Oils have been used for cooking since prehistoric times. In general, oils come from vegetable sourcesplants, nuts, seeds, etc. An oil is extracted from its source by one of two methods. In the solvent-extraction method, the ground ingredient is soaked in a chemical solvent that is later removed by boiling. The second method produces cold pressed oils, which is somewhat a misnomer because the mixture is heated to temperatures up to 160°F before being pressed to extract the oil. After the oil is extracted, it's either left in its crude state or refined. Refined oilsthose found on most supermarket shelveshave been treated until they're transparent. They have a delicate, somewhat neutral, flavor, an increased smoke point and a longer shelf life. Unrefined (or crude) oils are usually cloudy and have an intense flavor and odor that clearly signals their origin. Most oils can be stored, sealed airtight, on the kitchen shelf for up to 2 months. Oils with a high proportion of monounsaturatessuch as olive oil and peanut oilare more perishable. Once opened, they should be refrigerated if kept longer than a month. Because they turn rancid quickly, unrefined oils should always be refrigerated. See fats and oils listing for detailed information on hydrogenated, monounsaturated and polyunsaturated oils. See also almond oil; canola oil; chili oil; coconut oil; corn oil; grapeseed oil; hazelnut oil; mustard oil; olive oil; palm oil; peanut oil; safflower oil; sesame oil; sunflower seeds; soybean oil; trans fatty acids; vegetable oil; walnut oil.