Oil is any greasy substance that does not dissolve in water but can be dissolved in a liquid called ether. Many different kinds of oil exist. Most oils are lighter than water and are liquid at room temperature. A few oils, such as lard and butterfat, are solid at room temperature.
People obtain oils from animal, mineral, or plant sources. Oil from minerals, also called petroleum or crude oil, consists of a mixture of compounds that can be separated into fuel oils, mineral oil, and other petroleum products. This article discusses animal and plant oils, which differ chemically from mineral oils. For more information on mineral oils, see Petroleum.
Animal and plant oils consist chiefly of carbon, hydrogen, and oxygen. They belong to a family of materials called lipids. People classify animal and plant oils as fixed or volatile, depending on whether the oil evaporates under normal conditions.
Fixed oils,
which do not evaporate under ordinary conditions, are also called fatty oils or simply fats. They include all animal oils and many oils from plants. Butterfat, lard, and tallow rank as the chief animal oils. Margarine and salad oil consist mainly of fixed plant oils. Other products that are made from fixed oils include candles, linoleum, lubricants, paint, and soap.
Butter makers churn cream to produce butter. People render (heat) fatty animal tissues to obtain other kinds of animal oil. Lard is made from fatty tissues of hogs, and tallow from cattle, goats, or sheep. Oils from such sea animals as fish and whales are called marine oils.
Most fixed plant oils are extracted from such seeds as corn, cottonseed, and soybeans. Processors obtain olive and palm oils from the fruit pulp surrounding the seed. For information on the processing of such oils, see Vegetable oil.
Exposure to air causes fixed oils to thicken. Depending on the degree of thickening, they can be classified as (1) drying, (2) semidrying, or (3) nondrying.
Drying oils absorb oxygen from the air to form a tough film. They are widely used to make paints and varnishes. Important drying oils include linseed, perilla, tung or chinawood, soybean, oiticica, hemp, walnut, poppy seed, and sunflower. Linseed oil comes from flax seeds. It is one of the most important drying oils. It is used in making paints and varnishes. Tung oil is a valuable oil used in waterproof varnishes and quick-drying enamels.
Semidrying oils absorb oxygen from the air to become very thick, but not hard. Cottonseed, corn, and sesame oils are of the semidrying type.
Nondrying oils absorb oxygen from the air with little increase in thickness, but they often become rancid—that is, they develop unpleasant odors and flavors. Olive, peanut, and grape-seed oils, butterfat, and lard are examples of nondrying types.
Volatile oils,
also called essential oils, evaporate quickly, especially when heated. Some of these oils come from plants, and others are artificially created. People use volatile oils chiefly for their flavor or odor. The taste of such food flavorings as lemon, mint, and vanilla extracts results from the volatile oils they include. Volatile oils also flavor chewing gum, tobacco, and toothpaste. Most fine perfumes include fragrant oils obtained from roses and other flowers. Manufacturers use such volatile oils as lemon oil to give a scent to soaps and other cleaning products.
Natural volatile oils are extracted from various parts of plants, including bark, flowers, leaves, roots, seeds, and twigs. Methods of obtaining plant oils used for their scent are described in the Perfume article. Food-processing companies follow similar procedures in extracting natural volatile oils for flavoring.
Manufacturers use chemical processes to make artificial volatile oils from coal, petroleum, wood, and other substances. Some artificial oils duplicate natural oils. Others differ from any substance found in nature. Such products as artificial oil of rose and artificial vanilla extract cost much less than similar natural substances.
