Welcome to the ninth and final part of the series Terroir & the chemistry of wine. We have moved from the soil beneath the vine to acids, phenols, aromas, fermentation and stabilisation. Now we close the circle with the topic that ties the entire microbial and chemical story together: what happens when something goes wrong in the bottle or on the barrel.
For the experienced taster, the field of faults is the trickiest terrain. The line between fault and style is rarely sharp, and the same compound can be charming in small amounts and ruinous in large ones. In this final instalment we look at the chemistry and microbiology behind the classic wine faults, and how they are prevented in a well-tended cellar.
Hvad du lærer
- The most common chemical and microbial wine faults, and which organisms or reactions are behind them.
- The chemistry behind Brettanomyces and the formation of volatile phenols as well as volatile acidity.
- How you tell a genuine fault from a deliberate stylistic choice.
- The most important preventive measures in the cellar, from sulphur management to inert storage.
What defines a wine fault
Wine is by nature a microbial product. The surface of the grape carries a mix of mould, yeast and bacteria, and when the grapes are crushed and enter the tank, this microflora is subjected to natural selection. Filamentous fungi do not survive the fermentation conditions and disappear. Saccharomyces cerevisiae takes over during alcoholic fermentation and produces ethanol, fatty acids and metabolites that increase toxicity towards other organisms. The result is that finished wine is microbiologically more stable than must.
A wine fault arises when an unwanted organism or an unwanted chemical reaction nonetheless gains a foothold and pushes aroma, taste or texture beyond what the wine's style intends. The point is that faults are quantitatively and contextually determined. A small amount of a compound can contribute to complexity, while the same compound above a sensory threshold dominates and masks fruit and terroir. That is why expert assessment is less about searching for a single molecule and more about judging balance, intensity and whether the expression suits the wine's type.
Brettanomyces and volatile acidity
Brettanomyces bruxellensis is probably the most debated microbial actor in the modern wine cellar. The yeast forms volatile phenols, including 4-vinylphenol, 4-vinylguaiacol, 4-ethylphenol and 4-ethylguaiacol. The chemistry proceeds via a decarboxylation and subsequent reduction of hydroxycinnamic acids, namely ferulic acid and p-coumaric acid, which are present in the grape. When the total concentration of volatile phenols exceeds a threshold around 500 to 700 μg/L, they emerge clearly on the palate, often described as stable-like, animal or medicinal notes.
The risk rises when the Brettanomyces population goes above around 10³ CFU/mL, and particularly during barrel ageing, where higher cellar temperatures encourage activity. It is worth noting that the species is not uniform. Phylogenetic studies divide B. bruxellensis into three groups with differing SO2 sensitivity: highly sensitive, tolerant and resistant. Certain strains thus carry genomic traits that give them tolerance towards sulphur, which explains why standard measures do not always suffice.
Volatile acidity and oxidative yeasts
Volatile acidity is primarily about acetic acid. Acetic acid bacteria oxidise wine in the direction of vinegar, a classic spoilage route that requires access to oxygen. Related to this are the oxidative yeasts from the genera Candida, Pichia and Metschnikowia, which develop on the wine's surface in contact with air, form a biofilm and generate acetaldehyde. Common to all of these is their dependence on oxygen, which points directly to the preventive answer: exclude the air.
Mousiness, ropiness and acrolein
Brettanomyces and heterofermentative lactic acid bacteria can also form N-heterocyclic bases such as 2-ethyltetrahydropyridine, 2-acetyltetrahydropyridine and 2-acetyl-1-pyrroline, which give the so-called mousiness, a mouse-like odour that is often first noticed as an aftertaste because of pH conditions in the mouth. Ropiness, a slimy and elevated viscosity, is caused by an exo-polysaccharide (glucan) formed by certain Pediococcus damnosus strains that carry a plasmid encoding glucosyltransferase, which can form glucan from unfermented glucose. Finally, the glycerol dehydratase pathway in certain Lactobacillus species can break glycerol down into acrolein via a 3-hydroxypropionaldehyde intermediate. Acrolein is irritating and binds to polyphenols, giving a bitter taste.
If you want to dig deeper into the bacterial side, this connects to Malolactic Fermentation and Microbiology.
Cork taint, oxidation and reduction
Three types of fault are among the most discussed at the table, and they have widely differing origins.
Cork taint is typically caused by 2,4,6-trichloroanisole (TCA), a compound with a remarkably low sensory threshold. By way of comparison, analytical GC/MS methods can detect TCA right down to around two parts per billion. That underscores why even minimal amounts can dampen and blur a wine. TCA is a chemical contamination rather than a microbial process in the finished wine, and it is not removed by aeration.
Oxidation is the broad category where oxygen has had too much play. It is connected to the oxidative surface microorganisms and their acetaldehyde production, and more generally to a loss of fruit and freshness as well as a development towards brown tones. Reduction is the opposite extreme, where a lack of oxygen and the formation of sulphur compounds give closed, off notes. Here it is worth remembering that the yeast strain itself influences the formation of hydrogen sulphide, so reduction can have its roots all the way back in the fermentation choice.
The relationship between sulphur, oxygen and longevity is treated in depth in Stabilisation: Sulphur and Oxidation, and the role of barrel ageing itself in Barrel Ageing and Maturation.
Fault or style?
The expert judgement that separates fault from choice rests on context. A hint of oxidative character can be intended in certain wine styles, while in a fruit-driven white wine it is a fault. The question is always: does the compound contribute to the complexity, or does it dominate and mask the wine's origin. When the intensity clearly exceeds the sensory threshold and pushes fruit and terroir into the background, we are talking about a fault.
Prevention in the cellar
Prevention begins with understanding that most spoilage organisms either require oxygen or are sensitive to sulphur, low pH and ethanol.
Sulphur management is the central tool against Brettanomyces. It is the molecular SO2 that does the work, and maintaining around 0.5 mg/L molecular SO2 (corresponding to roughly 45 mg/L free SO2 at pH 3.75) inhibits most strains effectively. Note the dependence on pH: the higher the pH, the more free SO2 is required for the same molecular level. For sweet wines with residual sugar, stability against refermentation is a particular concern, since tolerant yeasts such as Zygosaccharomyces bailii and Saccharomycodes ludwigii can ferment the residual sugar. Here a free SO2 level around 50 to 60 mg/L is stated as necessary for stability, and heat treatment at 50 to 55 °C for 2 to 3 minutes is considered the most effective method against refermentation.
Against the oxidative routes, both acetic acid bacteria and surface yeasts, the answer is to exclude oxygen. Storage under inert gas, argon, CO2, N2 or mixtures thereof, prevents oxidative spoilage by inhibiting the oxygen-dependent microorganisms.
Finally, monitoring is decisive at the expert level. DNA-based molecular tools make it possible to detect and count spoilage organisms at specific population thresholds. Polymerase chain reaction (PCR) can detect Brettanomyces contamination at 10⁴ CFU/mL within a single day, far faster than culturing methods, and quantitative PCR enables precise counting at any process step. This means a well-tended cellar can intervene before a population reaches the threshold at which volatile phenols become a problem.
Kort fortalt
- A wine fault is quantitative and contextual: the same compound can contribute in a small amount and ruin a wine above its sensory threshold.
- Brettanomyces forms volatile phenols by decarboxylation and reduction of hydroxycinnamic acids, and the risk rises above around 10³ CFU/mL and with warmer barrel ageing.
- Cork taint (TCA), oxidation and reduction have entirely different origins and require different responses; TCA is not removed by aeration.
- Molecular SO2 around 0.5 mg/L controls Brettanomyces, while inert gas excludes the oxygen-dependent spoilage routes.
- Molecular methods such as PCR and qPCR make early detection and counting possible, so intervention happens before the threshold.
Ofte stillede spørgsmål
Can a Brettanomyces-marked wine save itself through aeration?
No. Aeration can volatilise some notes temporarily, but the volatile phenols are already formed in the wine. Prevention via SO2 management and temperature control during barrel ageing is the real lever, not after-the-fact treatment in the glass.
Why is TCA so hard to detect and remove?
TCA has a very low sensory threshold, and analytically it can be measured right down to around two parts per billion. It is a chemical contamination, not a process in the finished wine, so it does not disappear with aeration or by letting the wine stand.
Klar til næste skridt?
With this, the series Terroir & the chemistry of wine is complete. You have followed the wine from soil and climate through acids, phenols, aroma and fermentation all the way to the faults that can threaten it. If you want to revisit the foundation, Terroir: Soil, Climate and Geology is a good place to gather the threads, just as The Aroma Compounds of Wine and The Acids and pH of Wine illuminate what makes a healthy wine come alive.
This knowledge makes tasting richer, but it should not make you nervous at the table. The best pairing is still the wine you like with the food you fancy. Do drop by our selection when you want to put the theory to the test with a glass.