Aromatherapy uses a wide range of materials which contain the various smells, i.e. absolutes, phytoncides, hydrosols, infusions, carrier oils and essential oils. This brief article looks at the ways essential oils and absolutes are created from the necessary raw materials. Three main processes of distillation, expression and solvent extraction are covered.



Essential oils, by their very nature, hydrophobically clump together and do not mix with water. Typically these plant based aromatic compounds are volatile, being produced through distillation, solvent extraction or expression.

Not only do these oils get used by aromatherapists, but also in cosmetics, incense, medicine, cleaning products, food flavoring and perfumes.

The word 'essential' refers to the oil not as hugely important or indispensable, but as the aroma from the oil that reflecting the essence of the plant from which it was extracted.

Distillation

Lavender, eucalyptus and peppermint are commonly distilled for their oils. The plant species' wood, bark, peel, seeds, flowers, in fact anything containing the desired oil, is placed as raw plant material into distillation equipment over some water. When this water is heated the very high temperature water droplets that make up the rising steam pass via the raw material causing all the volatile oil compounds to become vaporized. These vapors are funneled through a coiled tube surrounded by cooling mechanisms, then condensing the vapor into liquid which is collected.

The majority of oils are distilled in this single simple process. Exceptions, as with Ylang-ylang (Cananga odorata), can take up to 22 hours to refine through a slightly more complex fractional distillation process.

Hydrosols are these condensed water and oil solutions that result from distillation. These have been sold, the most popular being orange blossom water, rose water and lavender water.

Expression

Citrus fruits tend to get their peels squeezed, mechanically or by cold-pressing, to remove the oils. Citrus fruit smells in general cost the least with this process since they are cheap to produce, the skins often collected as a byproduct from other industries. This process of extracting scents was typical before distillation was discovered.

Extraction of Solvents

Expression had its limitations, the main one being that flowers often simply did not have much volatile oils contained within them. High temperatures used with distillation meant that many oils had their typically delicate chemical structures denatured, loosing the scent. The development of a solvent based technique emerged as a more efficient way of extracting oils from plants. Supercritical carbon dioxide or hexane could be used to create 'concretes', basically mixtures of resins, waxes oils and other oil soluble plant materials.

Waxes and resins were largely non-fragrant and could be removed with a second solvent e.g. ethyl alcohol. Ethyl alcohol was able to only dissolve the pungent low molecular weight compounds to form a more concentrated oil. This alcohol was in turn removed through a second distillation, the so called 'absolute' remaining.

Supercritical extraction of fluid was a method of removing oils with a solvent of carbon dioxide. One of the advantages was that no petrochemical residues existed in the final product. Absolutes were not gained directly. Both the oils and waxes present in the various concretes reacted with the carbon dioxide and were released. Subsequent liquid carbon dioxide processes in the same machine, created through cooling, then separated the essential oils from the waxes. Lower temperatures used meant less oil denaturing improving the quality of the end product.

Once the extraction of the oils took place the pressure of the machine dropped to room level upon which the carbon dioxide could be released, returned to the atmosphere as a gas.