History and Method
Citrus materials (lemon, grapefruit, bergamot (in Japanese), bitter orange and others) are commonly processed by mechanically pressing the peel to obtain its essential oil. This method is called expression. For Key lime, another process uses steam distillation to recover essential oil from the mixture of peel oil and juice obtained by pressing. Because the composition changes during this process, expressed and distilled oils also differ in aroma.
Developed on the Italian island of Sicily in the nineteenth century, expression is an extraction method used only for citrus fruits. Citrus peel contains cells known as oil glands, where essential oil is stored. Their cell walls rupture easily under pressure, allowing the essential oil to be collected. In the past, the process was carried out by hand: the fruit was cut in half, the flesh scooped out, and the peel squeezed by hand in hot water; the oil floating on the surface was apparently absorbed into a sponge. For this reason, it was also called the sponge method.
Cold Expression
Because older expression methods required substantial labor and expense, new methods using modern science and technology have been developed. No heat is required during the process, and it is carried out with cooling, hence the name cold expression.
Citrus fruits arriving from the plantation are washed, and damaged fruit is removed. One of two methods is then used: the pélatrice method, in which “the whole fruit is processed by machine,” or the sfumatric method, in which “the peel, with the flesh removed, is processed by machine.” Since the extracted material is a mixture of essential oil and fruit juice, it is put through a centrifuge to obtain the essential oil.
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