Essential Oils

It is difficult to define essential oils precisely and concisely; for practical purposes they may be described as natural odoriferous compounds occurring in, or isolated from, plant materials. Normally liquid (in some instances semisolid; rarely solid), insoluble in water and volatile with steam, they evaporate at different rates under ordinary atmospheric pressure at room temperature. Hence the alternative term "volatile" or "ethereal" oils. (The common term "essential" originates from the Latin "essentia"- the "quinta essentia" which the medieval alchemists considered the characteristic and most important component of every natural substance.) It is in their relatively rapid evaporation and pronounced odor-aside from their chemical composition-that the essential oils differ fundamentally from the fixed, fatty oils. Of the many thousands of plant species known, a relatively small number yield essential oils. These may develop throughout the entire plant, or in certain parts only. Some oils occur solely in the root, or the wood, the bark, the leaves, flowers, or fruit. In some cases different organs of a single plant may contain essential oils of different chemical composition.

Several theories have been advanced to explain the biochemistry of the essential oils; however, none can be accepted as completely satisfactory. Perhaps the oils are merely elimination products in the" life processes of the plant. If so,' they resemble certain gums, balsams, and resins; some essential oils appear, indeed, to be precursors of such exudation products.

The essential oils vary widely in their physicochemical properties, and their chemical composition is usually complex. A few are composed almost exclusively of one component - for example, oils of wintergreen and sweet birch (methyl salicylate), and cassia oil (cinnamaldehyde). Most essential oils, however, contain a larger number of constituents, 50 and more not being unusual. These individual components belong to many classes of organic compounds, particularly the terpenes and sesquiterpenes, and their alcohols, esters, aldehydes, ketones, lactones, oxides, etc., some of them open chained, many cyclic and bicyclic. Members of the aromatic series, too, are present (for example, phenylethyl alcohol and benzyl acetate), and from recent investigations it appears that the azulenes play an important role in essential oils.

Assay is accomplished by conventional physicochemical tests, such as determination of specific gravity, optical rotation, solubility in alcohol, boiling range, etc.; determination of free acids, alcohols, esters, aldehydes, ketones, phenols and phenol ethers. Enormous progress in the examination of essential oils-particularly in the isolation and identification of individual constituents-has lately been made by the introduction of modern spectroscopic and chromatographic techniques, such as ultraviolet and infrared absorption, gas and thin-layer chromatography, nuclear magnetic resonance and mass spectrometry. Since odor will always remain an important criterion no assay of an essential oil will be complete without careful organoleptic tests which, however, require considerable experience.

The bulk of essential oils are isolated from the plant material (flowers, leaves, bark, wood, and roots) by hydrodistillation, partly in primitive, movable stills-in the interior of undeveloped countries - and partly in modern stationary distilleries. Only the citrus oils, which occur in the peel of citrus fruit, are obtained by the mechanical expression of the peel. Certain types of flowers are so delicate that their essential oils do not withstand hydrodistillation and are not amenable to expression; these must be isolated by extraction with volatile solvents (usually highly refined petroleum ether) yielding the so-called natural flower oils in concrete, solid form, which can be transformed into absolute, liquid form-jasmine, tuberose, acacia, mimosa, etc. Some flowers, rose and bitter orange blossoms among them, can be processed either by hydrodistillation or by solvent extraction.

The yield of essential oil differs with individual plant species-ranging in most cases from about 0.2 to 2.0%. As examples of extremes 0.025% for rose oil (otto of rose) and 17.0% for clover oil may be noted.

Essential oils are produced in many parts of the world, particularly in warm and temperate areas.

Less than one hundred essential oils have attained real commercial importance. These are employed widely for imparting odor and flavor to an almost unlimited variety of consumers' goods, such as, pharmaceutical and dental preparations, food products, beverages-alcoholic and nonalcoholic, confectionery, chewing gums, soaps, detergents, room sprays and insecticides, cosmetics and perfumes; and for masking of odors in synthetic products, such as, plastics, artificial leathers and rubber goods.

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