All cooking oils, particularly those that are plant-based, like canola, olive and peanut, can turn rancid over time. In some cases, cooking oil can also become contaminated with harmful microorganisms that cause botulism. To keep you and your family safe, read labels carefully for expiration dates and follow some simple storage tips.
Dangers of Bad Cooking Oils
Generally, rancid cooking oils don't cause immediate health problems. The real danger is that rancid oil may contain harmful free radicals associated with serious health issues, including stroke and liver disease. Cooking oils with added garlic or herbs have a different issue. The U.S. Food and Drug Administration traces a number of cases of the paralytic disease botulism to bottles of these commercial and homemade oil mixtures, especially when they are stored at room temperature.
Shelf Life of Cooking Oils
Most containers of cooking oil should have a "use by" or "sell by" date on the label. If they don't, a general rule of thumb is that oil will still be good for six to 12 months from the date of purchase unopened, and six months if opened. Exceptions are coconut oil and shortening, which are saturated fats and have a shelf life of a couple of years. Flaxseed oil, which is more delicate, spoils within weeks. Oils with added herbs last two to three months in a cool room and up to eight months when refrigerated.
How To Tell If Cooking Oil Is Bad
Many rancid cooking oils will have an off odor that may be bitter or industrial smelling, almost like motor oil. They may also have a darker color than usual or a stale taste. Some people like the taste of rancid oil so much they can't tell it's gone bad, according to a study from the Olive Center at the University of California, Davis. Sometimes the only way to tell is by buying a new bottle of the same oil and doing a comparison smell and taste test. Just don't use such sensory tests to check for botulism, as the spores that cause the disease don't necessarily alter the smell or taste of the oil.
How to Store Cooking Oil
Butter and flaxseed oil should be stored in the refrigerator, but all other oils can be stored in a cool, dark pantry. Keep the oils away from sunlight. If you don't have a pantry, then the refrigerator is fine for those oils. Make sure the container is tightly sealed. When buying cooking oil, look for unrefined, cold-pressed oils, and organic when possible. Heavily processed oils have antioxidants like vitamin E removed, making the oil go rancid faster. You can add powdered or liquid vitamin E or rosemary extract to processed oils to keep them fresh longer.
- Columbia University Health Center: Is Reusing Cooking Oil Safe?
- Colorado State University Extension: Flavored Vinegars and Oils
- University of California, Davis: Olive Oil Study Shows Some Consumers Like It Rancid
- American College of Healthcare Sciences: Base Oils -- Shelf Life and Storage
- Crisco: Frequently Asked Questions
- Photo Credit John Foxx/Stockbyte/Getty Images