group of organic substances that form an important part of the diet and also are useful in many industries. The fats are usually solid, the oils generally liquid at ordinary room temperatures. Some tropical products, liquids in their sites of origin, become solids in cooler climates; in commerce these often retain the name originally given, e.g., palm oil and coconut oil. Chemically fats and oils are either simple or mixed glyceryl esters of organic acids belonging to the fatty-acid series (see triglycerides ; fatty acids ). Fats and oils are derived from both plant and animal sources.

Commercial Processing of Fats

Among the vegetable oils of greatest commercial importance are cottonseed, linseed, olive, palm, corn, peanut, soybean, and castor oils. The method of obtaining the oils is similar for all: the fruits or seeds after being cleaned are crushed and pressed cold to obtain the highest grade of oil and then pressed warm, yielding a grade suitable for industrial use. Sometimes solvents are used to remove the remaining oil from the crushed mass. Edible oils are those used in foods, and for these the highest grade is utilized; these must be pale in color, free from disagreeable odor and taste, and wholesome. The lower grades are suitable for making soap and for other industrial purposes. The chemical property that makes fats solid and oils liquid is the amount of saturation in the ester (see saturated fats ). Animal fats are esters of saturated fatty acids; vegetable oils are esters of unsaturated fatty acids.

Conversion of liquid vegetable oils into solid fats is an important chemical industry. This process, sometimes called hardening, involves hydrogenation of the unsaturated fatty-acid portion of the oil molecule by heating the oil with hydrogen in the presence of a metal catalyst; by controlling the extent of hydrogenation, various products can be obtained. For example, controlled hydrogenation of cottonseed oil produces a solid vegetable cooking fat. Most fats become rancid upon standing; since a major factor leading to rancidity is air oxidation of double bonds (to form foul-smelling aldehydes ), saturated fats are much more resistant to rancidity than unsaturated fats.

Fats as Food

Animal fats used in foods include butter, lard, chicken fat, and suet. Cod-liver oil and some other fish oils are used therapeutically as sources of vitamins A and D. Nutritionally fats and oils are valued as a source of energy. Because they contain less oxygen than other nutrients, they oxidize more readily and release more energy. Fats are digested in the human body chiefly by the enzyme lipase (in the pancreatic juice) aided by the bile. There are several theories to explain the method of absorption of fats; favored by many is the view that they are absorbed by the epithelial cells of the lining of the small intestine in the form of the fatty acids and glycerol into which they are split by digestion and that a recombination to re-form the fat occurs within the cells. Most of the fat then enters the lymphatic system through the villi in the lining of the small intestine, although some is probably absorbed directly by the blood vessels of the villi. Medical research indicates the possibility that saturated fats in the diet contribute to the incidence of arteriosclerosis ; such fats may raise the blood's level of cholesterol, which is deposited in the arteries.

See oils ; petroleum.

The Columbia Encyclopedia, Sixth Edition Copyright© 2004, Columbia University Press. Licensed from Lernout & Hauspie Speech Products N.V. All rights reserved.

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