here i got this from GO ASK ALICE:
Reusing cooking oil has been done for ages. There really isn't a problem, if done properly. The greatest hazard is allowing the fat to become rancid (spoiled) and deteriorated to the point it produces undesirable flavors and odors. Besides ruining what would have been a perfectly good meal, rancid oils also contain free radicals that are potentially carcinogenic.
To understand how to best re-use oil, it is important to know about smoke points ? the temperatures at which oil begins to decompose. If you heat oil to a temperature that is too high, it produces smoke fumes. Acreolin, a substance that makes your eyes burn, is given off as well.
The optimal temperature to fry foods at is 190?C (375?F). At higher temperatures, the food will burn on the outside, and at lower temperatures, the food absorbs too much oil and tastes greasy. Different oils have different smoke points. Oils with higher smoke points are better for frying. The following is a list of smoke points:
Type of Oil Smoke Point Temperature
safflower oil 265?C
sunflower oil 246?C
soybean oil 241?C
canola oil 238?C
corn oil 236?C
peanut oil 231?C
sesame oil 215?C
olive oil 190?C
lard 183 - 201?C*
(*varies depending on the diet fed to hogs and the part of animal fat that is derived)
Each time the oil is re-used, the smoke point becomes lowered. This is due to:
foreign matter in the oil (such as batter)
the temperature to which the oil was heated
exposure to oxygen and light
length of time the oil remained heated
the number of times the oil was reused
Combining different types of oils also lowers the smoke point.
When oil becomes deteriorated, it appears dark and thick (viscous). It may have an off odor, and smoke appears before it reaches 190?C. If the oil smells funky and old, better to toss it away and replace it with fresh oil.
To re-use oil safely, use these tips:
Strain it through a few layers of cheesecloth to catch any food particles. Be careful with hot oil, though, because you can easily get burned.
Shake off excess batter from food before frying it.
Use a good thermometer to fry foods at 190?C.
Turn off the heat after you are done cooking. Exposing oil to prolonged heat accelerates rancidity.
Don't mix different types of oil.
Store oil in a cool, dark place.
Avoid iron or copper pots or pans for frying oil that is to be reused. These metals also accelerate rancidity.
Thread: Is reusing cooking oil safe?