Whether or not you know it, you encounter essential oils every day of your life, since they are widely used in food, candy, beverages, perfume, soap and cosmetics.
Essential oils were well known even in ancient times. The earliest references to them come from India, China, Persia and Egypt, and the Greeks and Romans conducted extensive trade in oils and ointments throughout the known world. They were extremely valuable products, often worth their weight in gold. This is explains why frankincense and myrrh were considered such valuable gifts in biblical times, though they're quite inexpensive today.
Most of the ancient products were probably prepared by long soaking of flowers, leaves, or roots in vegetable oils, or by enfleurage, because distillation was unknown. Arabian alchemists in the middle ages invented distillation and discovered the solvents that allow production of the essential oils we know today.
Knowledge of distillation spread to Europe, and essential oils became a specialty of the medieval pharmacies. By the middle of the 18th century about 100 essential oils had been discovered, and in the early 1900s, better technologies greatly expanded their production.
Natural essential oils are not cheap, and some of the rare or difficult to extract ones can cost several thousand dollars a pound. The high price and limited availability of natural oils encouraged chemists to search for substitutes. The major constituents of many have been synthesized and are now in everyday use. The commonest example of a synthetic flavoring is vanillin, which appears in literally millions of prepared foods. Most of the "fruit flavors" listed on labels are actually synthetic esters. The ready availability of cheap synthetic flavorings has expanded the number of food choices, but they lack the subtlety and of natural flavorings, which comes from the large number of compounds found in addition to the "primary ingredient".
Today, essential oils are usually extracted by steam distillation. There are three basic methods of steam distillation. The oldest and simplest method is to place chopped or crushed plant material in water, bring the water to a boil, and recover the oil and a hydrosol by condensing the vapors. This method is still used on a very small scale in laboratories and homes, but is very inefficient on an industrial scale. The more modern technique boils water in a separate vessel and passes steam through the plant material, which is held in a special chamber. The oil and hydrosol are collected in the same way. The third variant separately heats the plant material in its chamber so no steam will condense on it, achieving "dry" distillation. This can be useful if you are extracting plant material that will swell up or clump together when it gets wet.
Steam distillation is not the only method used to isolate essential oils. Where the oils are plentiful and the source is relatively soft and easily accessible (like citrus peels), the oils are often literally squeezed out, in a process called expression. The material is squeezed in huge presses and the oil either drips out or is extracted from the crushed mass with solvents.
Certain delicate oils are obtained by solvent extraction with as little heat as possible. When particularly strong extracts are required, removal of the solvents can yield a pure oil or a solid waxy substance called a concrete. Adding a little solvent to a concrete can make a semi-liquid material called an absolute.
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