The wide array of cooking oils available, ranging from corn and soybean to sunflower and sesame, all have a roughly similar shelf life, and none can be stored indefinitely in the same way that spirits or wines can. The key to extracting the maximum life from an oil is not to have more than you need sitting around in opened containers and to protect the oil from heat and light. Go for a handful of small bottles which can be used up before they have the chance to expire rather than large containers. When oil does go bad, it announces the change by darkening and giving off a strong odor.
Breaking the Seal
Unopened cooking oils can stay good for about two years. Bottles that have been opened, however, are generally good for only about a year. Some oils, such as olive oil or nut oils, need to be refrigerated once they are opened. While the temptation might be to keep a gallon jug of oil next to the stove, the heat and the build-up of air in the container as the oil level decreases degrades the oil more quickly than if the oil were kept in a smaller bottle away from the heat. Oils should never be reused for cooking, as heat speeds up the process of rancidity.
The more saturated an oil is, the less susceptible it is to rancidity. The most stable cooking oils are saturated tropical oils such as coconut and palm. Typically, these are partly solid at room temperature. Monounsaturated oils such as olive, peanut and sesame are more likely to turn cloudy or even solidify in the fridge but do clear at room temperature. These oils are relatively stable and keep longer, whereas polyunsaturated oils such as walnut, grapeseed and corn oils are sensitive to heat and light and are among the most unstable. They should only be used for cooking at low temperature and must be used within six months.
The Oxidation Process
Oil that has been kept for too long takes on a darker color and rancid flavor which renders it unpalatable. As the oil is exposed to air it oxidizes, and the oils catalyze to free fatty acids through oxidative rancidity, which not only ruins the taste but poses health risks through toxicity, contributing to premature aging, arterial damage and cancer. The key to preventing rancidity is to limit the supply of oxygen by keeping the bottle tightly sealed and to transfer small amounts of remaining oil to a smaller container.
Refrigeration extends an oil’s longevity and is essential for delicate oils, such as walnut, which do not last long at room temperature once exposed to air. As a rule, the more saturated an oil is, the more likely it is to turn solid during refrigeration, but this has no connection with the oil’s usability. Where refrigeration is not appropriate, the bottle should be kept in a cool, dark place away from direct sunlight. If possible, transfer the oil to a dark or opaque container instead of a transparent one. Machine-pressed oils have a shorter storage life, as the process initiates oxidation.