Brettanomyces is a yeast that can create a range of aromas and tastes that change the wine. It was first discovered in 1904 by the Carlsberg brewery who were investigating the reason for spoilage of British Ales, this lead to its name Brettanomyces, which is Greek for British fungus. The common aromas of Brett include: Stables, Barnyard, Band-aid, Medicinal, Cheesy, Sweaty, and Rancid.
As with most aromas in wine, different people have different sensitivities to Brett and the aromas are dependent on the type of wine. Red wines tend to have more of a problem with Brett than whites.
To some people, low levels of Brett can add complexity to the wine, while higher levels can detract from the wine. It is a case of personal opinion. The compounds responsible for contributing certain sensory characters to wine are;
• 4-ethylphenol: Band-aids, barnyard, horse stable, antiseptic
• 4-ethylguaiacol: Bacon, spice, cloves, smokey
• isovaleric acid: Sweaty saddle, cheese, rancidity
The Australian Wine and Research Institute (AWRI ) have started a research project into what has been an under researched area of wine production. But it is known that Sulphur is particularly potent at killing Brettanomyces. It appears that the recent trend for minimalist wine making and Natural Wine making (where no sulphur is added) has resulted in an increased incidence of Brett, particularly if the wine has high pH (low acidity). Apart from sulphur it can be managed after wine making by filtration, which removes the yeast. However, there is anecdotal evidence that filtered wines that are sound at the time of bottling can randomly become infected with Brettanomyces, probably as a result of the bottled wine containing residual sugar and being stored in warm conditions.
Brettanomyces is at its most dangerous after alcoholic fermentation, as it cannot compete with the wines alcoholic yeast (Saccharomyces cerevisiae). It is widely acknowledged that the majority of wines with Brett character, became that way during the period of barrel maturation, particularly if second use (or older) oak barrels were used. Barrels are not the only entrance path, Brett can also enter a winery by being carried by insects such as fruit flys.
If Brett lives in a glucose rich environment it tends to produce acetic acid, or vinegar. So sweet and semi sweet wines that have second fermentation in bottle can have a whiff of vinegar if they develop Brett in a bottle.