Brettanomyces

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 produce large amounts of acetic acid. Brettanomyces is important to both the brewing and wine industries due to the sensory compounds it produces.

Wine

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 affect 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 Chateau 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 so some find the compounds more unattractive than others.

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. The reality is that whilst these sensory compounds may be attractive at lower levels, from a winemaking perspective there is no guarantee that high levels will not be produced. Wines that have been contaminated with Brettanomyces taints are often referred to as "Bretty", "mousy", or as having "Brett character".

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. It has also been thought that Brettanomyces taint in wine is sometimes incorrectly identified as cork taint.^*

Origins in the winery

Brettanomyces is typically isolated from 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 method when en tirage. It is thought Brettanomyces can be introduced to a winery by insect vectors such as Drosophila melanogaster, 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. This option is less popular as it often strips out much of the flavour of a red wine, but may be more acceptable for a white. Wines that are vinified to low residual sugar levels, such as <1.0g/L, are also less likely to be spoilt 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.

Beer

In beer it is more common and is found in many Belgian styles, lambic and gueuze, Oud Bruin, and Flanders red ale in particular. Commercial examples include Liefmans Brown Ale, Rodenbach Grand Cru, Duchesse de Bourgogne and, prior to changes in the fermenation regime in the 1980s, Guinness's Foreign Extra Stout.

External links

Brettanomyces at wineanorak.com

References

Oenology | Brewing | Ascomycetes