In Introduction to wine faults: Part I, Westgarth Wines expert Maurizio Broggi introduced key faults like oxidation, cork taint, and volatile acidity and explained why they matter. In Part II Brettanomyces, reduction, mousiness, light damage, and smoke taint are in the spotlight.

Brettanomyces bruxellensis, commonly known as Brett, is a spoilage yeast that thrives in the challenging conditions of aging red wines. Its resilience is remarkable: it can grow with or without oxygen, survive at low nutrient levels, and persist in wooden barrels where it burrows into the staves. The sensory effects of Brett are polarizing. Low levels may impart spicy, smoky, or earthy notes, sometimes perceived as complexity. However, higher concentrations overwhelm fruit with aromas of barnyard, horse, medicinal bandages, or sweaty leather.

The primary flavor-active compounds are volatile phenols, especially 4-ethylphenol, which gives medicinal and animal notes, and 4-ethylguaiacol, which contributes smoky or clove-like aromas. As Brett populations grow, varietal character diminishes, tannins appear harsher, and wines take on a drying, metallic aftertaste.

Conditions favoring Brett include high pH, warm cellars, oxygen exposure, residual sugar, and delayed sulfur dioxide additions. Barrels, especially new ones, provide both a substrate and a hiding place for the yeast, making wooden maturation a major risk factor. Prevention, therefore, requires rigorous sanitation, including heat treatments of barrels where possible, as well as maintaining adequate molecular sulfur dioxide levels. According to the Australian Wine Research Institute (AWRI), maintaining molecular SO₂ at 0.625 mg/L is critical to suppress Brett populations (AWRI, 2005). Rapid completion of malolactic fermentation reduces the window during which Brett can establish itself. Lowering pH, maintaining cool storage, and avoiding residual sugar further reduce risk.

Once Brett has taken hold, treatment is difficult. Sterile filtration can remove cells but may affect texture. Dimethyl dicarbonate (Velcorin) kills yeast but leaves the taint compounds behind. Chitosan, a biopolymer derived from fungi or crustaceans, shows promise in removing Brett populations, though it does not eliminate volatile phenols already present. Reverse osmosis and adsorbents can reduce phenol levels, but the fruit lost to Brett is rarely recovered.

While some consumers accept Brett as part of traditional styles, modern winemaking largely views it as a fault to be avoided, emphasizing the importance of preventive management.

The opposite of oxidation, reduction refers to a lack of oxygen, leading to the formation of volatile sulfur compounds (VSCs). At low levels, some VSCs contribute desirable notes such as struck flint or gunpowder in Chardonnay or tropical thiols in Sauvignon Blanc. At higher concentrations, however, they produce foul-smelling aromas of rotten eggs, garlic, cooked cabbage, or burnt rubber.

Hydrogen sulfide, thiols, and disulfides are the main problematic compounds. Their formation is primarily linked to yeast metabolism during fermentation, especially when nitrogen levels in the must are deficient. Without sufficient yeast-assimilable nitrogen, yeasts divert to sulfur metabolism, releasing hydrogen sulfide. Additional sources include elemental sulfur residues from vineyard sprays and reductive conditions during lees aging or bottle storage under low-oxygen closures.

Prevention requires ensuring balanced fermentation nutrition. Yeast-assimilable nitrogen should be monitored and supplemented when necessary, using nutrients such as diammonium phosphate or organic yeast derivatives. UC Davis research suggests that a YAN (yeast-assimilable nitrogen) level of 200 mg/L or higher is ideal to prevent H₂S formation (UC Davis, 2014). Yeast strain selection also matters, as some strains produce more VSCs than others. Managing lees contact and providing controlled oxygen exposure during élevage reduces reductive risks, as does selecting closures with appropriate oxygen transmission.

If strong reductive odors develop, treatments include copper sulfate fining, which binds hydrogen sulfide and thiols, though it must be used cautiously since it can strip positive aromas and leave residual copper. Aeration techniques such as racking or sparging may dissipate volatile sulfides, particularly in robust reds. Disulfides, however, are more persistent and can revert to smelly thiols under bottle aging, making prevention the more reliable strategy.

Mousiness is a fault most often associated with natural wines and low-sulfur practices. It produces a distinctive mouse-cage or corn-chip aftertaste that appears retro-nasally after swallowing, making it undetectable on the nose. For those who perceive it—it is estimated that up to a third of people are anosmic to mousiness—the sensation is unmistakably unpleasant and renders wine undrinkable.

The compounds responsible are pyridines, particularly 1-acetyl-tetrahydropyridine, produced by lactic acid bacteria and possibly Brettanomyces. These compounds are not aromatic at wine pH but become perceptible when the wine mixes with saliva, which raises the pH.

Risk factors include high pH wines, extended lees aging, oxidative handling, and minimal filtration. AWRI has identified five major risk factors: high-solid fermentations, extended lees aging, high pH (above 3.5), oxidative aging, and limited clarification (AWRI, 2016). Natural wines with no or delayed sulfur dioxide additions are especially prone. Prevention relies on controlling pH, avoiding extended contact with solids, and ensuring small but sufficient additions of sulfur dioxide during fermentation. Once mousiness develops, there is no effective treatment. Blending may dilute the fault, but usually the wine is considered lost.

Light exposure can damage wine in two main ways: by accelerating oxidation and by producing light-struck aromas. These effects are most common in white and sparkling wines bottled in clear glass, which allows damaging wavelengths to penetrate.

Light-strike occurs when riboflavin reacts with sulfur-containing amino acids under ultraviolet or blue light, producing volatile sulfur compounds such as methanethiol and dimethyl disulfide. The resulting aromas resemble skunk, garlic, or cooked cabbage. Meanwhile, light also generates glyoxylic acid from tartaric acid in the presence of iron, which consumes sulfur dioxide and leads to browning.

Prevention is straightforward but often neglected for aesthetic or marketing reasons. Amber glass offers the best protection, followed by green, while clear bottles are the worst. Secondary packaging, such as colored sleeves or cartons, and careful control of lighting in storage or retail, can further reduce risk. Some producers are also fine with bentonite to reduce riboflavin concentrations. The Comité Champagne, working with lighting manufacturers, has even developed amber LED lighting to minimize risk in cellars (Comité Champagne, 2019). Once wine is light-struck, there are no remedies; prevention is the only defense.

In an era of climate change, smoke taint has become an increasingly pressing issue for wine regions prone to wildfires, including California, Australia, and parts of Europe. Grapes exposed to smoke absorb volatile phenols such as guaiacol and 4-methylguaiacol, which bind to sugars as glycosides. While the bound forms are not immediately aromatic, they are hydrolyzed during fermentation and aging, releasing smoky, ashy, or burnt flavors in the finished wine.

The sensory impact varies with grape variety, stage of ripening, and duration of exposure, but even brief smoke events near harvest can significantly affect quality. Some cultivars, such as Sangiovese and Semillon, appear particularly sensitive.

Prevention is challenging, as smoke exposure is beyond the winemaker’s control. Vineyard strategies may include removing leaves to limit transfer of volatile phenols or assessing damage through micro-ferments before harvest decisions. In the winery, limiting skin contact reduces extraction of smoke-derived compounds, making white and rosé styles less affected. Hand harvesting, gentle pressing, and low-temperature handling further help.

Remedial options exist but are imperfect. Reverse osmosis and resin adsorption can remove some volatile phenols, though they also strip desirable aromas and do not address bound glycosides that continue to release smoke character during aging. Activated charcoal can also reduce phenols in juice, especially for whites, but again at the cost of fruit character. UC Davis advises that washing grapes after smoke exposure has little effect, as phenols are absorbed within the skins (UC Davis, 2020). Blending with unaffected lots may be possible, but thresholds for smoke taint are low, limiting this approach. In some cases, producers rebrand affected wines under different labels or adjust styles to mask the taint, though this is more a marketing solution than a technical fix.

Wine faults underscore the fragile nature of wine, and the delicate balance required to guide it from vineyard to bottle. Each fault—whether oxidative, microbial, reductive, or environmental—demonstrates the interplay between chemistry, microbiology, and human decision-making. The effects range from subtle suppression of fruit to complete spoilage, while the causes often trace back to lapses in oxygen management, sulfur dioxide usage, sanitation, or vineyard conditions.

What emerges clearly is that prevention is always more effective than treatment. Once oxidation has dulled aromas, once Brett has consumed fruit character, or once smoke taint has embedded itself in grape skins, little can be done to restore the original wine. Vigilance, hygiene, and thoughtful choices throughout the process remain the winemaker’s best defenses. In this way, understanding wine faults is not simply about avoiding flaws but about preserving the fragile essence of fruit, place, and craft that defines great wine.

Want to read more? Take a look at some of our other blogs:

• • Sulfites in wine: Part I
  • Sulfites in wine: Part II
  • The serious side of Rosé