Ever heard of the bad boy in wine making? No, I am not referring to a winemaker and his mood at the end of harvest… Winemakers will often add yeast to grape must to complete the alcoholic fermentation, but not all yeasts are equal and good for the wine’s quality for that matter!
Brettanomyces is a type of yeast that can spoil wine and was first described in beer in Britain, hence the name. This yeast can sometimes grow in wine, mostly in red wines, when these are matured in barrels. It seems to be able to grow on very low sugar levels in the wine and in the process produce some off flavours. These off flavours include animal, medicinal and horse like aromas. These descriptors are caused by compounds called volatile phenols produced by the Brettanomyces. The two main types of volatile phenols most often produced by Brettanomyces in wine are 4-ethyl-phenol and 4-ethyl-guaiacol. Research has shown that at lower levels these compounds can give complexity to the wine, but when the levels are too high the above-mentioned off aromas are perceived and everyday consumers do not like it. In the past certain wine producing countries had problems with Brettanomyces, but new advanced in winemaking have led to lower incidences of this spoilage yeast occurring in wine. Certain wine producers from a famous wine producing region even called it part of their terroir, which is in my opinion nonsense, it is just spoilage of wine!
A winemaker can prevent Brettanomyces from growing in his/her wine by employing basic winemaking hygiene. This is especially true for barrel hygiene, where older barrels should be regularly washed and sterilized. It seems that heat treatment of barrels is especially effective in inhibiting Brettanomyces in the wood, as it penetrates the wood. The other options for the wine maker is the judicious use of sulphur dioxide (SO2).
Figure 1: Brettanomyces cells after SO2 has been added to the wine (0, 30, 120 and 360 minutes after the addition).
This preservative is taken up by Brettanomyces when added to the wine and can kill it quite quickly. I always tell my students that although both a Yorkie and Rottweiler are dogs, it would be a bit more daunting to get past the latter! The same applies to Brettanomyces, with different Brettanomyces strains occurring in wine with some of them being more SO2 tolerant than others. SO2 also functions more effectively at a lower pH in wine and should be rather added in one large dosage after malolactic fermentation than little bit at a time. It is thought that by adding SO2 in little increments in wine might contribute to resistance of Brettanomyces to this preservative. Filtration can also be employed to remove the Brettanomyces cells from the wine. However, most types of filtration will only lower or remove the Brettanomyces cells from the wine and not the volatile phenols concentrations. To lower the levels of these compounds in wine, reverse osmosis is required. However, prevention is better than cure, so a good wine maker will be on top of this nuisance in the wine industry!