Although we can use our homemade wine vinegar straight from the fermenting vessels, we also like to pasteurize batches of it every now and then to squelch the acid-producing bacteria. Left unchecked, they'll produce more and more layers of cellulose—the "mother of vinegar"—until eventually the whole vinegar vessel is a solid mass of mother.
Whenever we've pasteurized, it's been hard to evenly keep the heat at the recommended temperature for destroying bacteria (155°) for 30 minutes. Recently we came up with a way that maintains the temperature more easily: a water bath. Gently heating milk for cheese, in a pot immersed in a larger pot of hot water, gave us the idea.
Here's how you do it: Fill a big pot about halfway with hot water and bring it to a boil. (If you're pasteurizing a large amount of vinegar at once, you can use a boiling-water canner.) As you're sitting around waiting for the water to boil, line a plastic colander with a few coffee filters, set it over a second pot (big enough for your vinegar and small enough to fit within into your water pot), and scoop the vinegar into the colander. This will filter out all the messy bits. Taste the filtered vinegar; if it's breathtakingly strong, add water to taste. Keep tasting until it reaches the strength you like.
Insert a clean candy thermometer (with a clip) into the vinegar pot and clip it to the side. Ease the pot into the now-boiling water, turn the heat off, and heat the vinegar to 155°F; hold it there for 30 minutes. If it climbs above 155°F, lift the pot out and let it sit until the temperature drops to 155°F. If it sinks below that mark, turn the burner back on under the water bath, put the vinegar pot back in, and let it climb up again.
After 30 minutes of pasteurizing, let the vinegar cool. Then, if you want it to mellow, pour the vinegar into a sterilized container (we really like the smooth flavor it develops in these clay vinegar crocks) and let it rest for about month. Or you can bottle it and use it right away.