History and Method
Organic solvent extraction (volatile solvent extraction) is a method used since the eighteenth century in which plants are immersed in an organic solvent to extract their aromatic constituents. In the eighteenth century, explosions often accompanied the process, but organic chemistry advanced, and the method appears to have become established around the nineteenth century.
The raw material is placed in a stainless-steel vessel, and an organic solvent such as ethanol, hexane or benzene is added. Trays are stacked in multiple layers inside the vessel, allowing it to hold large quantities of raw material. Once the essence (essential oil) has dissolved in the organic solvent, the solvent is evaporated to produce a waxy paste. When extracted from flowers, this is called a concrete; when extracted from resins and similar materials, it is called a resinoid.
When the resulting concrete is washed repeatedly with alcohol, the essential oil dissolves into the alcohol (the wax is insoluble in alcohol). The scented alcohol obtained in this way is heated under reduced pressure to evaporate the alcohol. What remains is called an absolute.
The advantages of solvent extraction are that large quantities of raw material can be processed at once, and an essential oil with a scent close to that of the original plant can be obtained.
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