Climate Change Is Helping a Wine Spoilage Yeast Spread

Brettanomyces bruxellensis remains one of the most persistent microbiological threats in wineries, and its impact has grown as climate change has pushed grape pH higher in many regions, making wines more favorable to the yeast’s survival and growth. The issue is no longer limited to aged reds or to a few traditional cellar environments. Winemakers in Italy and elsewhere are now reporting contamination in younger wines as well, a shift that has changed how the industry approaches monitoring and sanitation.

The problem is not simply that Brettanomyces can spoil wine. It is that the yeast behaves differently depending on the strain, the wine’s chemistry and the stage of production. That variability makes it difficult to predict when a wine will be at risk and why one cellar treatment works in one case but fails in another. Researchers say that after more than two decades of study, Brettanomyces still resists any single, universal control strategy.

At the center of the concern is the yeast’s ability to survive in conditions that are hostile to many other microorganisms. Brettanomyces can tolerate ethanol levels as high as 13%-14% vol, can use carbon sources such as ethanol and glycerol when sugars are scarce and can persist on wooden surfaces and equipment where oxygen is present. Its genome, sequenced in 2011, contains about 3,000 genes, including genes linked to stress response that help explain why it survives so well in wine.

The sensory defect associated with Brettanomyces is usually traced to volatile phenols, especially 4-ethylphenol and 4-ethylguaiacol. These compounds are produced through a two-step enzymatic pathway. First, phenylacrylate decarboxylase, or PAD, converts hydroxycinnamic acids naturally present in wine into vinylphenols. Then vinylphenol reductase, or VPR, converts those intermediates into ethylphenols. The first enzyme is not unique to Brettanomyces; some strains of Saccharomyces cerevisiae also carry it and can produce vinylphenols during fermentation. But VPR is considered a functional marker exclusive to Brettanomyces and is what turns a mild chemical note into the familiar aroma described as barnyard, leather, wet animal or smoke.

Even there, the picture is more complicated than many winemakers assume. Not every Brettanomyces strain produces volatile phenols at the same rate. Some strains generate enough to create a clear sensory defect; others carry the genes but express them weakly or not at all under certain conditions. One study cited by researchers found that about 1 in 6 Brettanomyces strains did not produce perceptible volatile phenols despite having the genetic capacity to do so. That means the presence of the organism alone does not tell the full story.

Brettanomyces can also contribute to other faults. In the presence of oxygen, it may produce more acetic acid, raising volatile acidity. It has also been linked to tetrahydropyridines, compounds associated with mouse taint, although recent studies suggest that this defect often appears in mixed microbial communities rather than from Brettanomyces alone. The yeast has also been associated with biogenic amines such as histamine and tyramine, adding another layer of concern for both quality and safety.

The ecology of the yeast inside a winery makes control even harder. For years there was debate over whether Brettanomyces comes mainly from the vineyard or from the cellar itself. Genetic studies now suggest that strains from grapes and strains resident in wineries are not systematically different; the same strain can move between both environments. The highest-risk period appears to be after alcoholic fermentation ends and before malolactic fermentation begins, when sugars are nearly depleted, ethanol is present and sulfur dioxide has not yet fully stabilized the wine.

That window matters because Brettanomyces can begin multiplying even when residual sugar levels are low. It also forms biofilms on wood and can penetrate deeply into barrel staves, making sanitation difficult. Once established in barrels, hoses or fittings, it can persist from one vintage to the next.

The most troubling feature for cellar managers may be its strain-dependent resistance to sulfur dioxide. Studies have shown that some strains can grow even when molecular SO₂ levels are around 0.8 mg/L in aqueous conditions, while others are inhibited at about 0.4 mg/L. Ethanol changes that balance further. In practical terms, two wines with similar analytical profiles may behave very differently if they harbor different Brettanomyces strains.

That unpredictability extends to growth behavior as well. In one study of five strains tested across more than 50 red wines grouped by chemical profile, all five grew similarly in permissive wines with higher pH and lower free SO₂. In more restrictive wines, however, only one strain maintained aggressive growth while the others behaved very differently. The chemistry of the wine interacts with the genetics of the strain in ways that standard lab measurements cannot fully capture.

Another reason Brettanomyces remains difficult to manage is its ability to enter a viable but non-culturable state, known as VBNC. In this condition, cells shrink, slow their metabolism and stop forming colonies on Petri dishes even though they remain alive and capable of resuming growth once stress conditions ease. That means a negative plate count does not necessarily mean a wine is safe from future contamination.

Researchers have shown that cells exposed to SO₂ can disappear from conventional detection methods for days and then reappear once the sulfur dioxide pressure is removed. For wineries relying only on culture-based testing, that creates a false sense of security. Scientists increasingly recommend complementary tools such as quantitative PCR and flow cytometry with vital stains to detect cells regardless of whether they can be cultured.

For winemakers facing Brettanomyces today, the lesson from current research is clear: control depends on understanding both the wine and the strain. The yeast’s biology explains why it survives where others do not; its enzymatic pathways explain why it changes aroma so dramatically; and its hidden states explain why it can return after appearing to be gone.