In search of some guidelines on reusing oil, we fried chicken (thighs dredged in flour and cornstarch) and our Crunchy Kettle Potato Chips. We cooked both foods in identical Dutch ovens filled with 2 quarts of canola oil, strained the leftover oil after each fry, and used a kit to evaluate the oil, which turns it from blue to green in the presence of certain compounds that indicate degradation.
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The fried-chicken oil got progressively darker and stronger-smelling and turned increasingly green in the test kit vials, but the results were perfectly fine through four uses; only after the fifth batch did the chicken start to taste greasy and have off-flavors.
The potato-chip oil remained pale and clean-smelling and barely changed color in the test vials, producing identical chips through eight batches, at which point we stopped testing.
Our recommendation: With breaded and battered foods, reuse oil three or four times. With cleaner-frying items such as potato chips, it’s fine to reuse oil at least eight times—and likely far longer, especially if you’re replenishing it with some fresh oil.