Cooking oil is purified fat of plant or animal origin, which is liquid at room temperature.

Some of the many different kinds of edible vegetable oils include: olive oil, palm oil, soybean oil, canola oil, pumpkin seed oil, corn oil, sunflower oil, safflower oil, peanut oil, grape seed oil, sesame oil, argan oil and rice bran oil. Many other kinds of vegetable oils are also used for cooking.

The generic term "vegetable oil" when used to label a cooking oil product refers to a blend of a variety of oils often based on palm, corn, soybean or sunflower oils.

Oil can be flavoured by immersing aromatic food stuffs such as fresh herbs, peppers, garlic and so forth in the oil for a period of time. However, care must be taken when storing flavored oils to prevent the growth of Clostridium botulinum (the bacteria that produces toxins that can lead to botulism).

Health and nutrition

The appropriate amount of fat as a component of daily food consumption is the topic of some controversy. Some fat is required in the diet, and fat (in the form of oil) is also essential in many types of cooking. The FDA recommends that 30% or less of calories consumed daily should be from fat.^[1] Other nutritionists recommend that no more than 10% of a person's daily calories come from fat.^[2] In extremely cold environments, a diet that is up to two-thirds fat is acceptable and can, in fact, be critical to survival.

While consumption of small amounts of saturated fats is essential, excessive amounts of such fats has been shown to be correlated with coronary heart disease. Oils that are particularly high in saturated fats include coconut, palm oil and palm kernel oil.^[3] Oils with lower amounts of saturated fats, and higher amounts of unsaturated (preferably monounsaturated) fats, are generally healthier.^[3].

Health and particular oils

While such general principles can provide general dietary guidelines, it's also important to consider the dietary characteristics of individual oils. Olive oil, for example, raises "good" HDL cholesterol, a heart-healthy effect that need not be limited by an arbitrary figure.

Peanut, cashew and other nut-based oils may also present a hazard to persons with a nut allergy. A severe allergic reaction may cause anaphylactic shock and result in death.

Trans fats

Trans fats are unsaturated fats that are not required or beneficial for health. Hydrogenation, a process that adds hydrogen atoms to fat molecules to make them more saturated, is responsible for most dietary transfats. Oils are hydrogenated to increase their melting point (for example in making margarine).

Cooking with oils

Heating an oil changes its characteristics. Some oils that are healthy at room temperature can become unhealthy when heated above certain temperatures. When choosing a cooking oil, it is therefore important to note the oil's heat tolerance, and to match the oil to its use in cooking.^[4]. Oils that are suitable for high temperature frying (above 280°C/500°F) include:

• Canola oil
• Almond oil
• Apricot kernel oil
• High-oleic safflower oil or sunflower oil
• Peanut oil (marketed as 'groundnut oil' in the UK)
• Soybean oil
• Grape seed oil

Oils suitable for medium temperature frying include:

• Walnut oil
• Sunflower oil
• Sesame oil

Unrefined oils should be restricted to temperatures below 105°C/225°F.

Storing and keeping oil

Whether refined or not, all oils are sensitive to heat, light and exposure to oxygen. Rancid oil has an unpleasant aroma and acrid taste, and its nutrient value is greatly diminished. To delay the development of rancid oil, a blanket of an inert gas, usually nitrogen, is applied to the vapor space in the storage container immediately after production. This is referred to as tank blanketing.

It is best to store all oils in the refrigerator or a cool, dry place. Oils may thicken, but if you let them stand at room temperature they will soon return to liquid. To prevent negative effects of heat and light, take oils out of cold storage just long enough to use them. Refined oils high in monounsaturated fats keep up to a year (if they are olive oil, they'll keep up to a few years), while those high in polyunsaturated fats keep about six months. Extra-virgin and virgin olive oils keep at least 9 months after opening. Other monounsaturated oils keep well up to a high eight months; unrefined polyunsaturated oils only about half as long.

Types of oils and their characteristics