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Kirk-Othmer Encyclopedia of Chemical Technology. Copyright c John Wiley & Sons, Inc. All rights reserved.

OILS, ESSENTIAL
The volatile etherial fraction obtained from a plant or plant part by a physical separation method is called an essential oil. The physical method involves either distillation (including water, steam, water and steam, or dry) or expression (pressing). For the most part,essential oils represent the odorous part of the plant material, and therefore these oils have traditionally been associated with the fragrance and flavor industry (see Perfumes). Since essential oils frequently occur as a very small percentage by weight of the original plant material, the processing of large quantities is often required to obtain usable amounts of oil. As a result, expression of anessential oil is only employed in those cases where both the form of the natural plant material, such as a citrus peel, and the quantity of oil present make the process feasible. It has frequently been observed that the aroma of an essential oil is substantially different from that of the plant before processing. Because this phenomenon is largely the result of the treatment of the plant materialwith heat or hot water, various other methods have evolved over the years in an attempt to obtain a concentrate of the volatiles which more truly represents the aroma of the original. With the exception of the method of expression, almost all of these involve treatment of the plant material with one or more organic solvents (or mixtures thereof) followed by concentration of the extracted solute.Solvent extraction frequently yields, in addition to the volatile oil, various quantities of semi- or nonvolatile organic material such as waxes (qv), fats, fixed oils, high molecular weight acids, pigments, and even alkaloidal material. However, because solvent extraction often results in a product with superior and more representative odor properties to that of a distilled oil, many naturalproducts critically important to the flavor and fragrance industry are available as various extracts in addition to an essential oil. Some of the commonly used botanical extracts include the following.

1. Absolute
This is concentrated extract obtained by treatment of a concrete or other hydrocarbon-type extract of a plant or plant part with ethanol. It is usually liquid and should be totally solublein alcohol. By this method, waxes, hydrocarbons (including terpenoid), as well as most of the odorless material of the concrete are removed from the extract.

1.1. Absolute Oil

This is the steam distillable portion of an absolute. Frequently, the absolute oil possesses superior odor properties to that of the corresponding essential oil.
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OILS, ESSENTIAL

1.2. Aroma DistillateUsed by the flavor industry, aroma distillates are the product of continuous extraction of the plant material with alcohol at temperatures between ambient and 50◦ C followed by steam distillation, and, lastly, concentration of the combined hydro–alcoholic mixture. On cooling, terpenes often separate from the aroma distillate and are removed.
1.3. Concrete

Hydrocarbon extracts of plant tissue,concretes are usually solid to semisolid waxy masses often containing higher fatty acids such as lauric, myristic, palmitic, and stearic as well as many of the nonvolatiles present in absolutes.
1.4. Infusion

Infusion botanical extracts are tinctures that have been concentrated by either total or partial removal of the alcohol by distillation.
1.5. Oleoresin

Natural oleoresins are exudatesfrom plants, whereas prepared oleoresins are solvent extracts of botanicals, which contain oil (both volatile and, sometimes, fixed), and the resinous matter of the plant. Natural oleoresins are usually clear, viscous, and light-colored liquids, whereas prepared oleoresins are heterogeneous masses of dark color.
1.6. Pommade

These are botanical extracts prepared by the enfleurage method...
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