If you make vinegar long enough you have to tackle almost every problem. I sometimes get emails and questions about an “acetone” smell from vinegar, very similar to nail polish remover. What does this mean?
First, you must understand the process by which acetic acid bacteria turn alcohol into vinegar. The short chemical pathway is:
Alcohol (Ethanol) + Oxygen -> Acetaldehyde + Oxygen -> Acetic Acid + Water
The middle step is rarely discussed because it isn’t of use to the average vinegar maker. Acetaldehyde is an intermediate step on the way from alcohol to vinegar. And if you get my drift, acetaldehyde in concentration smells a lot like acetone.
So the acetone smell is what happens when the reaction to make vinegar isn’t completely finished. Many bacteria just make acetaldehyde and then start creating the pungent odor. The reasons for incomplete reactions can vary but the top two are lack of air (oxygen) and too much alcohol in the starting mash.
Vinegar fermentation requires oxygen and needs a plentiful supply. That’s why a porous (but not porous enough for fruit flies) cloth like cheesecloth is all you want closing up your jar or barrel. Having enough air usually prevents the issues with stalled fermentation.
The other reason is too much alcohol in your starting mash. As has been stated the starting mash should not have an alcohol content exceeding 10% ABV. I would not go higher than 8% ABV if you are using traditional methods. Too high an alcohol content puts stress on the bacteria. They spend too much energy trying to cope with the poisonous effects of high alcohol concentration and don’t have the resources to ferment vinegar correctly, leaving a lot of acetaldehyde. The solution to this is lowering the alcohol content.
Another thing you can do is put a packet of brewer or baker’s yeast for up to 5 gallons of vinegar to provide nutrients to help fermentation.