1. Volatile oils are light, mobile, highly odorous liquids, which can be distilled without decomposition.
2. They are volatile in steam and differ entirely in physical and chemical properties from fixed oils.
3. They are optically active and their specific gravity varies from 0.8–1.15 with most of them below 1.00. (Oil of clove and oil of cinnamon have specific gravity more than 1.00.)
4. They have a characteristic odor and taste and are freely soluble in ether and chloroform and fairly soluble in alcohol. They are slightly soluble in water.
5. When stained on paper they give a translucent stain (unlike fixed oils, which leave a permanent stain), which is temporary as it disappears when the oil volatilizes.
6. They undergo oxidation when exposed to moisture, light, or air, forming compound of altered/objectionable odor characteristics.
7. Oxygenated terpenes are odoriferous and much more soluble in water and alcohol than hydrocarbon terpenes.
8. When oils are specifically enriched with respect to oxygenated terpenes by the removal of hydrocarbons, such oils are called “terpene less” oils.
9. Extraction and isolation

Volatile oils are traditionally obtained by distillation of the plant parts containing the oil. Some volatile oils which undergo undesirable changes in the oil composition and organoleptic features due to distillation are separated by methods not involving heat application. In addition to these several newer methods such as super critical fluid extraction may now be conveniently applied for their isolation. Following are some of the usual methods of isolation of volatile oils.

1. Water or hydrodistillation: This method is applied to plant material that is dried and not subject to injury by boiling. The plant material is boiled along with water in a distillation chamber over naked flame until all the volatile oil distills over along with water. In some cases the plant material is allowed to macerate in water before being subjected to distillation. Turpentine oil, predominantly being constituted of hydrocarbons is unaffected by heat. It may be distilled from the fresh, un dried plant material by this method.
2. Steam distillation: This method is applicable to fresh plant material that may be subject to damage and loss of constituents by direct heat. Fresh plant material is directly taken to the distillation still in which it is charged on the perforated bottom. Steam generated in another boiler is passed through the still having the fresh material with sufficient moisture content. Droplets of the essential oil along with water vapor are carried through the condenser, whereby it condenses and collects in the receiver as a biphasic liquid of water and the immiscible essential oil. Ideally steam distillation enables maximal and rapid diffusion of steam through the plant cell membranes keeping hydrolysis and decomposition of constituents minimal. For example, peppermint and spearmint oils are isolated by steam distillation.
3. Water and steam distillation: This method is employed for the separation of volatile oils from plant material either dried or fresh, which tend to be injured by direct heat or steam. The plant material is charged under a layer of water over a perforated grid in the distillation chamber. Steam is passed through the macerated mixture, which is also heated underneath. The volatile oil vapor and water vapor arising from the charge, pass through the condenser to eventually get collected in the receiver. Because of different specific gravities, water and the oil get separated and the oil is drained or siphoned. For example, cinnamon and clove oil are isolated by water and steam distillation.
4. Expression and distillation: Essential oils such as oil of bitter almond and oil of mustard are not present in the natural state in the plant material. Instead they are present in a glycosidic form (called amygdaline in bitter almond). Crushing brings about contact with emulsion, an enzyme present elsewhere in the seed. This results in hydrolysis yielding the essential oil, which is eventually separated by distillation. Bitter almonds are first expressed when they yield about 30% fixed oil. The pressed marc is then macerated in water to initiate enzymatic action. This mixture is distilled to yield about 1% essential oil.
5. Expression: Some volatile oils such as essential oil of lemon cannot be distilled without decomposition and are usually obtained by expression or by other mechanical means. Ecuelle’s process is a newer mechanical method of separation of the volatile oil from citrus sources. Here the fruit is placed in a device lined with spikes and rolled in such a way that the glands present in the peel are punctured just at the level of the epidermis. The oil globules pass to the center of the device from which it is collected by draining from the bottom. In earlier days, citrus oils were separated manually by the sponge process in which the peel is separated and placed in warm water to make it more soft and pliable. It is then inverted to rupture the oil glands and the rind placed in contact with a sponge. The oil is absorbed into the sponge until saturation, which is then expressed to obtain the essential oil. Such oil is highly priced as it is labor intensive.
6. Enfleurge: This method is applicable to the isolation of essential oils that are present in very small quantities and also are heat labile. Hence, it is commercially not feasible to isolate them by the usual method of distillation or expression. Enfleurage makes use of a bland, odorless fixed oil to absorb the delicate essential oil. This fatty material is spread over flat glass plates on which the flower petals are placed in contact for a few hours. The fat absorbs the fragrant volatile oil. The charge is replaced with fresh material until the fat is fully saturated. The volatile oil taken into the fat is then extracted by treating it with appropriate solvent such as alcohol. This method was formerly used extensively for the production of rose oil, jasmine oil, etc., for perfumery.
7. Solvent extraction: In today’s perfumery industry, most of the modern essential oil production is accomplished by solvent extraction using volatile solvents such as petroleum ether, hexane, methanol, ethanol, etc. The plant material is brought in contact with the solvent at temperatures not exceeding 50°C. The solvent is then separated by filtration and evaporated under reduced pressure. Such oils are called absolutes or concentrates and resemble very closely the natural fragrance of the plant material. This is because temperature does not exceed 50°C and is maintained for the whole extraction period. Hence, the extracted oils have more natural aroma unmatched by distilled oils.
8. Super critical fluid extraction: This is a relatively newer method in which carbon dioxide at a hyper critical state is the solvent. At hyper critical pressure of 200 atmospheres, and at 33°C, CO2 gas reaches a state where it is a super fluid having both the properties of a liquid and a gas. At this state it has greater diffusion and hence acts as an excellent solvent in which the essential oil is completely extracted intact. The solvent is eventually removed by reversing the super critical conditions, when gaseous CO2 is regenerated and removed with much greater ease than any other solvent. Though expensive, in this method there is no solvent residue and the yield and quality of essential oil are good. It is a clean and environment friendly option for the isolation of number of plant products of medicinal, flavoring, and cosmetic interest. Super critical fluid extraction has also been successfully used for the extraction of a number of alkaloids, diterpenes, fixed oils, pigments, and sesquiterpene lactones. It has been adapted for the isolation of several essential oils such as those of calendula, peppermint, pepper, myrrh, and eucalyptus.