In Botany, the term “essential oil” refers to a class of secondary metabolites composed of volatile molecules. In aromatherapy, however, we use a somewhat more restrictive definition and say that a product composed of a plant’s volatile constituents should only be called an essential oil if it was removed from the plant by either steam distillation or, in the case of essential oils that come from the rinds of citrus fruits, cold pressing. The volatile constituents of a plant can also be extracted by other means, like enfleurage and solvent extraction but most people in the field of aromatherapy refer to the resulting products as absolutes, even though they are similar to essential oils. Enfleurage is used to extract essential oils from the most delicate flowers like Gardenia, Rose, Lily and Tuberose. In fact, with most of these really delicate flowers, enfleurage or solvent extraction are the only methods that can be used to get the volatile constituents out since most of these flowers will not hold up to steam distillation and will not yield their oils to pressing; rose is the exception in this list as it is possible to extract rose essential oil by distillation of the flower petals. Solvent extraction is used on a wide range of plant material but, while absolutes produced by solvent extraction are widely used in natural perfumery, they are not used for aromatherapeutic purposes when an essential oil for that plant is available.
Steam distillation of essential oils was invented by the Persian chemist Ibn Sina (known as Avicenna in the West) in the early 11th century. The invention of steam distillation contributed significantly to the development of perfumery, in perfecting the extraction of fragrances. The steam distillation technology significantly influenced Western scientific developments.
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Footnote: The above info on steam distillation was quoted from Wikipedia.