I answered this question a long time ago, but it turns out I only had half the answer. Does wine turn into vinegar? This web site says yes, while this web site says no. Both sides seem pretty authoritative, but it turns out that neither give a complete answer. To help me get it right, I've enlisted the help of Derek Shedd, enologist, winemaker, and all around Smart Guy who works in the lab at Dobbes Family Estate Winery.
Mattie: Can wine turn to vinegar?
Derek: In wine, acetic acid is an indicator of wine spoilage. If wine gets infected with acetobacter and other conditions are right, then acetic acid will be produced, along with lots of other bacteria (pediococcus, etc.).
We're talking about acetic acid because one definition of vinegar is diluted acetic acid. Derek says that acetic acid can be produced from wine under the right conditions. So, what's the confusion? It turns out that it's all a classic example of a verbal dispute. In philosophy, a verbal dispute is when two sides are arguing about something but they agree on the facts. The confusion stems from a definition, and in this case the definition for "vinegar."
Everyone agrees that wine can become acidic when it's been sitting around for a while. However, without adding the specific bacteria used to make vinegar (mycoderma aceti), this super acidic wine likely won't taste good, causing many to hesitate to call it vinegar. The verbal confusion here seems to be one between scientists and chefs.
A chef will say, No, wine cannot turn into vinegar because old wine doesn't taste like vinegar. It tastes bad.
A scientist will say, Technically acetic acid is created when bacteria, which is found everywhere in the air, comes into contact with alcohol for a long time. So wine can turn into vinegar (the scientific definition of vinegar is something like diluted acetic acid).
Here's what you need to know. Technically, wine can turn into vinegar when it is exposed to a lot of oxygen for a long time (months at least). Should you add a little to add richness to your tomato sauce? Sure. Should you use it for salad dressing? I wouldn't. It's highly unlikely that it will taste that good.
I asked Derek if he thought that a wine vinegar produced without a mother would be tasty. He said, "You would need to consult a better source than I, but the difference is what happens when you have an infection of acetobacter bacteria and what happens when you have an inoculation of a specific bacteria (the latter is a pure culture). The stuff that tends to infect wine can quickly turn acetic acid to acetone, which does not happen with a true mother. (Get a mother culture and keep it safe and happy and pure)."
I love science! If you know more about this, please set the record straight. But as it stands, I think this is all correct. It's all in a definition.