Chemically, essential oils are a combination of several organic substances, such as benzene derivatives, terpenes and various other hydrocarbons and straight-chain compounds. Essential oils have smaller molecules, ordinarily less than twenty carbon atoms and possess a characteristic aromatic fragrance due to their volatilization in contact with air. They can be extracted easily without any change in their chemical composition. They are distributed throughout the plant kingdom and occur in about 60 families of angiosperms. The important families which provide essential oils are: lamiaceae, Rotaceae, Geraniaceae, Apiaceae, Asteraceae, Lauraceae, Fabaceae and Poaceae.
Essential oils have a variety of industrial applications. Because of their high volatility and odour, they are widely used in the manufacture of perfumes, sachets, soaps and other toilet preparations. Several essential oils are used in medicines. Cedarwood oil, which has a high refractive index, is used as a mounting medium with oil-immersion lenses in microscopy. Eucalyptus oil is used in the floatation process for separation of minerals their ores.
Extraction of Essential Oils
Essential oils most frequently found as floral essences, but are not uncommon in leaves, bark, fruits and seeds, and are even found in the wood. They are extracted from plant tissues in various ways, depending upon their quantity and stability of the compound. The following methods are commonly used for extraction of essential oils.
Distillation is the most common for obtaining essential oils. Usually two types of distillation is practiced.
This is utilised where essential oils are immiscible in water. In this process, hot water or steam is passed into aromatic tissues under the influence of boiling water or steam. The essential oil is thus freed from oil glands in plant tissues. The distillate separates out as a layer of oil from. The oil is then condensed by passing condensers.
This process is utilised when essential oil is miscible with water. The simplest process involves a gradual increase in temperature of the mixture, during which more volatile oils are distilled first and less first and less volatile components progressively later. Sometimes, several distillations are required to attain a desired degree of purity.
[B] Enfleurage or cold fat extraction
This technique is employed when heat involved in steam distillation has deleterious effects on essential oils through hydrolysis, polymerization or resignification, or where delicate oils become lost in large volumes of water, or essential oils are no longer produced by flowers killed in boiling. The technique consists of applying fresh aromatic tissue to glass plates covered with fats. The plates are piled one above the other on square wooden; frames so that all fragrance emanating from flowers is held in an essentially air tight compartment proximate to the layer of fat. The tissues are allowed to remain on plates for several days till the fat absorbs all oils. Ultimately, the saturated fat is subjected to alcoholic extraction, which dissolves trapped essential oils but not insoluble fate. The alcohol solution is then suitably concentrated in order to get the perfume oil.
[C] Solvent extraction
This process is particularly useful for extracting essential oils from such perfume flowers which don’t continue to produce fragrance once they are picked. In this technique, fresh flowers are charged into specially constructed extractors at room temperature and treated with a carefully purified solvent (usually petroleum ether). The solvent dissolves out the essential oil. The solvent is then evaporated usually in vacuum, leaving a residue consisting largely of flower oil.
This process involves squeezing of plant tissues at great pressures in order to pressed out oils or other liquids. This is done by hand-operated presses or gigantic mechanical presses. The oil is then separated form the juice by centrifuging. This process is particularly suitable for extracting oils from the rinds of citrus fruits.
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