Brettanomyces is a non-spore forming genus of yeast in the family Saccharomycetaceae, and is often colloquially referred to as "Brett". The genus name Dekkera is used interchangeably with Brettanomyces, as it describes the teleomorph or spore forming form of the yeast. The cellular morphology of the yeast can vary from ovoid to long "sausage" shaped cells. The yeast is acidogenic, and when grown on glucose rich media produces large amounts of acetic acid. Brettanomyces is important to both the brewing and wine industries due to the sensory compounds it produces.
In the wild, Brettanomyces lives on the skins of fruit. The strain Brettanomyces claussenii was discovered at the Carlsberg brewery in 1904 by N. Hjelte Claussen, who was investigating it as a cause of spoilage in English ales. The term Brettanomyces comes from the Greek for "British fungus."
When Brettanomyces grows in wine it produces several compounds that can alter the palate and bouquet. At low levels some winemakers agree that the presence of these compounds has a positive effect on wine, contributing to complexity, and giving an aged character to some young red wines. Many wines even rely on Brettanomyces to give their distinctive character such as in Château Musar. However when the levels of the sensory compounds greatly exceed the sensory threshold, their perception is almost always negative. The sensory threshold can differ between individuals, and some find the compounds more unattractive than others. While it can be desirable at lower levels, there is no guarantee that high levels will not be produced. As Brettanomyces can potentially spoil a wine it is generally seen as a wine spoilage yeast, and its presence in wine as a wine fault.
Wines that have been contaminated with Brettanomyces taints are often referred to as "Bretty," "metallic," or as having "Brett character." Brettanomyces taint in wine is also sometimes incorrectly identified as cork taint.
Sensory compounds 
The compounds responsible contributing certain sensory characters to wine are;
- 4-ethylphenol: Band-aids, barnyard, horse stable, antiseptic
- 4-ethylguaiacol: Bacon, spice, cloves, smoky
- isovaleric acid: Sweaty saddle, cheese, rancidity
These compounds can impart completely different sensory properties to a wine when they are present in different ratios.
Origins in the winery 
Brettanomyces is most associated with barrel aged red wines, but has also been found in Chardonnay and Sauvignon blanc. In some cases the yeast has caused contamination in sparkling wines produced by the Méthode champenoise when en tirage. It is thought Brettanomyces can be introduced to a winery by insect vectors such as fruit flies, or by purchasing Brett contaminated wine barrels. The ability to metabolise the disaccharide cellobiose, along with the irregular surface of a barrel interior, provide ideal conditions for Brettanomyces growth. Once the yeast is in a winery it is hard to eradicate and is spread readily by unsanitised equipment.
Control measures 
The growth of Brettanomyces is best controlled by the addition of sulfur dioxide, to which the yeast is particularly sensitive. The addition of other sterilising compounds such as dimethyl dicarbonate often has a similar effect. Alternatively the wine can be bottled after sterile filtration, which physically removes the yeast. Wines that are vinified to low residual sugar levels, such as <1.0g/L, are also less likely to be spoiled as the main growth substrate has been limited. However growth has been reported at levels below this and it is assumed that the yeast can use other substrates.
In most beer styles Brettanomyces is viewed as a contaminant and the characteristics it imparts are considered unwelcome "off-flavours." However, in some styles, particularly certain traditional Belgian ales, it is appreciated and encouraged. Lambic and gueuze owe their unique flavour profiles to Brettanomyces, as do wild yeast saison or farmhouse styles; and it is also found in Oud Bruin and Flanders red ale. Commercial examples of these styles include Liefmans Brown Ale, Rodenbach Grand Cru, and Duchesse de Bourgogne. The Orval Trappist monastery is unique in crafting the only Trappist beer with Brettanomyces characteristics. In Orval's case, the brewers add the yeast to the beer at bottling.
Several American craft breweries use Brettanomyces in their beers. This use began with a renewed interest in Belgian style ales and later formed new styles altogether (Brewers Association, 2007 Great American Beer Festival Style Guidelines, section 13a, 16). Some breweries use 100% Brettanomyces for the fermentation of some of their beers, and omit Saccharomyces from the recipe. It is common for American brewers that use Brettanomyces to also include lactic acid producing bacteria such as Lactobacillus, and Pediococcus in order to provide sourness to the beer. While Brett is sometimes pitched into the fermenter, aging in wood barrels previously infected with Brettanomyces is another method used to impart the complexity and sourness contributed by these strains of yeast. A prime example of this is Captain Lawrence Brewing Company's Rosso e Marrone, an award winning Oud Bruin aged in wine barrels harboring these yeasts. Examples of American breweries that use Brettanomyces in their beer include DESTIHL Brewery (in their Saint Dekkera® Reserve Sour Ales), Ithaca Beer Company (in their Brute), Russian River Brewing Company, Deschutes Brewery, Lost Abbey, Captain Lawrence Brewing Company, New Belgium Brewing Company, Green Flash Brewing Company (in their Rayon Vert), Goose Island Beer Company (in their Matilda and Sofie), Boulevard Brewing Company (in their Saison-Brett), Allagash Brewing Company, Brewery Ommegang (in their Ommegeddon and Bière de Mars), Jolly Pumpkin Artisan Ales, Victory Brewing Company (in Wild Devil), Saint Somewhere Brewing, Seventh Sun Brewing, Surly Brewing Company (in their Pentagram and Five), Stillwater Artisanal Ales, O'Dell Brewing (Shenanigans), Avery Brewing Company (in Depuceleuse), Perennianl Artisan Ales (in their Savant Beersel), Epic Brewing Company (in their Elder Brett), Weyerbacher Brewing Company (in their Sour Black), Kane Brewing Company, and 4 Hands Brewing Company (in Cuvee Ange). Crooked Stave, in Colorado, has a Wild Wild Brett series, which brews a 100% brett beers for every color of the visible spectrum. Sante Adairius in Capitola California has quickly become a leader of the movement of Farmhouse Ales with their small batch experiments with Brett and Lacto finished Brews.
While most stouts achieve their sour tang through the use of acidulated malt, roasted barley, or — in the case of "milk stouts" — lactose and incipient lactic acid, some use Brettanomyces for the same purpose. Prior to 1980s-era changes in its fermentation regimen, Guinness Foreign Extra Stout is held to have been one such.
See also 
- "Oxford Companion to Wine — Brettanomyces". Also Heresztyn, T (1986). "Formation of substituted tetrahydropyridines by species of Brettanomyces and Lactobacillus isolated from mousey wines". American Journal of Enology and Viticulture (37): 127–32.
- Asimov, Eric (November 22, 2011). "Brews as Complex as Wine". The New York Times. Retrieved 2011-11-24.