Acid: Vinegar can corrode metals because it's acidic by nature (usually 4% to 6%), so you'll need to use glass or nonmetal containers and lids. On the upside, a high acid content also makes vinegar an unfriendly environment for bacteria, so you don't need to worry about spoilage.

Mother: If you buy an unpasteurized vinegar, however, don't be alarmed if a white film forms on the top. It's the harmless "mother" used to ferment vinegar. Just peel off and discard.

Flavoring: Drop a sprig of fresh rosemary or a few leaves of basil into some plain vinegar, and not very much happens. But gently crush the leaves between your fingers, and shazam! the "bruised" herb begins to release Pungent flavors that will seep into the vinegar as it ages. The results are dramatic. Just a little bit goes a long way.

Potency: Be sure, though, to match vinegars with the ingredients you add to them. Tart berries or strong flavored herbs (such as rosemary) can stand up to the boldness of red wine vinegar, while white wine or rice vinegars are best with milder fruits and herbs.

Shelflife: You'll also want to keep the batches small to keep flavors potent. Vinegars made with fruit, for example, stay at their flavor, peak for three to six months; vinegars made with herbs do so for about six months. But you can still use these vinegars beyond our recommended time periods. Their high acid content keeps them safe indefinitely. They just won't taste as intense.

~(Edited by Pat Hanneman 10/01/98)-

and Joanne Foran, authors of The Best 50 Flavored Oils and Vinegars.