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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)

2 Responses to “Minerality: Ioning It Out”

  1. Both kaolinite and gibbsite are much more soluble at pH 4 than at pH 6, which may simply mean that they are present at a higher concentration in acidic white wines. They and other minerals will likely bind to tannins in red wines, making them less perceived by the taster.

    This hypothesis clearly calls for experimental verification, say, with some Premier Cru Chablis for a baseline, and going on from there!

  2. Quite right Frank. At present it’s just an association, and these two minerals might just be acting as markers for some other substance which creates the minerally sensation. It would require proper investigation perhaps using gibbsite- and kaolinite-adjusted wines.