This article is the first in a planned series by accomplished winemaker Matt Brain on technical aspects of winemaking. Brain is the winemaker for Alpha Omega winery in Napa Valley, and has lectured in the enology and viticulture programs at both Cal Poly San Luis Obispo and Fresno State University in California. He earned bachelor of science and master of science degrees in biosciences at Sam Houston State University, where he was a decathlete. Matt also earned a winemaking degree from the University of California, Davis.
As a winemaker, people often ask me what I like to drink. I try to keep my palate sharp by exposing myself to as many different varieties and regions as possible while always searching for balance, expression … and complexity!
Minerality is my favorite kind of complexity. It can give the impression of rocks and earth, but also of the ocean or smoke-like aromas, and it has been a frequent topic in wine publications recently. Flint, wet stones, gun powder, oh my! Minerality is alluring and elusive because we aren’t always exactly sure what it is. We do know that its origins can differ and that it is almost exclusively found in the highest-quality wines.
Mineral notes tend to be subtle and are only expressed if a wine is crafted with precision and care. Starting with the fruit, dominant green characters (from underripe grapes) as well as plummy, raisin notes (from overly ripe fruit) are almost never found in unison with minerality.
Grapes must be ripe at the time of harvest but also must have retained good acidity, which almost always correlates with mineral wines. Wines that are overly oaked, highly volatile from stray microbial activity or oxidized from lack of attention almost never allow mineral aromas to be detected. Extremely fruity or floral aromatic profiles can also mask underlying minerality.
At the proper balanced concentration and unaltered by contact with air, sulfides are big contributors to the impression of minerality. They are produced by yeast during fermentation, as opposed to sulfites, which are added to control microbes.
Efforts to control oxygen, such as topping up barrels and gassing tanks with inert argon gas before moving wine into them, also set the stage for long-term aging benefits in bottle, along with bright acidity. Wines that age well also tend to be good candidates for mineral expression.
Glutathione, a sulfur-containing tripeptide, may also correlate with mineral expression, as it is a powerful antioxidant that helps keep aromatics in a reductive state. In my experiments boosting glutathione artificially in lab settings, I was able to produce some flinty mineral notes, although I often overshot the mark and produced quite off-putting, skunky, swamp-like concoctions when levels became too high.
Barrels are very important when expressing minerality in wine. Larger aging vessels like puncheons and foudres, tighter-grained oak from cooler forest regions and older oak tend to produce better minerality, as the larger volumes help slow oxidation and the pores of older wood tend to be more clogged with tartrates and polyphenols, further contributing to a reductive aging state.
Highly toasted barrels can definitely impart a smoky, flinty minerality if utsed correctly. Conversely, vanilla and coconut from wide-grain, new American oak will definitely mask mineral aromas.
ORGANICS, NOT ROCKS
But does minerality in wine come directly from the soil, as we have been long led to believe? Producers touting their own specific soils have long based their reputations on it, but the science says it’s much more complicated than that. The molecules in rocks are quite resistant to water as a solvent and extract very slowly over time, so they contribute only a very minor proportion of the minerals and nutrients found in soil. The decomposition of organic matter in the topsoil contributes the vast majority.
Also, the uptake of minerals by a grapevine is constrained by the root’s preference for, and ability to take in, certain molecules over others. Researchers motivated by defining terroir via chemical analyses have long sought to correlate soil minerals and grape minerals but have had very little success.
In short, minerality in the soil does not seem to be the major contributor to minerality in the glass. Soils that are thin or deprived of organic matter, however, may contribute to a less fruity and floral grape, allowing mineral characters to be more easily identified.
In fact, newer theories that propose microbiome differences between soil types have grabbed my attention. We know that microbes directly proximate to the vine root, like microbes in our own guts, highly impact the digestion and absorption of nutrients and minerals from their environment, and I anxiously await a study that correlates microbe populations with mineral-producing vineyard sites.
Winemakers in Chablis agree that the winemaking techniques described above help lead to mineral wines, along with clean racking, inoculation with commercial yeast and lees contact.
Finally, I believe that winemakers should aim to express minerality, and absolutely strive for it because minerality will always be hard to express on a high-volume or commercial scale. It will set those wines apart in the glass and also catch the next generation as we lure them with this technical concept that gives a framework for stories that will no doubt continue to include soil.
The winemakers I know get pretty excited and passionate when they find minerality in the wines they drink. They can do us all favor by trying even harder to express it.
– Matt Brain
