Brettanomyces: The wine fault that almost defined natural wine.

You often hear the same line in natural wine bars and from natural wine critics: “You know, it’s a natural wine”. It describes an animal’s taste and the absence of fruit flavors.

It’s usually said after someone notices an odd, animal like aroma and the absence of fresh fruit. You are at a diner, sniffing a glass of wine, and the waiter apologetically says to you, “It’s a natural wine, you know.” The phrase is used by fans and detractors alike, and it has become a caricature: natural wines should smell of the farm, the stable, or wild animals.

It is a caricature of a farm, cow, horse, fox, and mousse; this imaginary bestiary is used in a caricatural way to describe natural wine. Some will swear that it is the natural way of wine, while others will say the opposite.

Only a tiny minority of bottles fit that description, yet when they do appear in trendy bars in Paris, New York, London, or Amsterdam, they reinforce a cliché that can feel elitist.

To understand this reputation, we first need to know what causes those strange smells, then examine how the fault entered the short history of the natural wine movement and why it persists today.

The aromas of this wine fault that range from “farm” to “band aid” come from a single source: the rogue yeast Brettanomyces, commonly called Brett. There are two species most relevant to wine: Brettanomyces bruxellensis and Brettanomyces anomalus, with B. bruxellensis being the more common in cellars. Ironically, Brett was first associated with spontaneous fermentations in Belgian lambic beers rather than wine.

Unlike the familiar fermenting yeast Saccharomyces cerevisiae, Brett is extremely hardy and can metabolize a wider range of compounds than simple sugars, and can resist various adverse situations.

Brett produces volatile phenols and other compounds that create the spectrum of aromas around animals and farms.

• 4 Ethylphenol (4 EP) — horsey, barnyard, medicinal, “band aid”; the most commonly measured marker of Brett.
• 4 Ethylguaiacol (4 EG) — spicy, smoky, clove like.
• 4 Ethylcatechol — leathery, stable or “farm” character. Guaiacol — smoky.
• Isovaleric acid — sweaty, rancid cheese.
• 2 Phenylethanol — floral, honeyed notes.
• Isoamyl alcohol — solvent like, sharp.
• Ethyl decanoate — fruity, waxy esters.

On paper, some of these descriptors don’t sound so bad, some seem enjoyable, which is why low levels of Brett can be perceived as complexity rather than a wine fault.

Brett is a very strange wine fault. It’s not universal and binary, unlike other wine faults. A large proportion of people can’t detect it, and depending on the level, some people like it and some can stand it. It’s a fault, and, on a small level, it’s part of the identity of some wines.

Brett exists in every vineyard and cellar; it is not unique to natural wine. Studies and industry surveys suggest roughly 5–7% of commercial bottles show detectable Brett at fault levels (rejected bottles only, we can expect a higher percentage with bottles where Brett are not an issue), though figures vary by region and methodology.

Several conditions favor its growth: high pH (low acidity), warm cellar temperatures, high alcohol, turbidity and lees contact, porous oak barrels that shelter persistent colonies. Overripe fruit, that give too much residual sugar, and poor sanitation also increase the risk of having a bretty wine.

Because Brett is everywhere (from the cellar to the vine) and very resilient, limiting them can be challenging. Practical measures include harvesting at balanced ripeness to avoid excessive sugar and alcohol, preserving acidity. Cellar temperature is also important; maintaining cool cellar temperatures is crucial to limiting the effect of the rogue yeast (it prevents other wine faults and ensures good fermentation).

Rigorous sanitation in the cellar is also important. If Brett is already present in the Cellar, this is an important step. Stainless steel tanks are easier to keep Brett free than porous oak; barrels require rotation, testing, and sometimes reconditioning, and removing Brett colonies from them is almost impossible.

Every wine can have Brett at some level; it is a common fault. It occurs more in red wine, but also in orange wine, and less in white wine because of its high acidity. But what about the reputation of natural wine smelling the farm?

Early natural wine pioneers in the 1980s and 1990s often worked in cooler regions such as Beaujolais and the Loire and relied on traditional cellar craft to make low SO₂ wines with high acidity. Brett driven aromas were less common.

From around 2010, a new generation of natural winemakers experimented more widely, with orange wines, extended skin contact, longer fermentation, oxidation, along with looser cellar regimes. Some of those wines showed more farmy character. The result was a visible minority of bottles that shaped public perception. These years was also the moment where natural gained popularity, and natural wine get this label of farm smelly wine.

Today, there are no definitive global statistics, but detectable Brett (at rejection level) remains a minority phenomenon, even in natural wine. There are fewer and fewer natural wine with this kind of fault that triggers rejection, but the phenomena still exist.

With climate change, however, the risk is rising warmer vintages tend to produce riper fruit, higher alcohol and lower acidity, conditions that favor Brett growth across all winemaking styles.

However, natural wine is less associated with the farm/horse aromas, but the reputation still persists.

Brettanomyces is a complex, context dependent presence in wine: at low levels, it can add rustic nuance, but at higher concentrations, it erodes fruit and signals cellar issues. The stereotype that “natural wine smells like a farm” rests on a small subset of bottles and historical and stylistic choices, rather than an inherent truth about natural winemaking.