http://www.meadmadecomplicated.org/mead_making/troubleshooting/odor.html
What?
* The mead has a vinegar odor (acetic acid, figure). Sensory threshold: 175 ppm [B10].
acetic acid
Acetic acid [B2]:
Why?
* The acétobacter bacteria, naturally found in air, are responsible for the transformation of alcohol into acetic acid (acetic fermentation) according to the reaction [V6]:
C2H5OH + O2 -> CH4COOH + H2O
ethanol + oxygen -> acetic acid + water
What to do?
* Not much, except maybe find a book such as "What to do with 5 gallons of vinegar?"
* According to V3, it is possible to dissolve 2 tsp potassium bitartarate (cream of tartar) in 1 liter (1 qt) of the infected mead then add to the rest of the batch. If the flavor is not back to normal after a week or so, try again. After five unsuccessful attempts, bottle your vinegar. This treatment is at a short term and the mead will have to be drunk quickly.
* If damages are limited and if mead has no other flaw, V6 says it can be used for blending. J. Recht in V4 does not advise to do this and considers that the mead is lost.
* If a mead gets infected in a barrel (or any other vessel made of wood), the vessel is infected and hence can no longer be used.