Introduction to The Z-Enology Brett assay as presented in Wine Business Monthly

Spoilage of wines by Brettanomyces yeasts is a growing problem. This is due in part to currently popular winemaking practices such as extended “hang time,” post-fermentation maceration, barrel aging, and a desire to reduce the use of SO₂. Brettanomyces grows in finished wine, especially wine aged in barrels, often producing undesirable odors. The sporulating form of this yeast is known as Dekkera. Brettanomyces is a slow-growing yeast, unlike the fermentation yeast Saccharomyces cerevisiae, but it is a very tenacious yeast, being very difficult to eliminate from a winery once it becomes established.

Some strains of “Brett,” as it is known in the wine industry, grow in a mold-like way. Their hair-like hyphae can penetrate into the surfaces of oak barrels. This is the reason for the caveat in the wine industry not to buy used barrels for fear they may have become infected with Brett. This yeast is rarely found in juice or fermenting wines, so its levels, if present, must be below what would be detected by peering through a microscope or plating for S. cerevisiae (i.e., less than 10⁶ cells/mL).

After fermentation is complete, Brett can be found in high concentrations. This yeast is very alcohol-tolerant, with some strains able to use ethanol as a sole carbon source. Often, Brett is detected in barreled wine by the smell. It can produce several very aromatic substances, variously described as “Band-aid,” “horsey,” “sweaty” and “wet-dog-in-a-phone-booth.” One of the compounds, 4-ethyl guaiacol, can have a rather pleasant spicy aroma, but it is usually overpowered by the strong-smelling 4-ethyl phenol, or may not be considered a desirable or appropriate character for the wine. Brett is also involved in the formation of biogenic amines, which can cause headaches in wine drinkers.

Monitoring wine for Brett is not an easy process for most wineries. While there are currently several methods available for detection of Brett, because it is very slow growing (one to two weeks on agar plates), spoilage can and often does occur before the winemaker is aware of contamination. Rapid methods are available but are prohibitively expensive for large-scale use and require considerable investment in equipment, time and expertise. A simple, inexpensive method that could detect Brett directly from wine in a few hours would give the winemaker an “early warning” tool in his or her campaign against Brett. For those who like a little Brett in their winemaking, for stylistic reasons, this method could serve as a monitoring device to keep the levels of Brett in check.

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