How Should Cooking Oil Be Stored, and When Does It Turn Rancid?

Refined liquid oils like sunflower and corn oil, kept in a cool cupboard away from direct sunlight, hold their best quality for 1–2 years unopened and 4–6 months after opening. Light and heat are these oils' biggest enemies; both speed up the oxidation that makes oil go rancid. Unlike olive oil, refined oils have no issue sitting at room temperature and do not need to be refrigerated; what really matters is keeping the bottle tightly capped and out of the light. Why does cooking oil turn rancid? Rancidity happens when the unsaturated fatty acids in oil react with oxygen and break down into smaller molecules; the process speeds up with more light, heat, and air exposure. Oil left on a hot shelf right next to the stove turns rancid far faster than the same oil in a cool cupboard, because heat noticeably speeds up oxidation. Not screwing the cap back on tightly after every use also extends how long the oil sits exposed to air, which is another factor that speeds up rancidity. Where should an opened bottle of oil be kept? An opened bottle of oil should be kept in a closed cupboard, or at least on a shelf away from direct light, rather than near the stove or oven. Glass bottles let in less light than plastic ones, so oil in a dark glass bottle stays fresher a bit longer than the same oil in a clear plastic one. Leaving an oil bottle sitting on a well-lit counter constantly can lead to a noticeable change in taste within a few months. How can you tell oil has turned rancid? Rancid oil gives off a sharp, faintly metallic smell that a fresh bottle never has; that smell is most obvious the moment you open the bottle and it's right under your nose. It may taste slightly bitter, with a sometimes waxy aftertaste that can carry over into the food cooked with it. Rancid oil isn't dangerous, but it does spoil the taste of food; once you notice a change in smell or taste, stop using that bottle and replace it. How to extend the shelf life of your oil Screwing the bottle's cap back on tightly after every use keeps air exposure — the thing that speeds up oxidation — to a minimum. Buying a smaller bottle and using it up quickly also beats leaving a large one open for months; a big bottle used slowly can have its last portion tasting noticeably different from when it was first opened. Keeping oil in a closed cupboard rather than on a shelf right above the stove protects it from both light and the heat that rises while cooking. Does olive oil behave differently from refined oil? Olive oil holds up a little better against oxidation than refined oils, thanks to its natural antioxidants, but that doesn't mean the storage rules can be relaxed — olive oil needs protecting from light and heat just the same. Unlike refined oils, olive oil can turn cloudy or even solidify in the fridge; that's not spoilage, and it clears again back at room temperature. If frying oil will be reused a few times, strain it and store it in a dark, sealed container. Never reuse oil that has darkened or started foaming. Mixing different oils (olive oil, sunflower oil) in the same bottle isn't a good idea either — the blend both dulls the taste and makes it harder to tell which oil has gone rancid.