Brettanomyces (often abridged Brett) are unicellular yeast. Those yeasts can be found in any beverages resulting from an alcoholic fermentation: wine, cider or bier.

Brett are not toxic by ingestion. However, they produce volatile phenols which can modify the sensorial characteristics of the wine. In this area, 4-ethylphenol (stables, barnyards and sweaty saddles aromas), 4-ethylguiacol (smoky, spicy aromas) and isovaleric acid (rancid, horsey aroma) are the key molecules in order of sensory importance. 4-ethylphenol is the most prominent molecule in bretty wines and it is the one that most laboratories are looking for to indicate the presence of Brett.

The origin of Brett is not clear. Nevertheless, it seems that some aspects of wine-making do influence on the risk of contamination. Unsurprisingly,  hygiene appears to be a critical part, since yeasts have been found on organic leftovers in pumps used for the transfer of the must. 

Unfortunately, it seems that "modern" wine-making techniques used since the beginning of the Nineties, may favor the presence of Brett. Decreasing the added sulphur quantity is an aspect of these new techniques. Indeed, the most effective way of preventing Brett is to maintain an adequate concentration of free sulphur dioxyde (SO2). Moreover, the new style of extracted super-ripe grapes leads to a lower acidity in the must which decreases the effect of SO2.
An other problem is the residual sugar which allows Brett to be active after the fermentation.

Some recent studies show that oxygen could also be linked to the presence of Brett and that the microbullage technique favors its expansion. However this aspect is highly controversial since others studies have shown opposite results.

Nevertheless, even if the above mentioned aspects are minimized during the wine making, the number of contaminated wines seems to increase dramatically these days. This proves that there is still a long way to go to define precisely the behaviour of Brett in order to eradicate it.

The presence of Brett in wine is not always considered as a default. It all depends on its concentration. A little sensation of Brett can add some complexity to the wine. The famous Châteauneuf-du-Pape, Château de Beaucastel from the Southern Rhône area in France, is a perfect example of this assertion: bottles of 1989 and 1990, both considered as outstanding vintages for this estate, were controlled positive to Brett. Of course, it is also a matter of sensibility to the palate which is known to vary a lot from one wine taster to another.

Finally, one should be careful not to confuse Reduction with the presence of Brett. Reduction is characterized by rotten egg, onion, animal or cabbage odours. Reduction is a transitory phase which often disappears with airing. It is to be noted that aeration has no positive effect on the volatile phenols generated by the Brett.

Stephane Gorgerat, for Winemega

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