I purchase large jars of pickled onions imported from Italy and they are the best I have ever tasted. On the main (front) label it states "in balsamic vinegar" but on the ingredients list it states "wine vinegar".
When finished with the onions there is, of course, lots of vinegar left. Can I safely reuse this vinegar to pickle beets? Other ingredients listed are water, sugar, salt, citric acid and trace of sulphuric anhydride, whatever that is. These sometimes come in cans and then olive oil is also listed.
Does balsamic vinegar "go off?" I ask as the label states that pickles are to be kept refrigerated once opened.
Balsamic vinegar is wine vinegar. The vinegar begins with the trebbiano grape which is fermented into wine and then into vinegar.
The onions you purchase sound delicious. You could use the leftover liquid, but it likely is not your best choice, because pickled foods such as onions, peppers and artichokes are packed in vinegar as well as all of those other ingredients you listed. The issue that would most likely have an effect on your recipes would be the added salt.
Just use balsamic vinegar. Keep a bottle on hand and don't worry, it won't "go off" (although the pickled onions should be refrigerated as the package says).
Timothy S. Harlan, M.D.