Essential oil, highly volatile substance isolated by a physical process from an odoriferous plant of a single botanical species. The oil bears the name of the plant from which it is derived; for example, rose oil or peppermint oil. Such oils were called essential because they were thought to represent the very essence of odour and flavour.

Distillation is the most common method for isolation of essential oils, but other processes—including enfleurage (extraction by using fat), maceration, solvent extraction, and mechanical pressing—are used for certain products. Younger plants produce more oil than older ones, but old plants are richer in more resinous and darker oils because of the continuing evaporation of the lighter fractions of the oil.

Out of the vast number of plant species, essential oils have been well characterized and identified from only a few thousand plants. The oils are stored as microdroplets in glands of plants. After diffusing through the walls of the glands, the droplets spread over the surface of the plant before evaporating and filling the air with perfume. The most odoriferous plants are found in the tropics, where solar energy is greatest.

The function of the essential oil in a plant is not well understood. Odours of flowers probably aid in natural selection by acting as attractants for certain insects. Leaf oils, wood oils, and root oils may serve to protect against plant parasites or depredations by animals. Oleoresinous exudations that appear when the trunk of a tree is injured prevent loss of sap and act as a protective seal against parasites and disease organisms. Few essential oils are involved in plant metabolism, and some investigators maintain that many of these materials are simply waste products of plant biosynthesis.

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Commercially, essential oils are used in three primary ways: as odorants they are used in cosmetics, perfumes, soaps, detergents, and miscellaneous industrial products ranging from animal feeds to insecticides to paints; as flavours they are present in bakery goods, candies, confections, meat, pickles, soft drinks, and many other food products; and as pharmaceuticals they appear in dental products and a wide, but diminishing, group of medicines.

The first records of essential oils come from ancient India, Persia, and Egypt; and both Greece and Rome conducted extensive trade in odoriferous oils and ointments with the countries of the Orient. Most probably these products were extracts prepared by placing flowers, roots, and leaves in fatty oils. In most ancient cultures, odorous plants or their resinous products were used directly. Only with the coming of the golden age of Arab culture was a technique developed for the distillation of essential oils. The Arabs were the first to distill ethyl alcohol from fermented sugar, thus providing a new solvent for the extraction of essential oils in place of the fatty oils that had probably been used for several millennia.

The knowledge of distillation spread to Europe during the Middle Ages, and isolation of essential oils by distillation was described during the 11th to 13th centuries. These distilled products became a specialty of the European medieval pharmacies, and by about 1500 the following products had been introduced: oils of cedarwood, calamus, costus, rose, rosemary, spike, incense, turpentine, sage, cinnamon, benzoin, and myrrh. The alchemical theories of the Swiss physician and alchemist Paracelsus played a role in stimulating physicians and pharmacists to seek essential oils from aromatic leaves, woods, and roots.

Starting from the time of Marco Polo, the much-prized spices of India, China, and the Indies served as the impetus for European trade with the Orient. Quite naturally, such spices as cardamom, sage, cinnamon, and nutmeg were subjected to the pharmacists’ stills. By the middle of the 18th century in Europe about 100 essential oils had been introduced, although there was little understanding about the nature of the products. As chemical knowledge expanded in the late 1800s and early 1900s, many well-known chemists took part in the chemical characterization of essential oils. Improvement in knowledge of essential oils led to a sharp expansion in production, and use of the volatile oils in medicine became quite subordinate to uses in foodstuffs, beverages, and perfumes.

In the United States, oils of turpentine and peppermint were produced before 1800; within the next several decades oils of four indigenous American plants became important commercially—namely, sassafras, wormwood, wintergreen, and sweet birch. Since 1800 many essential oils have been prepared, but only a few have attained commercial significance.