Protect from Heat, Light and Air

• Cooking oil is a type of fat that remains liquid at room temperature. Common oils used for cooking include canola oil, corn oil, olive oil, safflower oil, soybean oil and sunflower oil. Although they come from different plant sources, cooking oils share the same vulnerability for storage. Exposed to heat, light and air, oil oxidizes as it ages and turns rancid, becoming both bad-tasting and dangerous to eat. Although cooking oil often comes in transparent plastic bottles, it's better to choose oils that come in dark-colored bottles. Olive oil is especially sensitive to light and air, which is why its purest form, extra virgin olive oil, comes in dark-colored glass bottles.

Aluminum Conducts Heat

• Given the storage needs for cooking oil, aluminum makes a poor choice as a container. Aluminum conducts heat very well, almost as well as copper, so it can't protect cooking oil from warm temperatures. Although the cans are opaque so that they keep out light, aluminum is what's known as a "reactive" metal. It can leach its metal into acids and oils, giving them a metallic taste. It can also discolor light-colored foods such as pale vegetable oils. Being a soft metal, aluminum can be scratched easily, making it more likely to react with its contents. Aluminum containers that have been anodized, such as small cruets for salad oil, can be used to store oil because the anodizing process creates a hard surface on the metal. The hard surface protects the can's contents from reacting with the metal. However, anodizing is rather expensive and typically is used more for cookware than containers.

Refrigerate Oils

• Refrigeration is another option for storing cooking oil in smaller quantities. Storing in the refrigerator can congeal oil and turn it cloudy, but this doesn't hurt the quality of most cooking oils. The oil often clears up when it warms to room temperature for use in cooking. While it won't damage its quality, refrigeration isn't recommended for olive oil because condensation can occur inside the bottle and affect its taste.

Check for Rancidity

• Large amounts of cooking oil that come in steel cans should be separated into smaller glass or plastic containers for storage. Even cooking oil that's properly stored should be used within a few months of being opened. All oils will turn rancid by 18 months whether opened or not, and should be discarded. Always test cooking oil for rancidity by tasting it before use.