essential oil(redirected from Aromatic herb)
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an odorous substance that imparts the characteristic odor of the plant from which it is derived. Essential oils are synthesized in special cells of various plant organs. The biological role of the oils has not been confirmed, but it is assumed that they reduce heat loss and serve as attractants or repellents.
Essential oils are multicomponent mixtures of organic compounds, mostly terpenes and their oxygen derivatives (alcohols, aldehydes, ketones, ethers, and esters). In some cases, only one or a few components predominate. For example, more than 200 organic compounds have been found in rose oil, but beta-phenyl-ethyl alcohol and terpene alcohols (geraniol, linalool, citronellol, and nerol) account for 80 percent of the oil’s mass. Peppermint oil has more than 100 compounds, but menthol, menthone, menthyl acetate, and cineole constitute more than 90 percent of the mass. The composition of essential oils often changes markedly during the development of a plant. For example, coriander oil obtained from flowers contains as much as 80 percent decyl aldehyde, whereas coriander oil derived from seeds contains 60–80 percent linalool.
Essential oils are transparent colorless or colored (yellow, green, or red-brown) liquids. In contrast to vegetable oils, many essential oils are volatile. Their density, as a rule, is less than 1.0. The oils are virtually insoluble in water, but they readily dissolve in ether, petroleum ether, benzene, and other low-polarity organic solvents. Essential oils are optically active. They are gradually oxidized and become resinous by the action of light and atmospheric oxygen, resulting in a change in odor.
Essential oils have been known since antiquity. They have been used in incense, cosmetic preparations, medicinal drugs, and embalming agents.
Of the approximately 3,000 plant species that contain essential oils, about 1,000 are found in the USSR. Only 150 to 200 species have industrial importance. Most essential oils are obtained from tropical and subtropical plants; only a few essential-oil plants, for example, coriander and anise, are cultivated in the temperate zone. In 1976, world production of essential oils was about 25,000 tons. Only a relatively few oils—citrus, citronella, lemongrass, peppermint, coriander, anise, and oil of cloves—are produced on a large scale of no less than 1,000 tons annually. The oil is obtained from raw (geranium, basil), dried (coriander leaves, the roots of sweet flag), or fermented (iris roots, oakmoss) plant material.
Steam distillation is the most common method for obtaining essential oils. Modifications of the method include the treatment of plant material with “dry steam.” The mixture of essential-oil vapors and water is condensed, and the oily layer is separated. For more complete extraction of the essential oils, the aqueous distillation layer is treated with active carbon or a highly volatile solvent, for example, ether.
Essential oils whose main components would decompose under the relatively vigorous conditions of steam distillation are obtained by extraction with organic liquids (for example, petroleum ether or benzene) or with liquified gases (for example, carbon dioxide). Extraction is used in the production of essential oils from jasmine flowers and iris roots. The residue after distilling off the solvent is usually a waxy or oily mass, which is treated with alcohol, usually by boiling. The solution obtained is cooled and filtered to remove waste matter. The essential oils remaining after removal of the alcohol solvent are said to be pure, or absolute.
Expression is used for the derivation of oils from the rinds of citrus fruits. Also common is enfleurage, a method by which the oil evaporating from flowers is absorbed by pure, scentless lard or beef fat applied in a thin film on a glass plate. Essential oils are extracted from the fragrant mass with a solvent. Maceration, a procedure involving the pouring of fat heated to 50°–70°C onto the flowers, yields low-quality oil and is rarely used.
The unavailability of some natural essential oils has led to their replacement by artificial compositions based on synthetic fragrances (artificial or synthetic oils).
Essential oils are used mainly in the perfume and cosmetics industry, where they serve as the raw material in the production of fragrances (menthol, citral, eugenol, geraniol, linalool). Some essential oils (peppermint, eucalyptus, and anise oils) are used in medicine, and some are used in the food, confectionery, and canning industries (peppermint, anise, orange, lemon, and mandarin oils). In these cases, the relatively toxic terpene hydrocarbons are removed by distillation or treatment with alcohol.
REFERENCESGoriaev, M. I. Efirnye masla flory SSSR. Alma-Ata, 1952.
Koral’nik, S. I., and L. Iu. Neiman. Sovremennye resursy i osobennosti proizvodstva efirnykh masel. Moscow, 1973.
Guenther, E. The Essential Oils, vols. 1–6. New York, 1948–52.
V. N. FROSIN