If a cork happens to pop out unnoticed and air reaches the wine for
several weeks, there is a good chance that bacterial action will begin
to convert the alcohol in the wine into acetic acid. Once the presence
of acetic acid can be detected (a vinegar-like odor) the wine will lose
its appeal as wine. A usable vinegar can be retrieved by encouraging the
process to go to completion.
Vinegar produced from an undiluted wine will be overly strong, so an
equal volume of water should be added. The container should be less than
three-quarters full and closed with a loose cotton plug or covered with
a piece of light cloth to keep out fruit flies.
If wine vinegar is your desired goal and no wine has started to sour,
use a vinegar starter. A selected strain of vinegar starter can be
purchased from some winemakers' shops, or a wild starter may be used.
Frequently the water in an air-bubbler will have a vinegar-like smell.
This can be used to start a batch of vinegar. The wine is diluted with
an equal volume of water and the container partly filled and covered as
above.
A warm, but not hot, location will speed the process. In a month or two
the vinegar should be ready. The clear portion of the vinegar can be
poured or siphoned off for use. If another batch is wanted, more of the
wine-water mixture can be added to the old culture.