Some of you are already wincing. No topic has wrought more confusion and ruffled more feathers among dedicated enophiles than the incessant bandying about of the lofty sounding “M” word. For some (myself included), asking whether minerality exists is like asking whether the sky is blue. Yet for many, the term isn’t nailed down.

The current confusion about minerality is hardly surprising. The well-known confusion about sour-bitter is another example of the poverty of linguistic palate training that most Americans receive in youth. Yet no one doubts the existence of these sensations. Why is this particular descriptor so elusive?

One source of confusion about minerality is that there are at least three aspects of wine flavor for which some tasters can latch on to no better term. Sometimes it refers to the aroma of wet stone, an odor for which I prefer the term “petrichor,” the smell of new rain as it liberates natural oils from rock in the desert. The second use is for flavors in the mouth that can also resemble stone or well water, a common attribute of Semillon. My definition represents a third sense, which differs entirely.

In these circumstances, inconsistency of usage can hardly be viewed as evidence that minerality does not exist. Science is supposed to embrace confusion and organize inquiry. That its existence is questioned in scientific circles says plenty about what passes for science today. In the quasi-scientific technocracy of today’s winemaking priesthood, enthusiastic inquiry into such mysteries is often replaced by an authoritarian circling of the factual wagons.

 In all fairness, though, I have to admit that the phenomenon hasn’t yielded any easy explanation to those of us who have been poking around its edges. I have danced around this topic throughout this series. Here I want to bring together in a single article my take on this elusive phenomenon. (Therefore, forgive me when I repeat myself.)

The world according to Clark
Now, nobody has appointed me the new Noah Webster, but if we were sitting in the same room, I would simply pop some corks and show you what I mean by minerality. What follows is my personal point of view about how that palate sensation connects to viticultural practices and behavior in the bottle, and I’ll speculate as to where it comes from.

In my lexicon, minerality is not an aroma, nor is it a flavor by mouth, though it could be argued to be a taste. It is an energetic buzz in the wine’s finish, almost like an electrical current running through the throat—a nervous raciness similar to acidity, with which it is often confused, but further back in the mouth.

I got my definition originally from Randall Grahm, who once sat me down and showed me what a big difference a tiny drop of a soil amendment mixed-mineral tincture could make in the finish of an otherwise ordinary glass of plonk. It didn’t hurt that my late wife Susie was crazy about minerally wines. Once I got the hang of it, I found the trait easy to spot, and since I taste wines from a lot of different regions and circumstances, I’ve been able to observe tendencies in its source and behavior.

I hope it will not be considered crassly commercial for me to mention that I make some very minerally wines, particularly my WineSmith Faux Chablis and Diamond Ridge Vineyards “Aspects.” Other reliably minerally wines include Chardonnay from the Pinnacles or Santa Cruz Mountains, any Corton, any Sercial from Madeira. Mosels usually have it, and California’s Riesling counterparts usually don’t. The varieties Cabernet Franc and Roussanne tend to be strong in this attribute if not overripe. You can compare true Portuguese ports, always from schist and always minerally, with just about any New World Port (not). And despite the general dearth of examples among big brands, Blackstone Merlot is usually pretty minerally.

My French tutors at Oenodev, the cutting-edge winemaking pioneering firm, focused on this attribute as an indicator of longevity as well as its flipside—reductive problems in youth. It is found to be an attribute of limestone, schist, slate and granitic soils. There is clearly something special about these soils. In 1756, when the Marquêz de Pombal mapped out the world’s first delineated winegrowing region in Portugal, he based the region’s borders on schist and went so far as to behead cheaters passing off grapes from adjacent terroirs such as Douro. Even today, despite soaring prices for Côte d’Or wines, potatoes are chosen over Pinot Noir on soils just a few meters off the limestone.

Worm’s eye view
It turns out that excellent minerality can be obtained on any site if living soil principles are applied. According to French terroir expert Claude Bourguignon, the development of mycorrhizal fungi will facilitate trace mineral uptake by expanding the grape rootlet surface a hundredfold, facilitating a symbiotic exchange of minerals for sugar. Sure enough, when Dr. Stephen Krebs, ace viticulturist at Napa Valley College, worked with me on a project to make Chablis-style Chardonnay at their south Napa site, we worked by promoting a living soil through a no-till policy, and exceptionally minerally wines came into being from a Yolo sandy loam.

Mycorrhizal fungi are tricky to grow. They are fastidious organisms, sensitive to a wide variety of herbicides and pesticides. They depend on a complete established soil ecosystem to flourish. According to Soil Food Web, a healthy earthworm population is the simplest reliable indicator of a balanced soil ecology.

Nobody has pinned down what exactly is happening. In his thesis, Tuscan winemaker Paolo DiMarchi measured high iron content in his very minerally Nebbiolos. But composition by itself doesn’t explain things very well. I worked with California State University, Fresno, sensory guru Susan Rodriguez and famed analytical chemist Barry Gump to try to nail down what minerals might be responsible. It was easy for Rodriguez’s tasting panel to differentiate the wines into minerally and non-minerally groups, but Gump’s broad-spectrum atomic adsorption analysis, which scanned the whole periodic table, couldn’t find any simple compositional driver.

Dr. Heymann at UC Davis has speculated that sulfur compounds cause minerality, but I believe this is backwards: minerality causes reduction, not the other way ’round.

What’s i t all mean?
It is possible that minerality is actually very similar to acidity. An acid is any compound containing hydrogen atoms that are easily detached as ions. Since a hydrogen ion (the atom minus its electron) is just a proton, we can as easily say that acids are substances that can contribute one or more protons to solution. When it enters the mouth, those protons are neutralized by saliva, and our taste buds tell us how much of this reaction is going on through the sensation of tartness. Acidity can be described as a flow of discharged protons sensed by the tongue.

Most of the protons in wine are bound to weak acids like tartaric, malic and lactic, which are in equilibrium with the free protons. These act like a series of reservoirs of acidic protons that get depleted sequentially because the acids are of different strengths, thus forming a kind of ladder of buffer capacities. When wine enters the palate, the first to be depleted is the strongest acid, tartaric, which you taste on the tip of the tongue. Then malic is in the mid palate, then lactic and lastly acetic (which clings to its protons most) is neutralized in the throat.

Just as titratable acidity is sensed on the palate as a flow of protons discharging from binding sites on weak acids, minerality may be a flow of electrons released from various elements of the periodic table as they move to higher valences. Elements such as zinc, aluminum, magnesium and many others have an oxidized state as well as a reduced state. They can move from the former to the latter by giving up an electron, for example Fe2+ —> Fe3+ + e-. This is the rust reaction, and also the main mechanism through which soils retain oxy-genation after tilling.

There are dozens of important mineral redox couples, so you can have a series of electron reservoirs, just like acids except that they donate an electron instead of a proton. Acidic discharge of protons, however, occurs very rapidly, but oxidative discharge is likely to be kinetically inhibited, and it may occur very slowly in the bottle. That’s a good thing, because it would allow minerals to oppose slow oxidation and confer the added longevity we actually observe in minerally wines.

Minerals can play a second role. Redox reactions sometimes require tiny amounts of other mineral catalysts to flow freely. We don’t know all the ins and outs of the right mix of minerals, but it’s plausible that they can essentially constitute a battery in the wine.

But that’s not enough. To get the electrons flowing, the battery needs to be charged. There are a variety of sources of reductive energy, though we don’t know which are strong enough to be effective. Proper ripeness, which results in aggressive phenolic reductivity, is probably such a source. Oak tannins and lees stirring may also contribute to charging up the battery by moving the mineral redox pairs into the electron-loaded reductive state. Fermentation, which every winemaker knows has plenty of reductive strength to create sulfides, may not have the energy differential to reduce metals.

The analogy is clear: electron flow vs. proton flow. It seems plausible that if we can sense one, we can sense the other. It’s the same sensation you get when you lick a fresh battery—a static discharge.

I’m no taste physiologist, and I doubt that anyone really understands how acid perception in taste buds really works, let alone minerality. We do know that acid perception (i.e. tartness and salivary response) is clearly related to titratable acidity, not pH (the free proton count), and not total acidity (the total anion count). Analogously, sensory minerality can be imagined to relate to mineral discharge rather than redox (sometimes called rH) or total mineral composition.

This is, of course, all speculation. It’s the first step in the scientific method, not the last. A hypothesis that is not too outlandish provides some tangible theoretical basis for the phenomenon to be considered and tested. Meantime, I hope we can all stretch our credibility towards an openness to the concept and a curiosity that can only be satisfied by tasting appropriate wines. Experience and training is the only path towards a more unified use of the term.

Forever young
We may not know what causes minerality, but much can still be said about how it behaves. When wine is in this anti-oxidative condition, it will gobble up whatever oxygen gets to it, but the absence of oxygen will cause closing of aromas and formation of H2S and other sulfur compounds favored in the reduced state. That’s why it takes six years for my Faux Chablis to come around and be drinkable.

If this hypothesis is correct, we would predict a decrease in the characteristic during ageing, and this is exactly what we observe. Acidity does not diminish, but minerality does. Over time, the wines become flatter and less vibrant.

Bottom line: If a young white wine has less aroma than you think it should, maybe even a little stinky, but you see lots of flavor by mouth and it tingles in the finish, you might lay some of it down to see if it improves with age. But consumers are largely new to wine and don’t have cellars, and thus are untrained in evaluating its ageing trajectory.

We have a problem, Houston. As we move towards curtailed petrochemicals, reduced tillage, deeper roots, cover crops, proper ripeness and screwcaps, we will need to do what we can to educate consumers that youthful reduction is a mark of quality.

By contrast, red grapes subjected to excessive hangtime will have much lower reductive strength and will not appear minerally. Moreover, the bitterness of high alcohol will obscure the sensation even if it is present. These wines, which exemplify current trends in Napa Cab top-end collectibles, will lack both phenolic vigor and mineral energy; thus, they will be short lived. P.T. Barnum said, “Nobody ever went broke underestimating the intelligence of the American people.” But is it wise to bet the farm that nobody will notice the Emperor has no clothes?

The advantages of California for growing grapes are innumerable, yet we have three major disadvantages. The first is that our days are too short in summer, owing to our latitude, which may inhibit the development of great Rieslings (there’s not much to be done about that). Second is our lack of limestone, slate and schist, though we do have granite. The third is our scarcity of water, which limits us from dry farming and p resents challenges to maintaining living soil.

There are many lovers of distinctive, terroir-driven wines who have quite simply given up on California wines. I believe their judgment is premature. As some of us begin to take more seriously the challenge to make interesting wines with vitality and life, bottled poetry is on the rise. Cracking the code on minerality is an essential milestone on that path.

Clark Smith is winemaker for WineSmith, founder of the wine technology firm Vinovation. He lectures widely on an ancient yet innovative view of American winemaking. To comment on this article, e-mail edit@winesandvines.com.