Brett beer is a style of beer that makes explicit reference to its yeast.
The term “brettée” can have 2 meanings: either the beer has been fermented with brettanomyces type yeast or the beer has been contaminated with wild yeast. In the first case, it is a popular beer, in the second case it is a defect.
There are indeed 2 types of yeasts: saccharomyces and brettanomyces, also called "wild yeasts".
Originally, all beers were inoculated with wild yeast present in the air. With Pasteur and the invention of bacterial cultures, it was now possible to choose the yeasts used.
The best-known examples of beers fermented with wild yeast are lambics and gueuzes, which are typical of the Senne valley, that is to say, Brussels and its surroundings. Thus Cantillon, Oud Beersel, Tilquin or even Drie Fonteinen are well-known names for lovers of gueuzes.
Another, much better known beer, Orval also contains brettanomyces. We thus find its characteristic imprint on the nose.
Conversely, the presence of brettanomyces may be a defect. We then speak of contaminated beers. Contamination is the fear of brewers because its consequence is often the loss of several hours of work and thousands of liters poured into the sewers. There are happy contaminations (slight presence of brettanomyces) and unpleasant contaminations on the pallet. In any case, even contaminated beer will not make you sick, at least if it is drunk in reasonable quantities.