Many winemakers and tasters will exclaim using a vulgar descriptor to state their displeasure of barnyard smell-infected wines.

Personal taste and preferences notwithstanding, if a barnyard aroma is subtle, it is considered acceptable, and is often desired by many winemakers who assert that it adds complexity to certain styles of wines. But when the aromas are overpowered by a strong barnyard smell, then it is considered a fault. And such is the nature of the indigenous Brettanomyces yeast, more affectionately known as Brett; it can have both a positive influence on wine, for aficionados who are partial to it, or it can spoil wine outright.

Brettanomyces bruxellensis yeast is the anamorph (non-sporulating form) of Dekkera bruxellensis yeast, which causes like-it-or-not barnyard aromas, or medicinal, sweaty, “Band-Aid” and rancid odors; these are the results from three main compounds, 4-ethylphenol and 4-ethylguaiacol, often referred to as 4EP/4EG, and 3-methylbutyric acid, also known as isovaleric acid, which can be detected at very low concentrations.

4EP, the most significant compound as it is responsible for the barnyard smell, is the product of the decarboxylation (COO is removed) of p-coumaric acid and reduction of its intermediate. The main source of p-coumaric acid in Brett-affected wines is toasted oak wood, as yeasts start feeding on cellobiose, a sweet disaccharide produced from cellulose hydrolysis during the oak toasting process. The reaction is very slow and can take several months or longer, making detection difficult. Sulfite is not an effective treatment against Dekkera/Brettanomyces in barrels as the yeasts can hide deep in wood cracks and crevices where sulfur dioxide (SO2) cannot reach. Therefore, it is highly recommended to sterile filter barrel-aged wines prior to bottling to remove the yeast, particularly when positive identification cannot be achieved.

Commercial wineries use dimethyl dicarbonate (DMDC) to treat contaminated barrels; however, this is beyond the domain of home winemaking and small wineries as specialized equipment that can only be handled by trained personnel is required.

Brettanomyces/Dekkera are glucophilic yeasts, which means that they also feed on glucose, very small amounts of glucose, that is, and  convert it into acetic acid, making dry wines at risk too. At high glucose concentrations, such as in sweet wine, volatile acidity (VA) can increase significantly and cause acetic spoilage, detected as the familiar smell of vinegar.

The challenge with Brettanomyces yeast is that it easily thrives throughout the winery and is difficult to eradicate, particularly that it seems to adapt to changing environments. It is an anaerobic microorganism, meaning that it thrives in the absence of oxygen. Specifically, it thrives in oak barrels feeding on cellulose, or in wines with residual sugar, high pH or high polyphenol concentration. Red wines are inherently more susceptible to Brett owing to their higher pH and high polyphenol concentration. Residual sugar is usually not a problem in whites because (if) they are well protected with sulfite; in reds, however, where the style is meant to be a dry wine, residual sugar can be a source of food for Brett.

Eradicating Brettanomyces is nearly impossible except by sterile filtration. But Brett is sensitive to free SO2 (from sulfite), and so it is easily preventable. Here are some tips on how best to avoid a Brett infection in your winery and wines.

o Ferment red wines to dryness as much as possible for dry-style red wines.

o Maintain a low pH in the recommended range for your style of wine.

o Use SO2 judiciously, taking into account the wine’s pH, being extra vigilant with high-phenol wines.

o Do not age wine on the lees for too long as they are a source of nutrients for Brett.

o Store wine in a cool cellar, ideally at 13° C (55° F).

o Keep carboys, tanks, barrels and all other containers well topped up.

o Sterile filter wines prior to bottling.