More Chowhounds are rendering their own fat at home these days—jen223 has a refrigerator bulging with jars of home-rendered beef, duck, and chicken fat. But how long does it last? How can you best keep it fresh, and how will you know if it's rancid?
Commercially rendered animal fat tends to keep forever, JungMann says. Unfortunately, home-rendered fat doesn't last as long, as tiny amounts of impurities in the fat (like water, protein, and blood) can cause rancidity. Since the bits of meat and impurities tend to settle to the bottom, seamunky says, sprinkling a layer of salt at the bottom of your storage jar before pouring in the fat will keep those bits from spoiling the whole batch. Or let the fat settle, spoon off the snow-white fat from the top, and toss the bits at the bottom, hotoynoodle says. (Or you could use those bits right away to flavor another dish.)
For long-term storage, the freezer is best, hotoynoodle says. Fat can get freezer burn or otherwise lose its oomph, but it won't get rancid. And since fat doesn't freeze as hard as water, you can freeze it in bags and chip off bits as needed, hotoynoodle says.
As for recognizing when it's rancid, you'll know from the taste—melpy says it'll make your tongue tingle from the acidity. You won't necessarily be able to smell anything wrong with it, JungMann says, but the taste will definitely be horrible.