Typical Methods of Extraction
In the true sense, essential oils are either distilled or expressed (Arctander 1960). The most common ways of distillation can be completed via water, water and steam or steam alone. Distillation involves using a still to heat the plant material to a temperature high enough to produce a vapor then cooling that vapor to cause condensation into a liquid form.
Water distillation – plant material is covered by water and slowly brought to a boil
Water-Steam distillation – plant material is mixed with water and subjected to steam
Steam distillation – plant material placed on grid and pressurized steam blown through it
Cohobation – the water used in an initial distillation is added back to the still with the plant matter for re-distillation, used to recover water soluble components lost in the first run – used to make Rose otto.
Fractional distillation – usual distillation with interruptions at various stages to collect only the portion that has been produced to that point in the process – Ylang ylang essential oil is produced by this process – can be stopped and restarted up to 4 times with the first fraction being Ylang ylang Extra, and the following fractions referred to as Ylang 1, 2 and 3
Dry Distillation – plant material distilled, without water or steam, in a vacuum
Expression – citrus peels are scraped then the essence is collected by centrifugal separation
Additional Methods
There are other methods of extraction that produce products often incorrectly referred to as essential oils. These are created by the use of solvents and applied to plants whose aromatic substance would be degraded by usual distillation.
Absolutes – this involves a three step process: 1) solvents are used to extract the essential oil of delicate flower petals, thus creating a concrete (a mixture of about 50% essential oil and 50% plant wax); 2) ethanol is then used to dissolve the wax; 3) the resulting mixture is filtered and the alcohol removed by distillation in a vacuum.
CO2, Hypercritical Carbon Dioxide – an excellent solvent that can extract essential oils quickly at a low temperature without any chemical reaction creating a pure plant extract. The disadvantage of this is some of the chemical changes created by steam extraction do not occur and the end product is not the same as the ones that have been researched and used in aromatherapy.
As more research and trials are conducted, this may become an accepted part of aromatherapy.