When I'm done with a batch of coconut oil that I've used for frying, can I filter it and save it for another batch of frying or should I discard it? Is it too oxidized after it's been used to fry stuff?
Hope so, because I do that often and haven't really thought about oxidation at all!
Saturated fats are less prone to oxidation, though, and have a very high melting point since they are completely saturated with hydrogens. This is what makes them solid at room temp and a great fat for cooking with.
Coconut oil has more saturated fat than either lard or tallow, further protecting it from oxidation.
Ultimately, it probably depends on your cooking temperatures. If you're cooking at a very high temp often, you may want to change your oil more often just to be on the safe side. If you're at lower temps, however, you should be fine!
Here's a link to Kurt Harris' views http://www.paleonu.com/panu-weblog/2009/6/22/fats-and-oils.html
yes to tallow and lard. keep both in properly sealed containers after properly straining - cheesecloth works best; I find coffee filters cumbersome.
Dump when they start to smell rank - you'll know.
With highly saturated/stable fats like coconut oil and animal fats I wouldn't be overly concerned as long as you're not doing any high temperature frying, which isn't a good idea in the first place IMO (nutrient loss, carcinogen formation). The only thing I'd be worried about are leftover bits/particles of food from your previous dish getting burned or tainting the flavour of the oil.
Huh. I wouldn't dream of wasting my cooking fat by tossing it out. Not thrifty, lol ;)
I have 3 containers on my counter (mason jars) one for lard/beef drippings, one for pork/bacon grease, and one for coconut oil. I don't even "properly" filter them - I put my tea strainer over the container to catch the big stuff.
I've fortunately never had a problem with any of it going rancid!