Does sherry go bad

Answer 1

Cooking sherry or any cooking wine for that matter is already bad by design when you buy it: Only the worst wines that can't be legally sold as sherry or drinking wine are used to make cooking wine as a way to still make big bucks on these defective products that would be used for vinegar otherwise.

Cooking wines have salt and a bunch of other flavorings or chemicals and whatnot added, but that does not prevent them from spoiling. It probably has to do more with the fact that cooking sherry is the least fruity sherry, and is therefore less likely to keep for long, although this can be prevented by adding more sugar if it is sherry.

It does not turn "bad" though, it just turns into vinegar and after it is fully turned into vinegar in a few months, it can be used in French dressing, to deglaze fried onions or gravy, to marinate meat, etc.

It will still be a low quality vinegar, because garbage in, garbage out and that sort of things, but not a total waste.

Sherry vinegar is a much sought after product. Quality sherry vinegars sell between $15 and $30 a bottle, depending on their age and origin (search for Banyuls vinegar for instance.)