in a minute) but more the effect it has on the ‘shape’ or ‘feel’ of the wine.

This “taste sensation” leads to an almost inevitable confusion between minerality and acidity, one that is confounded by the fact that minerality tends to be a feature of cool climate, high-acid white wines – think Chablis, Sancerre and Muscadet, for example. But whatever the cause of this ‘minerally’ sensation, I think we can be clear that it is not the same as acidity. The two feel very different on the palate, and our persistent inability to identify an explanation for this minerally sensation should not lead us to draw overly simplistic conclusions regarding a potential role for acidity. Indeed, some wines may be acidic but not minerally. And probably (although I’m struggling to think of one off the top of my head) vice versa.

Is minerality merely flavour or “taste sensation” though? I would contend it is not, as in many minerally wines I sense this minerality in the aromatic profile. This is a statement that is likely to incite some debate, because I have noted some wine professionals state with certainty that minerality cannot be smelt. This to me seems to be patently incorrect, however, for three reasons:

(a) a major rationale for the statement is that minerals such as feldspar are not volatile, and therefore can not be smelt. But we have already agreed (I hope!) that what we call “minerality” is not the presence of actual “minerals” in the wine. In the same vein, old Rioja does not really contain strips of leather and roasted game, and maturing left bank Bordeaux does not really contain cigar boxes, pencil shavings and the inside of a dusty old cabinet. I realise these analogies are a little flippant, because minerality as a structural element is perhaps more complex than these ‘mere’ aromas and flavours, but you get the point I hope. Minerality isn’t minerals, so let’s stop fencing in ‘minerally’ characteristics according to the physical properties of minerals.

(b) there seems to me to be a significant cross-over between minerality and what we call reduction. The issue of reduction in wine is another can of worms, as like minerality what we call ‘reduction’ probably isn’t true full-on reduction but something else, namely the presence of volatile mercaptans and complex sulphide molecules. This is something I will explore in more detail in the future, so for the moment suffice to say that “reduction” is very aromatic, as are “minerally” aromas in wine, and in some wines – especially Pouilly-Fumé I have found – I can experience some difficulty teasing them apart. That wouldn’t be the case if ‘minerality’ weren’t aromatic.

(c) in my experience, time and time again I come across wines which display aromatic minerally elements. The classic cool-climate white wines that display minerality I have already mentioned, but it can be found in red wines too. A recent memorable example was when tasting with Jonathon Maltus at Château Teyssier earlier this year; he likes to compare and contrast Le Carré (clay over ‘ordinary’ limestone) with Les Astéries (clay over calcaire à asteries, the classic hard, fossil-encrusted limestone of St Emilion). Just nosing the wines, it was as clear as day that the latter had a much more minerally aromatic profile than the former, and it wasn’t necessary to taste them to tell which was which. Some aromatic distinction – which I found worthy of description as ‘minerally’ – allowed me to distinguish between the two wines.

Before I finish I just want to return to the statement I made – regarding the ‘flavour’ of a pebble – which I wrote was flawed. To understand the problem with describing the flavour of a pebble we must return to the my first minerality post when I laid out some minerally chemical formulae for a small selection of minerals including feldspar, muscovite and biotite. These are large, inorganic molecules that are insoluble in water and, as discussed in (a) above, are non-volatile. As such, these substance cannot be ‘tasted’. That doesn’t upset our notion of what might cause minerality, because we have already discounted any thought that these giant mineral molecules are somehow absorbed by the roots and deposited in the grapes ready for the fermentation vat. But it does beg the question – pebbles smell and taste of something, so if it isn’t ‘pebble’ just what is it? Is it the cause of minerality? Or is ‘reduction’ more important? Or is ‘minerality’ something else altogether, some aspect of winemaking not yet covered? These are questions I will have to come back to on another day.

Minerality: Ioning It Out

In this personal exploration of minerality I started here with a look at how rock minerals, such as feldspar (an important constituent of slate, gneiss, and countless other familiar rock types) might be transported into the root and then up to the fruit, giving us a fabulously mineral sensation when drinking the wine. My conclusion, in short, was that such a concept is nonsense (to be fair, this is what many others have also concluded, including many with greater knowledge of plant biology than I). You don’t have molecules of feldspar and quartz, imparting subtle suggestions of slate and gneiss, in your most recent glass of Muscadet regardless of how appealing the thought might be.

There are other much more likely candidates which might account for our perception of ‘minerality’ in wine. And we also need to explore exactly what I (and you) mean when we use the term; is it, for example, an aroma (or flavour) within the wine, or is it a comment on its texture? Or something else? Before getting on to these questions though, I wanted to return briefly to something raised in my first post. And that is the suggestion that, if minerality is not down to these large ‘geological’ minerals which are far too large to be transported into the root cells, could it be down to the ‘chemical’ minerals, the more elemental constituents of these ‘geological’ minerals, i.e. the ions of calcium, magnesium, potassium and so on that certainly are actively taken up by the roots.

Of all the nutrients the vines require, only three – carbon, hydrogen and oxygen – come from the air, being permitted to enter the tissues through open pores on the underside of the leaves. All other substances essential for life, health and growth – including nitrogen, phosphorus, potassium, calcium, magnesium, sulphur, iron, manganese, copper, zinc, chlorine, boron and molybdenum – are absorbed through the roots. The notion that these ions might have a role to play in ‘minerally’ sensations seems, on first inspection, a tempting idea, as some of those ions – calcium, for example – look like good candidates for engendering such a characterictic. Surely, calcium has to taste at least a little chalky, you might think?

My experience with a variety of mineral waters suggests that different concentrations of ions dissolved in the water can influence its flavour; presumably, this is why the water from one spring tastes different to water from another. One of my favourites is San Pellegrino (a bit of a cliché I know, but I hope you will forgive me) and a glance at the label tells me that there is, for example, 52 mg/l of magnesium and 179 mg/l of calcium in the bottle I’m just about to open to accompany my lunch.

Minerality in wine - schist, Clos des Allées, Domaine Luneau-Papin, June 2013

Even so, my pre-existing prejudice was that the metal-ion concentrations in wine would be too low to have any influence on flavour. I didn’t feel that these ions were likely to be the explanation for minerality, a term which has only been a common part of the wine writing lexicon for 10-20 years, when mankind has been drinking presumably ion-rich wines for millennia. I still think ions are not the answer to the ‘minerality’ question (it just couldn’t be that simple!) although as it turns out I was wrong about their potential impact on wine flavour. The ion concentrations in many wines match or exceed flavour thresholds, which means it is conceivable that these ions may have some influence on flavour.

Published flavour thresholds vary, this variation the result of differing temperatures of the solution, what salts the metal might be bound within, the sensitivity of the individuals (some authors having claimed 6400-fold variation in sensitivity between tasters), and so on. In addition, published concentrations in wine also vary (from wine to wine, obviously), so here are a few typical figures (some sources are listed at the foot of the article):

  Sodium: reported threshold up to 200 mg/l, concentrations in wine can be higher, varying from 10 to 300 mg/l.

  Potassium: reported threshold up to 680 mg/l, concentrations in wine can be higher, varying from 200 to 2000 mg/l. It is no surprise that potassium concentrations go high, this being the major intracellular ion of living organisms, and those grapes were alive once!

  Magnesium: reported threshold 100 mg/l, concentrations in wine about this level, so possibly detectable.

  Calcium: reported threshold up to 125 mg/l, concentrations in wine probably lower, up to 125 mg/l, one of the few relatively commonly occurring ions that doesn’t seem to hit the threshold.

  Iron: reported thresholds are very variable, but range up to 0.3 mg/l (I’m sure there are some very different numbers out there), but concentrations in wine can be higher, varying from 1 to 10 mg/l.

  Copper: reported threshold perhaps 1 to 2 mg/l, concentrations in wine thankfully generally lower at 0.1 to 0.3 mg/l

So are minerals such as these simple metal ions responsible for minerally sensations in wine? I doubt it very much, and even if they contribute a little, they surely aren’t the whole story. My doubt about a potential role for ions in minerality is based on a number of holes in this theory. First, if these ions were responsible then wines have always been ‘minerally’, we would have been talking of minerality for centuries, not the last one or two decades. Second, it would be easy to test, as classically ‘minerally’ wines – Muscadet, Chablis, Sancerre, and so on – would have high levels whereas non-minerally wines – big, blobby Châteauneuf du Pape and other turbocharged ooze-monsters – would not. And studies don’t seem to bear this out, as all wines seem well endowed with these minerals, even from warm climates such as Greece and Australia.

There is one fly in the ointment, however, which comes from some research correlating ion concentrations and tasters’ perception of the mineral character of the wine. The work, by Oze, Horton and Beaman of the University of Canterbury (in New Zealand), has only been published in abstract form (and thus not in a peer-reviewed journal) following a meeting of the American Geophysical Union in 2010, nevertheless their results still pique my interest. First, there was no significant correlation at all between perceived minerality and the concentrations of aluminium, silicon, potassium, sodium, calcium, magnesium, sulphur and iron, in keeping with my thoughts above. There was, however, a tantalising result for kaolinite (a clay mineral) and gibbsite (a form of aluminium hydroxide), both of which may form in the wine depending on the pH, or possibly even in the mouth under the influence of the much higher pH of saliva compared to the wine. There was a strong correlation between these compounds and the perception of minerality in the wine. However, the finding that minerality might be engendered by the creation of minerals in a higher-pH, less acidic milieu seems at odds with the general perception that it is the acidic, cool-climate wines that are more likely to display minerality. For the moment, I think this isolated result remains an acaemic curiosity, one that certainly doesn’t offer a convincing or conclusive answer (but which perhaps deserves further exploration). For the moment I will put these results to one side, and continue my search to uncover the cause of minerality in wine.

Sources:

Boulton, R.B., V.L. Singleton, L.F. Bisson, and R.E. Kunkee. Principles and practices in winemaking. Chapman & Hall, New York (1996).
Lockhart, E.E., Tucker, C.L. and Merritt, M.C. The effect of water impurities on the flavor of brewed coffee. Food Res., 20: 598 (1955)
Lazos, E.S., ajd Alexakis A. Metal ion content of some Greek wines. International Journal of Food Science and Technology., 24,39-46 (1989)
Cohen, J.M., Kamphake, L.J., Harris, E.K. and Woodward, R.L. Taste threshold concentrations of metals in drinking water. J. Am. Water Works Assoc., 52: 660 (1960)

Minerality: Gneiss-y Nonsense

There has been a lot of words written on minerality over the past few years; what it means, where it comes from, is it real, and so on. I think the terms minerally and minerality are useful descriptors for wine; I accept, however, that it can be difficult knowing exactly what someone (including me) means when we use one of these terms in a tasting note. I wanted to share some of my own thoughts on minerality over the next few days. In particular I will try and explain my own take on the term, although I will warn you now I’m not going to be as precise as you might be hoping.

Before I get to that though, I first wanted to consider the notion that minerality in wine reflects minerality in the soil.

The major problem with minerality is that some may have assumed that its presence in a wine reflects a direct relationship between the wine and/or fruit from which it was made, and the soil or rock in which the vines are planted. In other words, minerals from the ground somehow find their way into the grapes, and then to the wine. This has always seemed like nonsense to me. We probably all know that plants absorb water, nutrients and minerals through their roots; this is why the health and growth of plants can be influenced by adding fertiliser to the soil. The fertiliser contains nutrients and minerals valuable to the plant, which can be absorbed across the huge surface area offered by its extensive network of fine, branching roots. It is important to understand exactly what we mean by ‘minerals’ though. It is tempting to think they are rock minerals, a molecule of gneiss here, a granite molecule there. That’s always seemed like nonsense to me, for several reasons. Take gneiss – a commonly encountered metamorphic rock in the Muscadet region – as an example; this is made from a variety of minerals, but is likely to be a mix of the following:

  Feldspar, chemical formula : (Ca,Na)AlSi3O8 or KAlSi3O8

  Quartz, chemical formula : SiO2

  Muscovite, chemical formula : KAl2(AlSi3O10)(F,OH)2 or (KF)2(Al2O3)3(SiO2)6(H2O)

  Biotite, chemical formula : K(Mg,Fe)3(AlSi3O10)(OH)2

Gneiss, as an aside, is not defined by a specific mineral composition (so it can be a mix of the above, and other minerals too) but by a texture; it is a metamorphic rock, often granite-derived in the Muscadet region I believe, where the minerals are banded into dark and light strands a result of extreme heat and pressure.

No, I haven’t surreptitiously pulled out a geology degree you never knew about; I simply pulled these formulae off Wikipedia, which only seems fair as they have been mining Winedoctor for wine-related facts ever since a wiki author decided that Latour and Le Pin warranted entries just as much as Miley Cyrus and all the episodes of South Park and The Simpsons ever made did.

Looking at the above minerally formulae, several things spring to mind:

  1. Gneiss is made up of some huge molecules; it would be very difficult (impossible I think – but I’m not a plant scientist and I’m trying to remain open and balanced) to transport them across a cell membrane.

  2. There looks as though there is a lot of potential toxicity there – do plants really want to be hoovering up Aluminium-containing minerals?

  3. Active transport into the cell requires energy – why would the plant use energy to absorb such huge molecules, when it is the constituents (iron, manganese, etc.) that might be useful. Why not just transport in iron and manganese ions from the surrounding soil?

  4. Transport into the cell requires the nutrient or ion to be water soluble; any gardener knows this. In high-pH (alkaline) soils plants suffer from chlorosis. This disease was a big problem for those planting experimental grafted vines in the early years after phylloxera, as some rootstocks when planted on limestone couldn’t handle the conditions. In such conditions much-needed iron is sequestered as a solid, bound with calcium. Solids cannot be absorbed by roots, the ‘minerals’ need to be dissolved. There are two solutions; alter the pH, for a long term solution, or add chelated iron (iron in soluble compounds) for a quick fix. It is impossible to imagine the molecules of stone, described above, dissolving in water and being absorbed. Gneiss, and other rocks, just aren’t water-soluble. They erode, yes, but they don’t dissolve into water like sugar.

  5. Discount all of the above; assume gneiss is soluble, and magically taken up by the roots. How and why would this gigantic minerally molecule be ascended through the structure of the plant to be deposited in the grapes. Would the normal xylem channels achieve this? What would be the purpose of this energy-expensive process?

It all seems like gneiss-y nonsense to me, and seems to discount both the notions that ‘minerality’ originates with the absorption of minerals, and also – on a related issue – that a particular terroir might be expressed in a wine through the absorption of the very minerals that comprise that terroir.

More minerally thoughts on another day.