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Essential Oil are any of a group of volatile odorous oils occurring in certain plants. Essential oils, also known as volatile oils or ethereal oils, are used to give odor or flavor to a wide range of products, including cosmetics and perfumes, foods, soaps and detergents, chewing gum, dental products, pharmaceuticals, lacquers, and dyes. They also serve to mask the natural odors of such materials as plastics, rubber goods, artificial leathers, and paints.
There are about 150 commercially important essential oils, originating in about 1,500 plants from warm or temperate areas all over the world. Some of the best-known ones are the oils of bitter almond, cinnamon, clove, eucalyptus, lavender, lemon, sweet orange, rose, sandalwood, peppermint, spearmint, and wintergreen. Some oils are found throughout the whole plant, while others are localized, occurring only in the roots, leaves, flowers, or fruit.
Properties of Essential Oils
In general, essential oils are volatile in steam and are most commonly recovered by steam distillation. Most of the oils are only slightly soluble in water but soluble in organic solvents, such as petroleum ether, benzene, and alcohol. They have characteristic odors and are flammable. Most of the essential oils have a high index of refraction, and many of them are optically active.
Most of the essential oils are mixtures, some of 50 or more constituent compounds. However, a few consist of virtually a single compound. For example, oil of wintergreen consists of methyl salicylate and cassia of cinnamaldehyde. Some essential oils, particularly those from citrus fruits and coniferous or eucalyptus trees, contain high proportions of organic compounds known as terpenes. The terpenes tend to oxidize in air or to polymerize on standing to form compounds with a strong odor or flavor of their own. For this reason terpenes are frequently removed from the essential oils by fractional distillation under vacuum or by extraction. Other components of essential oils include alcohols, aldehydes, acids, esters, ketones, phenols, lactones, and nonterpene hydrocarbons.
Production of Essential Oils
Steam distillation is the major method of production of essential oils. However, several other processes are used, including extraction with volatile solvents, expression, en-fleurage, and maceration, especially where high temperatures would have an undesirable effect on the oil.
Modern steam stills are stainless steel or copper vessels that hold about 600 gallons (2,270 liters). They are charged with the flowers, stems, berries, chipped roots, twigs, or woods to be distilled and heated by perforated and closed steam pipes beneath a perforated false bottom. The condensate produced is divided into an aqueous and an oily layer. The aqueous layer is frequently recycled for more complete recovery of the product. Distillation is usually done at atmospheric pressure, although vacuum processes are used if the oil is subject to hydrolysis.
Solvent extraction is a sophisticated modern method that yields a high-quality product. The most commonly used solvents are purified petroleum ether and benzene. They do not react chemically with the oil, are selective in solvent action, and have low boiling points so that they can be easily separated from the oil by distillation. The residue left after solvent extraction contains waxes and resins from which the desired components can be recovered by extraction with cold alcohol. The alcohol is removed from the extract, leaving the essential material. As a solid, the material is referred to as the concrete form; as a liquid, it is called the absolute form.
Certain delicate flowers, such as jasmine and violet, are extracted by enfleurage. In this process the fresh flowers are placed in wooden frames supporting glass plates that are coated on both sides with a purified mixture of tallow and lard. After 24 hours at room temperature in the stacked frames the flowers are replaced by fresh ones, and the process is repeated daily for two months or more. The fat, which is not changed during this time, becomes saturated with the essential oils from the flowers. The oils are then recovered from the fat by the method of alcohol extraction.
Expression is a process used mainly for the recovery of orange and lemon oils. It may be done either by hand or by machine. In expression the peel is removed from the fruit, soaked in water for several hours, and pressed against a sponge, which absorbs the oil that is released. The sponge is pressed dry at intervals to recover the oil.
Maceration involves extraction of the essential oil from shredded material with hot fats. However, this process is little used.