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OIL. Oil is liquid fat, usually plant-derived, used as a cooking medium, as a lubricant to keep food from sticking to pans, and as a source of flavor. Oil from animal sources, particularly fish oil, is also used as a nutritional supplement. Oil is also ubiquitous in processed foods.

The difference between oil and fat is that oil is liquid at room temperature while fat is solid. Chemically, both are composed of hydrocarbon chains and are potent sources of energy. The strong molecular bonds of fats and oils make them relatively resistant to heat and thus suitable for high-heat cooking methods such as deep-frying. Oil can be used in conjunction with other fats, such as butter, to raise the temperature of the other fat at which it would otherwise begin to break down and smoke.

History of Oil

Olive oil and sesame oils are among the most ancient oils in the Western world, dating back to 4,000 years or more. Olive trees are relatively simple to cultivate and, once the olives are prepared, the oil can be obtained by pressing. Both olive oil and sesame oil were used in southern Europe, while northern countries typically used animal fats such as lard or goose fat.

In the Orient, oil was pressed from soybeans, while sesame, mustard seed, and safflower oils were used in India and ancient Egypt. Peanuts, corn, and sunflower seeds were available in the New World, but oil was generally extracted only from squash seeds, especially squash belonging to the species Cucurbita pepo.

The nineteenth century saw the rise of international trade in tropical oils, particularly palm and coconut oil. The raw materials, which included hearts of palm and dried coconut meat (copra), were exported from Africa and the Pacific islands to industrial countries to be pressed for oil. The vast production of corn in the United States provided a source of oil to be used in cooking and in the manufacture of oleomargarine.

In nineteenth-century Africa, both the French and British introduced larger-scale peanut farming specifically for peanut oil, which was used as an adulterant in cheap grades of olive oil, and as a base ingredient in soap.

Composition of Oils

Oils, like other fats, consist mostly of triglycerides, which are three fatty acids attached to a molecule of glycerol (an alcohol built around three atoms of carbon). A fatty acid consists of a carboxyl group (carbon, oxygen, and hydrogen linked together), which in turn is linked to a hydrocarbon chain. The more the hydrocarbon chain is filled with hydrogen atoms, the more chemically stable it will be. A fatty acid fully loaded with hydrogen is said to be "saturated," while a less hydrogen-rich acid is "monounsaturated" or "polyunsaturated," depending on its structure. Oleic acid is a common monounsaturated fat, while linoleic acid is the most common polyunsaturated fatty acid.

The more saturated a fat is, the more likely it is to be solid at room temperature (such as lard or shortening). Oils are liquid because they are less saturated than fats. They are also less shelf-stable because their chemical structures are more likely to be affected by exposure to oxygen, causing them to become rancid or develop offflavors. To make the product more stable, food processors pump hydrogen through the oil to fill in the gaps in its chemical structure, a process known as "hydrogenation." Highly hydrogenated oil is creamy or solid at room temperature, useful for making oleomargarine or other processed food products.

Oils generally have about the same caloric value of approximately 120 calories per tablespoon whether the fatty acids are saturated or not. More important to health-conscious consumers is the role of fatty acids in raising or lowering the presence in the bloodstream of the high-density lipoprotein (HDL) or low-density lipoprotein (LDL) associated with cholesterol. HDL (the "good cholesterol") is a beneficial substance that helps the body get rid of excess cholesterol, while LDL (the "bad cholesterol") builds up in the arteries and can increase the risk of heart disease. Eating foods high in monounsaturated fatty acids is believed to help lower LDL cholesterol levels and decrease the risk of heart disease, while the consumption of saturated fats may increase levels of LDL and total cholesterol. The consumption of polyunsaturated fats in place of saturated fats decreases LDL cholesterol levels. The American Heart Association recommends the consumption of oils that have no more than two grams of saturated fat per tablespoon.

Unsaturated fat content is a major selling point for household oils. One of the major oils on the market, canola oil, was developed specifically to appeal to consumers concerned about fat content. Canola—short for "Canadian oil"—is a variety of the rapeseed plant developed in Canada with less fatty acid than the traditional variety and thus a lower level of saturated fat than most other oils.

The chemical structure of oils makes them relatively stable at high temperatures, but at some point oil begins to break down and give off smoke; beyond this point there is danger of fire. The smoke point for most oils is around 410°F (210°C), although some oils have even higher smoke points. The smoke point gets lower as oil is reused due to degradation of chemical bonds and contamination of the oil with food particles. Commercial operations that re-use oil will pass it through a filter to take out the contaminants.

Sources of Oil

In addition to olives, oil is obtained from legumes such as peanuts and soybeans; from the seeds of many plants, including corn, rapeseed (canola), sesame, cottonseed, sunflower, palm, safflower, coconut, grapeseed, mustard, pumpkin, and avocado; and from tree nuts such as walnuts,

Smoke Points of Common Oils
Oil	Smoke point (degrees F)
Sunflower	392
Olive	410
Corn	410
Peanut	410
Soybean	410
Cottonseed	435
Avocado	435
Canola	437
Grapeseed	446
SOURCE: The Simon and Schuster Pocket Guide to Oils, Vinegars, and Seasonings

Fat Content of Major Household Oils
Oil	Calories per tablespoon	Saturated fatty acids (grams per tablespoon)	Polyunsaturated fatty acids (grams per tablespoon	Monounsaturated fatty acids (grams per tablespoon)
Olive	119.3	1.8	1.1	9.9
Corn	120.2	1.7	8.0	3.3
Canola	123.8	1.0	4.1	8.2
Peanut	119.3	2.3	4.3	6.2
Sesame	120.2	1.9	5.7	5.4
Soybean	120.2	2.0	8.0	3.2
Soybean, hydrogenated	120.2	2.0	5.1	5.8
Sunflower, 70% oleic and over	123.8	1.4	0.5	11.7
Sunflower, less than 60% linoleic	120.2	1.4	5.5	5.5
Sunflower, 60% linoleic and over	120.2	1.4	8.9	2.6
Sunflower, linoleic, hydrogenated	120.2	1.8	4.9	6.3
Grapeseed	120.2	1.3	9.5	2.2
Cottonseed	120.2	3.5	7.1	2.4
Safflower, over 70% linoleic	120.2	0.8	10.1	1.9
Almond	120.2	1.1	2.4	9.5
Rice bran	120.2	2.7	4.8	5.3
Avocado	123.8	1.6	1.9	9.9
Palm	120.2	6.7	1.3	5.0
Fish oil, menhaden, fully hydrogenated *	112.7	11.9	0.00	0.00
Unlike oils from plant sources, fish oil contains cholesterol. Fully hydrogenated menhaden oil contains 62.5 milligrams of cholesterol per tablespoon. SOURCE: U.S. Department of Agriculture, Agricultural Research Service. 2001. USDA Nutrient Database for Standard Reference, Release 14. Nutrient Data Laboratory Home Page, http://www.nal.usda.gov/fnic/foodcomp.

almonds, hazelnuts, and pistachios. Tree nut oils are usually expensive and do not respond well to high heat, so they are used primarily to dress salads or to add flavor to baked goods.

The finest oils are simply extracted from the raw material (such as olives or nuts) by pressure. This is called "cold pressed" or "first pressed" oil. Oil that remains bound up in the raw material can be extracted by heat or chemical solvents.

Oils labeled by specific names, such as peanut oil, are obtained from those particular plants; a product labeled merely "vegetable oil" is a blend of various oils. In the production of vegetable oils, seeds are cracked, cooked, and run through a press to extract readily obtainable oil. The pulp is further processed to obtain the rest of the oil, which is neutral and tasteless; if flavor is desired, the oil can be mixed with the product of the first pressing to restore flavor and color.

Uses of Oil

Oil has a variety of uses in cooking, most of them based on its ability to transfer heat to the food while remaining stable itself. Asian food is often stir-fried in a little hot oil, just enough to keep the food from sticking to the pan. Chicken can be sautéed or fricasseed in a few tablespoons of oil, while pieces of breaded fish can be fried in shallow oil—perhaps a quarter of an inch deep.

Frying large pieces of food in deep oil requires a temperature of 350° to 375°F (177° to 191°C). At that temperature, the hot oil will sear the surface of the food being fried, trapping moisture within the food. The food thus cooks in its own moisture rather than in the oil, which is why properly fried food is not greasy. Greasiness results from frying at a temperature lower than optimal.

In Italy, olive oil is used as a dipping sauce for bread at the table. Olive oil is preferred as a salad dressing because it has its own flavor to contribute to the dish.

In food manufacturing, oils are used in a host of products, ranging from soups and gravies, salad dressings, bread and rolls, and fried foods to nondairy toppings and frozen desserts, coffee creamers and cocoa mixes, candy bars and cakes, and in most processed snack foods.

Fish Oil

Some species of fish such as Atlantic menhaden have high levels of certain essential fatty acids, called omega-3 fatty acids, which the human body is not able to synthesize by itself and must obtain from food. Oil made from these fish is sold as a nutritional supplement. Menhaden oil is used in the production of margarine and shortening in Europe and has been approved for use in the United States.

See also Butter; Cooking; Fats; Fish; Frying; Snacks.

Richard L. Lobb