Acid is existential to wine, essential to a wine’s lifecycle from grape to glass. Without acid, wine wouldn’t ferment properly, wouldn’t age well, and ultimately wouldn’t refresh — which is the job it’s hired to do.
A single glass of wine contains many different acids, and each makes a unique contribution to the wine’s charm. Some acids are stable, while others are highly volatile. And it’s that latter category that has many wine lovers scratching their heads. Isn’t volatile acidity bad?
In short: no. At length: read on.
Wine’s acidity starts in the grapes
Grapes have abundant natural acidity, although the amounts vary by variety. Nero d’Avola, Cinsault, and Pineau d’Aunis are examples of highly acidic red grapes; Albariño, Chardonnay, and Grüner Veltliner are highly acidic whites.
The acidity develops over the season as the berries mature, and this process is influenced by heat. Cooler climates, cooler seasons, and early harvests preserve acidity, while warmth and late harvests tame it. Even high-acid varieties become flabby if there’s a heat spike right before picking, an issue that’s a growing concern during climate change.
Once grapes arrive at a winery, a winemaker labors to preserve their acidity through fermentation, aging, and bottling.
Firm acidity helps prevent the growth of unwanted microorganisms while promoting beneficial strains. It’s often permissible to add acid, but it’s generally better just to pick the grapes when they’re still tart.
A complex blend of acids
Once a wine has finished fermenting and aging, it contains a mix of different acids. These fall into two major categories: fixed, or stable acids, and volatile acids.
The fixed acids are so-called because, physically speaking, they don’t evaporate when a liquid is heated — they stay in solution. Specifically, these are tartaric, malic, lactic, citric, and succinic acid. People sense fixed acids texturally — in other words, with their tongues, not their noses. A wine lacking fixed acids will feel flabby and flat, and maybe even soapy.
- Tartaric acid gives wine structure and astringency.
- Malic acid gives wine a tangy bite.
- Gentler lactic acid is produced by converting malic acid via a bacterial process called malolactic fermentation. Nearly all red wines and some white wines, like creamy Chardonnay, go through that softening step.
Meanwhile, the volatile acids are present in much smaller quantities. People sense volatile acids with their noses. Although technically wine may contain eight different volatile acids, the principal and most common is acetic acid, and it’s present in all wines. This is the same acid found in vinegar, but even vinegar has only a small amount of it, usually about 40 g/L. In wine the fraction is smaller still, usually less than 1 g/L.
So, yes, all wine has some volatile acidity
Volatile acidity, or VA, is produced at many stages of winemaking, by both microbial and chemical processes. Acetic acid is created during the main fermentation by the very microbes the winemakers seek to encourage to convert sugar into alcohol. In other words, you can’t make wine without making some acetic acid.
However, acetic acid can also be generated by spoilage organisms that come in on the grapes or riding on the backs of fruit flies. Oxygen is key to acetic acid production, and overexposure is a major reason wines end up with too much of the stuff.
Fault or finesse?
An elevated level of volatile acidity is considered a fault, and it’s certainly off-putting. Who wants to drink a wine that smells like salad dressing?
But at low levels, VA can be attractive. Chianti Classico, Barolo, and similar Italian reds are sometimes described as having a pleasant balsamic note, which is earned from oxidative barrel aging.
Often, high levels of acetic acid are accompanied by an excess of another volatile molecule, ethyl acetate. It has the pungent aroma of nail polish remover. It’s technically not an acid, but it often gets lumped with acetic acid when we speak about VA in wine.
At high concentrations, these volatile compounds conspire to make a wine that’s aromatically distracting at best and downright unpleasant at worst. Elevated levels can even deliver a burning sensation in the throat.
But again, it’s not all bad news. VA is often present at high levels in sweet wines, especially botrytised wines like Sauternes, but then it’s usually considered part of the wine’s typicity rather than a fault.
Keeping it in balance
Although winemakers can remove some volatile acids after the fact, via filtration or reverse osmosis, those treatments are expensive and can deaden a wine.
The best strategy is to prevent them from spinning out of control in the first place and turning the wine faulty. That’s done by using unblemished grapes, maintaining spotless equipment, fermenting at cooler temperatures, keeping the pH low, using sulfite judiciously, and limiting oxygen exposure. Call this the original clean wine.
Producers who work naturally, especially those who forgo all sulfite additions, must attend even more ferociously to those principles. Think about that next time you pour yourself a glass of flawless natural wine.