If a wine is exposed to air long enough it will eventually turn into vinegar. It's a two-part process: the wine's alcohol reacts with oxygen in the air and converts first into aldehyde and then into acetic acid (the cause of that sharp, vinegary smell). Wines with alcohol levels above 15% cannot be converted into vinegar (one of Mother Nature's rules). This is the raison d'ĂȘtre behind the production of fortified wines, which have alcohol levels at 15% or above. For example, fino sherry is one of the lighter fortified wines, but it must be at least 15.5% alcohol in order to prevent it from turning into vinegar.