I will be deep frying apple fritters this weekend and am curious what to do with the oil when I am done. Can you reuse it?
For an apple fritter, yes. Let the oil cool, strain it and refrigerate it.
If you invite me over, I'll show you. : ) Yum.
First, make sure you use corn, soybean or other vegetable oil with a high smoke point. Unless you can find high oleic canola oil, nearly impossible in the average grocery store, stay away from canola oil, which acquires an off-putting odor, almost fishy, after one use, which transfers to delicate foods.
If you make apple fritters by dipping apple slices into a plain batter and deep frying them, let the oil cool, strain it through a coffee filter placed inside a mesh sieve, pour the oil into a container with a tight-fitting cap and store at room temperature in a dark cabinet. (I use the original oil bottle, and I place the sieve inside a funnel and pour the old oil directly into the bottle.) You can use this oil for french fries or anything else you deep fry.
If you make apple fritters by dicing apples and adding them to a cinnamon-flavored dough-like batter, you can save the oil but you shouldn't use it for fries or beer-battered fish unless you want them to be infused with the scent of cinnamon.
An awful lot of professional cooks won't deep fry in pure fresh oil; they mix a little old oil with fresh oil, not for flavor but for better fry-ability--things brown quicker and more evenly if about 1/4 of the oil is old.
You can tell when oil has broken down too much to be any good for frying: it gets thick, dark and foamy. Toss it if it reaches this point, unless you want to try to make your own plastic, in which case you can keep heating it.
I'm not in the camp that reuses oil. There's a rancidity factor that frankly I just don't like. Used up oil I funnel into empty Pelligrino bottles (I drink a lot of sparkling water). After that use your imagination.