Are There Rocks in My Wine?

A down-to-earth examination of “minerality”

Early in this century, rather like a new comet appearing in the night sky, the phenomenon of minerality in wine arrived,” observes Alex Maltman in his literally groundbreaking book, “Vineyard, Rocks, & Soils: The Wine Lover’s Guide to Geology.” In fact, describing a wine’s smell or flavor as minerally, has been around at least since the mid-20th century — personally, in wine notes from March of 1976, referring to a 1967 Chablis Grand Cru Vaudésir from Domaine Mary Drouhin, I even used the term. So, what’s the big deal?

We know Pinot scented with violets or roses, or a rosé tasting of strawberries and watermelon doesn’t mean these flowers and fruits actually flavor the wines. And yet, many people seem to think any perceived minerality originates from said minerals.

Evidence supporting this belief comes from the rise in “terroirism,” the idea that the land, among other factors, influences the taste of wine. Here, for example, is the description of a 2017 Saumur-Champigny “Les Mémoires” in the March 2019 Kermit Lynch Newsletter: “And not only raspberries, but blood orange, fine tannins and incredible minerality. The stone that once built a château now fills bottles.”

With a series of articles starting in 2013, Maltman, a retired professor of earth sciences at Aberystwyth University in Wales who also grows grapes and makes wine, has been the leading voice in challenging the notion of “rocks” in your wine. He insists, “Stones are inert…so scientifically, it’s hard to see how they can imbue a wine with certain flavors.” With the publication of his 2018 book, Maltman offers winemakers and enthusiasts a crash course in geology with the goal of dispelling misconceptions.

He begins by breaking down a vineyard into the atoms that constitute the chemical elements, of which oxygen, silicon, aluminum, iron, calcium, magnesium, potassium and sodium dominate. These elements form the geologic minerals from which rocks are made. The rocks are classified by how they were created — for example, igneous rocks from molten lava; sedimentary from the aggregation of particles; metamorphic from a variety of physical changes.

Vineyards rest on bedrock, which contributes to the formation of subsoil above. The top layer is soil, a mixture of loose sediment resulting from the weathering of rocks and decaying biological material called humus; this is the arena where the main action takes place. Maltman explains, “It is the moisture and humus that make it soil, as both are required by plants.”

The vines receive essential nutrients for growth from humus in the soil, not sediment. Some essential nutrients are extracted from geologic minerals by mycorrhizal fungi on the root hairs and absorbed by the roots. Others, confusingly called mineral nutrients, are released into the soil’s water. One way this happens, Maltman tells us, is by “the weathering processes that unlock and dissolve the constituent elements of geologic minerals … at or near the ground surface … most nutrition typically comes from the top few centimeters … of soil.” Vine roots go farther down in search of water, not nutrients. 

Cation exchange is the process by which nutrients are absorbed by the vines’ fine roots. When an atom or molecule loses one or more electrons, it becomes a positively charged ion called a cation. An anion is a negatively charged ion. “Minerals are usually left with some superfluous charge on their outermost surfaces, and this is particularly the case in clay minerals,” writes Maltman. The water in the soil contains cations of different minerals that jockey to achieve neutrality by attaching mainly to the surface of a humus or clay particle, which hosts free electrons. A more strongly positive cation will shove aside a weaker one and be held more firmly. A vine pushes hydrogen cations into the soil water, which, because of their small size, knock loose the other ions that are then selectively taken in by its roots.

O.K., so are these mineral nutrients the source of minerality in wine? Maltman enlightens us: “Although we usually think of nutrients as mineral in origin, in practice, much of the nutrition … comes from organic material.” Furthermore, most of the nutrients contain no smell and little taste. Chlorine is an interesting exception. As a gas molecule, its smell is readily recognized, but as an anion, it is odorless and will impart a salty taste if sodium is also present. In addition, the process of turning grapes into wine alters the nutrient content in the finished product. Moreover, the concentration of these nutrients are measured in parts per million. “In normal wines, mineral nutrients typically comprise less than 0.2%, in total,” asserts Maltman. It is the organic compounds in wine that can be detected in even smaller quantities and generally overwhelm any impact the nutrients might have. The detection thresholds for inorganic compounds in wine have not been determined. Nevertheless, Maltman concludes, “Describing a wine as mineral or as possessing minerality should not be referring to actual minerals — geologic or nutrient — but should be recalling … some mental association.” In other words, there are no rocks in your glass, but metaphorically-speaking, there may be some in your head.

So, what is it that we all seem to be sensing? While science is at work examining what the story is, very little is known. Maltman admits, “A pervasive difficulty is isolating which factor or factors are most significant in nature: Is it necessarily the geology that’s causing the effect?” He summarizes some recent findings, including one relating soil pH in the Willamette Valley to the flavors of Pinot Noir. It was found that the lower the pH, the rounder, more complex the flavors. A compelling connection between bedrock and wine taste was published by German researchers in 2011. Rieslings from two vineyards, one on basalt and the other on sandstone, were compared over two vintages and displayed systematic differences, but it is not known how the rocks cause them.

Throughout his indispensable book, Maltman reminds us that rocks don’t dissolve and, despite the ubiquitous use of the term “minerality” in tasting notes, cannot impart any taste or aroma to a wine. This is known and easily demonstrated. What remains mysterious is how else geology can influence a wine. So, while part of the joy of wine tasting is identifying the various sensations, such as salinity and chalk, you should realize the descriptors are not meant literally as evidence that these are actually in your glass. You are certainly entitled to your own metaphors but not your own facts.