There are several different ways in which essential oils can be extracted from plants. Two of the main methods are distillation, which is used for the majority of essential oils, and expression, which is used for most of the citrus oils. Special oils and absolutes may require the use of other methods utilising more advanced equipment.
Steam distillation involves heating plant material until a vapour is produced, using an injection of steam to help complete the process as quickly as possible to minimise the damage to heat-sensitive essential oils. The vapour is then cooled to condense it to a liquid or solid distillate. This method isolates the volatile and water-insoluble parts of a plant. The resulting oil may sometimes be re-distilled to remove any remaining non-volatile matter or unwanted constituents.
A similar process to steam distillation, this method entails covering plant material completely with water before bringing it to the boil. Water distillation is a slow process best suited to essential oils that are not damaged by prolonged contact with hot water. To ensure that the essential oils can be easily released from plant material such as seeds and woods, they are first broken down into small pieces or into a powder.
This method is carried out in the same way as normal distillation but instead of the essential oil being continuously collected, it is gathered in batches, or fractions. Fractional distillation is used to produce ylang ylang essential oil, which is collected in four fractions. The first fraction, referred to as ylang ylang ‘extra’, contains the most volatile constituents and is highly favoured in perfumery. The following fractions are known as 1st and 2nd grade, which contain larger quantities of less volatile constituents, and 3rd grade, which is composed of the least volatile constituents.
Only the essential oils of citrus fruits are extracted by expression. One of the original expression methods was the sponge process, which involved cutting fruit into halves before removing the pulp and leaving it to soak in warm water to soften and elasticise the pith. The peel was then exposed to the air for several hours before being inverted and placed around a sponge. The compression of the peel resulted in the rupture of many of the oil cells and the release of the essential oil, which was absorbed by the sponge and squeezed into a collecting vessel.
In a more modern method known as the écuelle à piquer process, the whole citrus fruit is rotated against spikes to pierce the oil cells. The essential oil is consequently released, along with other cell contents such as sap, protoplasm and pigment, and is collected into a vessel before being separated from the aqueous layer.
The most commonly used method of expression is machine abrasion, which uses the same procedure as the écuelle à piquer process but on a much larger scale.
Solvents such as petroleum ether, methanol, ethanol or hexane may be used to extract the essential oils of fragile plants such as jasmine. A complex mixture of volatile and non-volatile substances – known as an extract – is dissolved out by the solvent before being gently distilled to boil off the solvent. The result is a concentrated solution, known as a concrete if it is not resinous, and a resinoid if it is.
Concretes can be further processed by solvent extraction by melting them in warm alcohol, causing the essential oil – along with a small amount of waxes, fixed oils and fats – to dissolve. The alcohol solution is then distilled, leaving behind a substance known as an absolute, the most concentrated and costly form of fragrance.
The simplest form of solvent extraction and one of the original methods of essential oil extraction, maceration involves submersing flowers in hot oil until the essential oil dissolves into it. The mixture is then strained to remove the plant material and repeated using fresh flowers and the same oil. The end result is a highly fragrant fatty solution, which is known as a pomade when solidified. Alcohol is then used to extract the essential oil from the pomade.
A similar method may sometimes be used to extract the essential oil from delicate flowers, using cold fats instead of hot, in a process known as enfleurage.
Hypercritical Carbon Dioxide
This is a fairly new method of essential oil extraction which produces high quality oils, however, it requires very expensive equipment. It uses carbon dioxide, which possesses qualities of both a liquid and a gas at 33˚C, to extract the essential oil. This method is considered advantageous because it does not use organic solvents, neither does it require a high temperature, thus preventing damage to essential oils.