In cooking, it is extremely important to select the best oil. Cooking an oil above its smoking point can result in poor flavor and burning. Oil is said to reach this point when it begins to emit smoke, and many oils have a flash point very close to the same temperature, meaning that the oil can potentially catch fire. This is not usually a desired effect when cooking, so most cooks keep a library of oils in order to enable the selection of the best oil for the cooking job.
When oil reaches the smoking point, it begins to degrade. It will taste bad, adding an acrid smell and taste to the food. The smoke forms substances that may be carcinogens, and should probably not be breathed or consumed. In addition to tasting bad and potentially being dangerous, oil which has reached this temperature is not suitable for cooking because it will burn the food.
Cooks need to select an oil based on the type of cooking job. In general, unrefined oils that are harvested using cold extraction have a lower smoking point than other oils. Unrefined oils include olive oil, avocado oil, and many nut oils. They tend to have a better flavor, and are excellent for use in sauteing, some baking, and for dressings. This type of oil should be used if the cooking vessel is not going to exceed 350°F (176°C), and should be selected on the basis of the flavor it will lend to the food.
Refined oils are extracted using heat and are therefore more able to withstand high temperatures, with a smoking point that is often double that of unrefined oils. In theory, any unrefined oil can also be made in a refined form. Refined oils tend to be less flavorful and will also be clear and free of sediment. They should be used for high heat cooking, including deep frying.
If oil in a pan does start smoking, the pan should be removed from heat and allowed to cool. The overheated oil should be discarded and replaced with one with a greater heat tolerance. Cooks should be aware that some blends of refined oil have a lower heat tolerance, while the best refined oils for high heat cooking are peanut, canola, safflower, and soy. Many cooks prefer peanut oil because it has a lighter feel when used in deep frying applications. Most oils will perform better if heated gradually, rather than dropped into an intensely hot pan. If heated and cooled slowly, some oils can be reused, although they should be discarded if they begin to discolor or smell rancid.