Volatile acidity and mousiness - subtle faults worth knowing
Some wine faults are obvious, like cork taint or sharp oxidation, but two of the most interesting are subtle and insidious. Volatile acidity gives notes of vinegar and nail polish remover, present in every wine in a small dose, but becoming a fault once a threshold is crossed. Mousiness is even more treacherous, because you cannot smell it, only sense it in the aftertaste, and some people cannot perceive it at all. Both stem from the microbiology of wine and both balance on the line between a defect and a trace of style. Knowing these two subtle faults is a mark of a mature taster. Here is what volatile acidity and mousiness are, where they come from, their detection thresholds, why you cannot recognise mousiness by smell and when it is a real fault versus a tolerated nuance.
What volatile acidity is
Volatile acidity is a group of wine acids that readily pass into a volatile state and are perceptible in the aroma. The main component is acetic acid, the same that gives vinegar its character, and its associated ester, ethyl acetate, which smells of nail polish remover or glue. Importantly, every wine contains some volatile acidity, because it is a natural byproduct of the metabolism of yeast and bacteria. In small amounts it is imperceptible or even adds complexity. It becomes a fault only when its level crosses the detection threshold and the wine begins to smell of vinegar. Understanding that volatile acidity is a natural component of wine that becomes a problem only in excess is the starting point. It is a matter of quantity, not mere presence.
Where volatile acidity comes from
Volatile acidity rises mainly through acetic acid bacteria, which exploit the wine’s contact with oxygen. Bacteria such as Acetobacter and Gluconobacter convert alcohol or sugar into acetic acid. Acetobacter oxidises ethanol into acetic acid, and Gluconobacter converts glucose into acetic acid. The key enabling factor is oxygen, because it lets these bacteria work. That is why poorly stored wines, in leaky vessels or with excessive air exposure, are more at risk. So volatile acidity is mainly the effect of bacterial activity in the presence of oxygen. Understanding this mechanism explains why care for a tight seal and oxygen control is so important in production. It is a fault whose source is microbiology driven by air. Oxygen and bacteria are its two culprits.
The thresholds of volatile acidity
Volatile acidity is a good example of a measurable fault with specific thresholds. Even clean, sound wines contain about 0.36 to 0.48 grams per litre as a normal byproduct of metabolism. The problem starts when bacteria raise the level above the sensory detection threshold, that is roughly 0.6 to 0.9 grams per litre, when the wine begins to smell of vinegar. At a level of about one gram per litre the wine is already in danger of spoiling. These numbers show that the line between norm and fault is fairly narrow and concrete. That is why producers watch the level of volatile acidity so that it does not cross the threshold. Understanding these thresholds lets you appreciate that volatile acidity is not black and white but a scale. A small dose is normal, a medium one perceptible, and a large one spoils the wine. It is a fault with concrete numbers.
Ethyl acetate versus acetic acid
It is worth distinguishing the two main faces of volatile acidity, because they smell different. Acetic acid gives a straight note of vinegar, sharp and sour, familiar from every kitchen. Ethyl acetate, the ester formed from acetic acid and alcohol, smells completely different: of nail polish remover, glue or solvent. Both belong to volatile acidity but give different aromatic impressions. In practice, a wine with raised volatile acidity may smell of vinegar, of solvent, or of both, depending on the proportion of these compounds. Recognising whether you sense vinegar or solvent helps diagnose the type of problem. This distinction shows that volatile acidity does not have one smell but a whole palette of notes. Vinegar and solvent are two faces of the same fault. Knowing both makes it easier to recognise in the glass.
What mousiness is
Mousiness is a far more treacherous and mysterious wine fault. It is caused by volatile nitrogen compounds from the tetrahydropyridine group, mainly a compound called 2-acetyltetrahydropyridine, and related ones. The smell of mousiness is described as a note of mouse cage, stale bread, popcorn or something animal and unpleasant. The fault is associated with natural and low-sulfur wines, because the lack of protection favours the microbes that cause it. Mousiness is special, because it does not behave like a typical aroma fault. Recognising it requires understanding its unusual perception mechanism, which sets it apart from all other faults. It is one of the most fascinating and confusing faults in the wine world. Mousiness is a fault that hides from the nose.
Why you cannot smell mousiness
The most unusual feature of mousiness is that you do not smell it but sense it only in the aftertaste. The compounds responsible for mousiness are volatile only at a suitably alkaline pH, and wine itself is acidic. Only in the mouth, when the wine mixes with saliva of higher pH, do these compounds release and become perceptible. That is why mousiness appears only in the aftertaste, after swallowing or spitting, as a growing, unpleasant impression. This explains why you will not sense it by smelling the glass, but only a moment after tasting. This unique mechanism, dependent on the wine’s reaction with saliva, makes mousiness quite different from other faults. Understanding the role of pH and saliva is the key to grasping why mousiness is so insidious. It is a fault that reveals itself only in your mouth.
Why not everyone senses it
The second surprising feature of mousiness is that not all people perceive it. This stems from individual differences in saliva pH and sensitivity to these specific compounds. People with more acidic saliva may not release the volatile mousiness compounds at all, so they do not sense it, while others perceive it clearly. This leads to situations where one person calls a wine faulty, while another, drinking the same wine, notices no problem. This individual variability makes mousiness one of the most contested wine faults. It is hard to discuss it objectively when not everyone perceives it. Understanding that the perception of mousiness is individual explains many of the misunderstandings around this fault. It is a fault whose existence can be a matter of your own body. Not every taster is an equal judge here.
The link with brettanomyces
Mousiness and volatile acidity have links with the activity of microorganisms, including the yeast brettanomyces. This yeast, known mainly for notes of stable and sticking plaster, may also be capable of producing the compounds responsible for mousiness. Mousiness is also linked with lactic acid bacteria. This shows that subtle wine faults often have a common, microbiological source. Brettanomyces is itself a fascinating topic, because it is regarded as a fault or as a desirable element of character, depending on the style and amount. You can read more about this controversial yeast in the post on brettanomyces. Linking mousiness and volatile acidity to specific microorganisms shows that wine faults are largely a matter of controlling microbiology. It is a world of invisible authors of flavour.
Fault or trace of style
Both these faults balance on the line between a defect and a tolerated nuance. Very low volatile acidity is normal, and in certain styles a light note of it is regarded as adding complexity, though clear vinegar is already a fault. Mousiness is harder, because it is usually considered an unambiguous fault, but in the world of natural wines it is debated whether it is a defect or an element of raw character. Context, amount and expectations of the style decide the judgement. This shows that the line between a fault and a feature can be fluid, especially with subtle faults. Judgement requires feel and knowledge of the style, not a rigid rule. You can read more about the full spectrum of wine faults in the post on wine faults. The ability to tell a fault from a nuance is a mark of a taster’s maturity.
How to recognise them
In practice, recognising these two faults requires different senses and attention. Volatile acidity you catch by nose: look for notes of vinegar and nail polish remover or glue, especially when sharp and dominant. Mousiness, by contrast, you do not look for in the smell but in the aftertaste, a few seconds after tasting the wine, as a growing, unpleasant, animal impression. If a wine smells clean and yet leaves a strange, stale aftertaste, suspect mousiness. Remember that not everyone perceives it, so it is worth comparing impressions with others. This exercise teaches how differently wine faults manifest. If you want to systematically learn to recognise subtle faults, record your tastings in the app and note both the smell and the aftertaste. These two faults are a great test of a taster’s attentiveness.
What it means in the glass
For the drinker, knowing volatile acidity and mousiness is a higher level of knowing tasting. It lets you recognise subtle problems that are easy to miss or confuse with style. Light volatile acidity can be acceptable, but clear vinegar or solvent is a signal of a fault. Mousiness, if you sense it, is usually a defect, though in natural wines it is contested. The most important thing is to know what and where to look for: vinegar in the smell, mousiness in the aftertaste. This knowledge protects against a mistake and deepens your understanding of wine. It is also a good reason to taste wine attentively, not only to smell it. Volatile acidity and mousiness are two subtle, fascinating faults that separate the beginner from the knowing taster. Mastering them rounds off the knowledge of how wine can go wrong. It is a refined end to the study of faults.
The key points
Volatile acidity and mousiness are two subtle wine faults of microbiological origin. Volatile acidity is volatile acids, mainly acetic acid smelling of vinegar and ethyl acetate smelling of solvent, present in every wine but becoming a fault once a threshold of about 0.6 to 0.9 grams per litre is crossed; they rise mainly through acetic acid bacteria in the presence of oxygen. Mousiness is nitrogen compounds produced, among others, by brettanomyces and bacteria, giving a note of mouse cage and stale bread. Its peculiarity is that you cannot smell it, only sense it in the aftertaste when the wine mixes with saliva of higher pH, and some people do not perceive it at all. Both balance between a fault and a trace of style, especially in natural wines. Volatile acidity you recognise by smell, mousiness by aftertaste. It is a refined test of an attentive taster.