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Brettanomyces in wine - fault or character? Horse leather and barnyard in the glass

Some red wines smell of something hard to call fruit: a barnyard, horse leather, a sticking plaster, smoke, and sometimes downright sweat. For some it is a disgusting fault, for others a fascinating depth and character. Behind these notes stand yeasts called Brettanomyces, Brett for short, one of the most controversial microorganisms in the world of wine. They divide winemakers and tasters into two camps: those who see spoilage in them, and those who value the complexity they add. The truth, as usual, lies in the dose. Here is a guide to Brettanomyces: what they are, what notes they give, where they come from, and where the thin line runs between character and fault.

What Brettanomyces is

Brettanomyces is a genus of wild yeast, different from the ones that normally lead the fermentation of wine. The most important species is Brettanomyces bruxellensis. Unlike the noble fermentation yeasts, Brett does not make wine but modifies it, producing characteristic, strongly smelling compounds. These yeasts can survive in wine where others cannot, and act slowly, often after fermentation, during ageing in the barrel. This is why Brett is mainly associated with red wines aged long in wood. Brettanomyces is one of the most recognisable and most discussed microorganisms in winemaking. Understanding that it is a wild yeast that modifies wine, rather than creating it, is the starting point for the whole discussion of whether it is a fault or character.

Where these notes come from

The characteristic Brett aromas come from a particular chemistry. Brettanomyces metabolises certain compounds present in wine and turns them into volatile phenols, that is fragrant substances responsible for these controversial notes. The most important are two: one giving barnyard and animal smells, the other giving smoky and spicy notes. It is these volatile phenols, and not the yeast itself, that we smell in the glass. They form when Brett develops in wine, especially during slow ageing in the barrel, where it has time and favourable conditions. This is why the intensity of these notes depends on how far Brett has developed. The more of these compounds, the stronger, more dominant the character. Understanding that particular phenols stand behind the smell helps grasp why Brett varies so much in intensity.

The two key compounds

The character of Brett is mainly down to two volatile phenols. The first is a compound giving notes described as barnyard, horse leather, animal or sticking plaster, and at higher concentrations downright medicinal. The second is a compound giving smoky, spicy notes resembling clove. It is their presence, proportions and concentration that decide how a wine smells. In different wines these compounds occur in different amounts and ratios, creating a wide range of aromas, from a subtle, spicy depth to an overwhelming barnyard. Brett aromas are described very colourfully: from barnyard, bandage and bacon to the smell of a sweaty horse saddle. These two compounds are the chemical heart of the whole phenomenon. Recognising them helps understand why one wine with Brett smells intriguingly spicy and another repulsively animal.

A table: Brett compounds and their notes

Let us gather the most important compounds in one place:

The table shows that Brett is not one smell but a mixture, in which the proportions of the two compounds decide whether a wine comes off as rather spicy and deep, or animal and off-putting.

Fault or character - the great debate

Whether Brett is a fault or a desirable complexity is one of the most polarising debates in winemaking. The argument of one side is that, at low concentrations, Brett compounds can add complexity to wine and even an impression of terroir, a spicy, wild depth that makes the wine more interesting. The argument of the other side is that Brett obscures the fruit and the character of the grape and the expression of place, so the same wine without it would be better. Both sides are right, because it all depends on the dose. A subtle trace of Brett can be fascinating, and its excess overwhelming. This is why the debate never ends. Wine with Brett divides people like little else, and the same glass is a masterpiece for one and spoilage for another.

A matter of dose and threshold

The key to the whole debate is the dose. At low concentrations Brett adds a subtle, spicy, wild note that many value as depth and character. But once its compounds cross a certain threshold, they begin to dominate, drown out the fruit, and the wine tips toward a clear fault. Research indicates that once the total concentration of the key compounds crosses a certain level, the Brett character, that is the barnyard, animal, spicy or smoky notes, becomes undesirable. Below this threshold it can be interesting, above it spoils the wine. This is why one cannot say Brett is simply good or bad. It all depends on how much of it there is. The same substance in a small dose adds, and in a large one takes away. The line between character and fault is fluid and subjective.

Where Brett comes from in wine

Brettanomyces is a wild yeast that dwells in the cellar, on equipment, and especially in old, porous barrels, where it is hard to remove. It develops most readily in red wines ageing long in wood, because there it has time and favourable conditions. It is favoured by insufficient cellar hygiene, low doses of sulfur and slow, long maturation. This is why Brett appears more often in wines of less control, including many natural wines, where intervention and sulfur are limited. We cover this movement more in natural, organic and biodynamic wine. Brett is partly the effect of the philosophy of minimal intervention, which consciously allows wild processes. For some it is a risk, for others a source of character.

Brett and wild yeast

It is worth linking Brett to the broader subject of wild yeast. Brettanomyces is precisely a wild yeast, only undesirable in the classic sense, because it does not ferment wine but modifies it after the fact. This shows the thin line in winemaking focused on naturalness: the same wild processes that give complexity and expression of terroir also carry the risk of Brett and other faults developing. A winemaker betting on wildness must therefore balance between character and spoilage. We cover this choice more in wild and cultured yeast. Brett is an extreme example of how nature can both enrich and spoil. It is a reminder that in wine the realm of wildness is fascinating but also dangerous, and the line can be as thin as a hair.

How to recognise Brett in the glass

For a taster, recognising Brett is a valuable skill. Look for notes that do not fit the fruit: barnyard, horse leather, sticking plaster, smoke, sometimes sweat or medicine. If a red wine smells of something animal and wild, and not just fruit and barrel, that is a strong clue Brett stands behind it. The mere presence of these notes does not decide whether it is a fault, because the intensity and your own preferences matter. It is worth consciously comparing a wine with clear Brett to a clean, fruity wine, to learn to catch it. We cover recognising faults more in wine faults. Over time you will learn not only to recognise Brett but also to decide whether, in a given wine, you like it or it bothers you.

The essentials in brief

Let us gather it up. Brettanomyces is a wild yeast that does not make wine but modifies it, producing volatile phenols with notes of barnyard, horse leather, plaster, smoke and spice. Two compounds are mainly responsible: one animal, the other spicy-smoky, and their proportions and concentration decide the character. Whether Brett is a fault or an asset is a great, unresolvable debate, because it all depends on the dose: in small amounts it adds complexity, in large ones it drowns out the fruit and becomes a fault. Brett dwells in the cellar and old barrels, more often in red wines aged long and in natural wines. Now you know where these wild, animal notes come from and why they divide wine lovers like little else.

Note every wine in GustoNote - the style, the notes you sense and your own judgement of whether you like them. Over time you will learn to recognise Brett yourself and decide where, for you, character ends and fault begins.