If a wine’s smell reminds you a bit of nail varnish, Airfix glue or car touch-up paint, then that will be volatile acidity, often abbreviated to VA. If you are not sure what I am talking about then try some Chateau Musar, which is an excellent example of a wine with this character. A high level of VA is regarded as a wine fault and the wine is said to be volatile, but lower levels can be quite pleasing, and often lead the wine to be described as lifted or high-toned, indicating that the smell of VA seems to be higher in some sense than other wine aromas. Chateau Musar levels of VA probably lie at the upper end of what most people would consider acceptable, even though lovers of this wine see this as a very positive aspect of its character.
I believe the term volatile acid was coined because its volatility has important consequences for how it can be isolated for measurement. Steam distillation is normally used for the purposes of analysis, but if you simply boil a wine the volatile acids are also given off as a vapour. On the other hand, the non-volatile acids, mainly tartaric acid, will remain in solid form after the wine has been boiled dry. There are a few different volatile acids that can be found in wine, but by far the most prevalent is acetic acid (AKA ethanoic acid), which is what gives vinegar its distinctive smell and taste. Thus, effectively the term volatile acidity refers to the presence of acetic acid.
So why doesn’t volatile acidity in wine smell like vinegar, I hear you ask? Well, a proportion of the acetic acid reacts with the ethanol in the wine to create an ester called ethyl acetate (AKA ethyl ethanoate), and our noses are a lot more sensitive to ethyl acetate than they are to acetic acid. So at low levels of volatile acidity we will only detect the ethyl acetate, and at higher levels the ethyl acetate tends to drown out the smell of vinegar, even if it might still be noticeable in some instances. And if you have not already guessed by now, ethyl acetate is a solvent that is used in nail varnish, and the other products I mentioned above.
In principle acetic acid can be a straightforward oxidation product of ethanol. But in practice, whenever oxygen is present in large quantities, bacteria and yeasts may grow, and it is these microorganisms that are largely responsible for the production of acetic acid. Some is even produced by the yeasts responsible for wine’s primary fermentation.
And now for the bit about what VA isn’t. Normally it is not necessary to say what things are not, but many people describe VA as smelling of acetone, and there are a number of articles on the internet that link acetone with VA, one even saying that acetone and ethyl acetate are different names for the same thing. So hereby I declare what VA isn’t: acetone.
Acetone is (or at least was, speaking from personal experience) a very common laboratory solvent. Although I find it hard to recall it as I write now, the smell is unpleasant and irritates the nose, and it is nothing like acetic acid or ethyl acetate. I am not sure about the basis in science for this, but when I have the misfortune to smell a mouldy orange, my mind is taken back to the acetone bottle in laboratories of old, so that might give you some clue as to how it smells. And in addition to acetone’s smell being nothing like VA, as far as I can determine (it is difficult to prove a negative) wine never contains acetone in practically significant quantities. The only reason for the confusion seems to be that both acetone and ethyl acetate are sometimes used as a solvent in nail varnish removers, and I vaguely remember being told in school chemistry lessons that nail varnish remover actually is acetone. But that use of acetone seems to have practically died out. Many years ago I struggled to find a nail varnish remover that was acetone-based to check what acetone was like. And when I found one, even that smelled mainly of added fragrances and other stuff, rather than the acetone itself. Also note that nail varnish remover does not have to contain ethyl acetate; it can be any solvent that will do the job.
So if you want to learn about volatile acidity in wine in practical terms, forget about acetone – and sit yourself down with bottle of Chateau Musar, and a bottle of nail varnish. Enjoy.