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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)

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