Whether frying oil can be reused depends not only on its condition but also on what was fried in it. Fish, garlic- or spice-flavored batters, and sauced or marinated meats leave a lasting odor in the oil. Do not use that oil again for sweet or neutral foods, but it can be used for the same type of savory frying. The question is what the oil pairs with.
During frying, oil does more than transfer heat; it also absorbs aromatic compounds from the moisture and surface of the food being fried. Some of these compounds dissolve in the oil and cannot be removed by straining. Neutral foods such as potatoes leave almost no trace in the oil, while fish leaves the strongest trace, and the next food fried in that oil will smell like fish. Straining removes crumbs, but not odors; that is why the decision depends on the oil’s history, not on the strainer. If you are going to fry two different foods in the same oil, fry the neutral food first and the strongly flavored food second.
For this reason, separate oils according to what they have fried. Oil used for frying fish and seafood should be used only for fish again, and never for food served to someone with a fish or shellfish allergy. Oil used for frying garlic-, onion-, or spice-flavored batters, meatballs, and coated meat should be reserved for savory fried foods. Oil used for frying potatoes, plain batter, and vegetables has the widest range of uses. Doughs that will be soaked in syrup, such as lokma and tulumba, need either clean oil or oil that has been used only for potatoes; oil left from a savory frying will transfer its flavor to these doughs.
Some fried foods also change the oil in ways other than adding odors. Coatings made with flour or breadcrumbs shed crumbs into the oil; when these crumbs settle at the bottom and burn, the oil turns bitter and dark. Remove the crumbs with a skimmer during frying as well; oil like this can be used only after straining and then only for another coated food. Meats marinated in a sugary, honey-based, or tomato-paste sauce leave sugar and tomato paste in the oil; they burn quickly at high heat, and the oil darkens and begins to foam. Oil that smells burned should never be used for frying again.
A practical way to remember these pairings is to label the oil. Write what was fried—fish, meatballs, potatoes—on the bottle or jar of strained oil. Add the date, too, so you can see how long the oil has been stored. Do not combine 2 oils with different histories in the same bottle, because the mixture takes on the history of the most strongly flavored oil. In a kitchen that does little frying, the most practical option is often to discard the oil left from a strongly flavored frying without storing it separately and use fresh oil for neutral foods.
This pairing rule does not say how many times oil can be used; it only says what it can be used for. Whether the oil has deteriorated is a separate question, answered by its own signs: oil that starts smoking early, produces persistent foam, darkens, and smells sharp should not be used, whatever its history. Ask both questions. Check the oil’s condition first; if it is still in good condition, check its history. If one of the 2 answers is no, do not return the oil to the frying pan; dispose of it properly.
A Note for Readers
This content is for informational purposes only and is not a substitute for medical advice. Consult your doctor before making any decision about your health.