Wine Arcana

Concept

Brettanomyces

Also known as Brett, Brettanomyces bruxellensis

Brettanomyces is a genus of yeast, usually discussed in wine through the spoilage species Brettanomyces bruxellensis. Its activity can produce smoky, medicinal, sweaty, or animal-like aromas and diminish a wine's fruit character. The shorthand Brett may refer to the organism or to the sensory character associated with its products. Those are distinct things to measure: a wine can contain cells before an obvious aroma develops, and aroma compounds can remain after the cells have been removed.

How the aromas form

Brett transforms grape-derived hydroxycinnamic acids, a group of phenolic compounds, into volatile phenols. Among the important products are 4-ethylphenol, commonly associated with medicinal or adhesive-bandage notes, and 4-ethylguaiacol, associated with smoky or spicy notes. A third, 4-ethylcatechol, can contribute sweaty or animal-like character.1. Australian Wine Research Institute, “Brettanomyces,” frequently asked questions, accessed 11 September 2026.

The proportions vary with the available precursors and the yeast. The rest of the wine also changes perception: oak-derived aromas or abundant fruit may partly mask Brett-related compounds. As fruit fades during ageing, an aroma previously less apparent can become easier to recognize. A single numerical threshold therefore cannot define when every wine will seem affected.

Continued microbial activity can change the wine further during storage. Smell alone gives limited information about that risk.

Conditions that favour growth

Brett can exploit residual sugar and persist in the winery through contaminated wine and equipment. Barrels are one possible reservoir, but transfers and wine used to top up casks can also spread cells. Control consequently includes the whole route the wine takes through the cellar.2. Australian Wine Research Institute, “Controlling Brettanomyces during winemaking,” updated February 2023.

The interval around malolactic fermentation can be particularly vulnerable. Winemakers often limit sulfur dioxide while the desired bacteria complete their work, leaving less antimicrobial protection. Higher pH further reduces the proportion of sulfur dioxide present in its most active antimicrobial form.

Strains also differ in their tolerance to sulfur dioxide. An Australian research project comparing isolates collected in three periods between 2000 and 2018 found greater tolerance among the most recent group. The researchers cautioned that broader sampling was needed to establish how widespread the change was.3. Wine Australia, “Brett on the rise again?”, 9 August 2019, reporting Caroline Bartel's Australian Wine Research Institute research. This variability helps explain why cellar control combines hygiene, monitoring, and appropriate protection.

What testing reveals

Chemical analysis measures volatile phenols already produced. Microbiological testing asks whether Brett cells are present. Culturing a sample on suitable nutrient media detects cells able to grow under the test conditions; slow growth means the result takes time. DNA-based testing can identify Brett more rapidly, though a conventional test may also detect DNA from dead cells. Affinity Labs explicitly notes this limitation for its PCR test, which uses the polymerase chain reaction to amplify a target DNA sequence.4. Affinity Labs, “Brettanomyces testing,” accessed 11 September 2026.

The methods answer complementary questions. Elevated volatile phenols record past activity; detecting cells can identify a developing problem before it becomes obvious to a taster. Interpreting either result requires the method, sampling circumstances, and the wine's treatment history.

Appropriate filtration can remove yeast cells before bottling, while sanitation reduces recontamination. These measures address future activity. Removing aroma compounds already dissolved in wine requires other treatments, which can also affect desirable aromas. Preventing further growth and correcting an existing sensory defect consequently require different decisions.2. Australian Wine Research Institute, “Controlling Brettanomyces during winemaking,” updated February 2023.

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