Oils, Plant-Derived

Plant oil sources are typically the seeds or seed coats of plants. Plant breeding and genetic engineering have made available many plant oils with fatty acid compositions quite different from the typical values cited in the accompanying table.

Oils are extracted from plants by using pressure or solvents, usually the petroleum fraction hexane. Olive oil, for example, is a typical seed coat oil and is extracted by multiple pressings of the fruit pulp. The oil from the first pressing has the best quality and is termed virgin oil. Oilseeds may be extracted with pressure in a mechanical expeller but usually are cracked and pressed into flakes for extraction with hexane. The hexane is removed from the extracted crude oil by distillation.

Crude oils contain small amounts of undesirable pigments , phospholipids, and free fatty acids (i.e., fatty acids not chemically linked to glycerol) that make the oils dark, hazy, and smoky, respectively, on heating. Olive oil has a good flavor and is typically sold without treatment other than filtering or centrifugation for clarity. However, most other oils are refined. Refining involves mixing with water to wash out phospholipids (degumming), treatment with lye solutions to remove free fatty acids, bleaching with absorptive clays to remove pigments, and a vacuum steam treatment (deodorization) to remove undesirable flavors. Plant oils also contain small amounts of sterols and fat-soluble vitamins. These may be partly removed by deodorization and are regarded as harmless or desirable components of the oil.

Although most plant oils are used as food they are also used to make such things as paint and surface coatings, detergents, linoleum, and plastics. Some plant oils, such as castor and tung, contain special fatty acids used to make surface coatings.

Oils from plants are chiefly triglycerides, which are made up of one glycerol molecule linked to three fatty acids. The fatty acids have linear carbon chains varying in length, generally from six to twenty-two carbon atoms, with various amounts of hydrogen linked to the carbon. Carbon chains that hold all the hydrogen that they can are called saturated, and those with less hydrogen are unsaturated. Where the unsaturation occurs, the carbon chain is linked by double bonds.

Most plant oils are clear liquids at ambient temperatures rather than fats, which are plastic solids at room temperature. Butters, such as cocoa butter (chocolate fat), melt around room temperature. The solidification temperature of an oil depends on the length and saturation of its fatty acid chains. Short chains and double bonds (less saturated) decrease the solidification point. To change liquid oils, such as soybean oil, to a shortening or margarine, the oil is treated with hydrogen under pressure and a nickel catalyst. The resulting more saturated fat is said to be hydrogenated. During hydrogenation some of the double bonds are converted from their native cis form to trans isomers.

Fats and oils provide the most concentrated source of calories in the human diet, about nine calories per gram. Certain fatty acids produced in plants are nutritionally required. These essential fatty acids contain multiple double bonds and are called polyunsaturated. They come in two families called n-3 or n-6 based on the position of the first double bond counting from the tail of the fatty acid chain.

see also Economic Importance of Plants; Lipids; Seeds.

Earl G. Hammond

Bibliography

Hammond, E. G. "The Raw Materials of the Fats and Oils Industry." In Fats and Oils Processing, ed. P. Wan. Champaign, IL: American Oil Chemists' Society, 1991.

Hegarty, Vincent. Nutrition Food and the Environment. St. Paul, MN: Eagen Press, 1995.

Stryer, Lubert. Biochemistry. New York: W. H. Freeman and Company, 1995.

Ulbricht, T. L. V., and D. A. T. Southgate. "Coronary Heart Disease: Seven Dietary Factors." Lancet 338 (1997): 985-92.

HEALTHY FATS AND OILS

Animal fats contain the sterol cholesterol. The human body naturally produces all the cholesterol it needs; therefore, overconsumption of fatty foods rich in cholesterol is believed to encourage artery disease. Artery disease is also influenced by the fatty acids we consume. Fats and oils are considered healthy if they contain low proportions of saturated fatty acids with chain lengths of twelve to sixteen carbons. The animal fats lard, tallow, and milk fat and the plant oils palm, palm kernel, and coconut contain significant proportions of these less-desirable fatty acids. An atherogenicitiy index (AI) for fats and oils has been proposed to predict their tendency to cause artery disease.

AI [%12:0 4 (%14:0) %16:0] / % all unsaturates where %12:0 represents the weight percent of a fatty acids with twelve carbons and no double bonds, and so on.

A low index value is desirable. The AI of animal fats range from 0.6 to 4. Some believe that consumption of the fatty acids with trans double bonds formed during hydrogenation also predis-poses us to artery disease. Our diets should contain adequate amounts of the unsaturated n-3 and n-6 fatty acids. We should consume less than 30 percent of our total calories as fat.