The term “vinegar” refers to a liquid that contains 5% acetic acid, and in turn, acetic acid is made by fermenting drinking alcohol (ethyl alcohol). There are two basic ways to create vinegar: either (a) a fruit juice can be fermented into a type of “wine” that contains alcohol, and then it is fermented a second time so that the alcohol becomes acetic acid, or (b) pure ethyl alcohol created from corn, wheat, or another starch (or from petroleum) is diluted in water, and then fermented until it becomes acetic acid. Type “a” vinegar will usually be referred to by the fruit juice (or other item) it was created from, and will have a relatively unique taste. For example, apple cider vinegar, malt vinegar, wine and balsamic vinegar, or rice vinegar, refer to vinegar made from apple juice, malted barley, grape juice, or rice respectively. Type “b” vinegar is called “white distilled” vinegar because it is colorless and the ethyl alcohol is purified/concentrated via the process of distillation.
Wine vinegar and balsamic vinegar made from stam yayin are not kosher, and are even assur b’hana’ah (except in cases of hefsed merubah). Accordingly, those types of vinegar obviously are only acceptable when bearing reliable kosher certification. In addition, other vinegars – whether made from other fruit juices (type “a”) or white distilled (type “b”) – require hashgachah because they are commonly made in the same plants as wine vinegar and because many of them include kosher-sensitive “nutrients” and/or enzymes. [Although vinegar is produced at below yad soledes bo, the ta’am from the wine vinegar can transfer to the other vinegars through kovush or if/when the vinegar is pasteurized to remove the “mother of vinegar”.]
For Pesach, vinegar needs special hashgachah, because (a) some vinegars are made from chametz ingredients; for example, malt vinegar (made from barley) or a white distilled vinegar which was made from wheat, (b) many vinegars are made from kitnios; for example, rice vinegar and most white distilled vinegar made in the United States, and (c) the nutrients and enzymes used in producing a vinegar might be made from chametz or kitnios. As a result, vinegar and all pickled items, require Pesach certification.
Vinegar has a sharp taste that qualifies as being “charif”, and therefore the following 4 halachos apply to vinegar or items pickled with it. [1] One may not store a kosher liquid in a non-kosher container for 24 hours (kovush), but if one did so, the general halacha is that the kosher food is b’dieved permitted because by the time the non-kosher ta’am comes out of the container, it is already aino ben yomo. However, that leniency does not apply if the kosher liquid is charif, because a charif liquid “reinvigorates” an aino ben yomo taste. Therefore, if canned peaches had been stored in a non-kosher container for 24 hours, the peaches would b’dieved be permitted, but if pickles had been stored in that same container they would be forbidden.
[2] There is a machlokes as to whether kovush of a davar charif requires 24 hours (like all other liquids) or just 6-18 minutes (like brine or other overly-salty liquids). It is generally accepted to follow the lenient opinion on this issue, and therefore kovush will not occur if pickles are stored in a container for less than 24 hours. [That said, as we saw earlier, once kovush does occur after 24 hours, the davar charif will reinvigorate the aino ben yomo taste which was absorbed into the container.]
[3] The general halacha is that one may not place hot food into a non-kosher kli sheini, but if one did then b’dieved the food remains kosher. That leniency does not apply to vinegar or other davar charif. [4] Before kashering, one must always remove all non-kosher residue from the item being kashered, but when kashering equipment that held vinegar or some other food that has a very strong flavor, they must also remove all odor from the equipment before kashering.