There are numerous methods of obtaining aromatic substances from plant material, most of which are described below, but strictly speaking, essential oils are only those obtained by distillation or expression.
Distillation is the most widely used and the most economical method of extracting essential oils. Many historians attribute the discovery of distillation to Avicenna, the Persian physician and scholar, although it is possible that the Egyptians were aware of the primitive process.
There is a great deal of skill involved in the process of distillation if the precious essential oil is not to be lost or altered in its composition. Some plants are distilled immediately after harvesting, whereas others may be left for a few days or even dried prior to extraction.
During distillation the plant material is heated either by placing it in water which is brought to the boil or by passing steam through it. The heat and steam cause the cell structure of the plant material to burst and break down, thus releasing the essential oils.
The essential oil molecules and steam are carried along a pipe and channelled through a cooling tank, where they return to liquid form and are collected in a vat. The emerging liquid is a mixture of oil and water, and since essential oils are not water soluble they can be easily separated from the water and siphoned off.
Essential oils which are lighter than water will float on the surface, whereas heavier oils such as clove will sink.
The water travelling around the distillation plant becomes impregnated with aroma and is recycled. It may be used as perfumed water such as lavender water or rose water.
During the process of distillation only the extremely small volatile molecules are able to evaporate. Essential oils which contain a high proportion of the smallest (most volatile) of these molecules are referred to as 'top notes'. Those which are composed mostly of the heaviest (least volatile) molecules are known as 'base notes'.
Essential oils which are in between are called 'middle notes'.
Top note oils are the most volatile - the aroma disappears within twenty-four hours. Examples are basil, grapefruit, lemon, lime and eucalyptus. They tend to be stimulating and uplifting.
Middle note oils have an aroma which lasts for two to three days. Examples are chamomile, geranium and lavender. They are generally balancing and primarily affect the general metabolism and the systems of the body such as digestion and menstruation.
Base note oils are the least volatile. The aroma will last at least one week. Examples are frankincense, myrrh, neroli, patchouli and vetivert. They have a relaxing and sedative quality.
The first distillation is usually the best quality. If essential oils are redistilled this process is known as 'rectification'.
The second and subsequent distillations will produce a cheaper oil unsuitable for aromatherapy.
This method is reserved exclusively for members of the citrus family such as bergamot, grapefruit, lemon, lime, mandarin and orange. The essence yielded is found in small sacs which are located under the surface of the rind. The majority of citrus oil is now expressed using mechanical presses.
Citrus oils for therapeutic aromatherapy use are best obtained from organically or naturally grown fruit. Unfortunately some citrus oil factories distil the peel after expression in order to release more oil. Obviously this essential oil is of an inferior quality but it is often added to the expressed essential oil to increase the quantity and thus make more profit.
The process of solvent extraction does not yield essential oils. This method is employed for flowers, gums and resins and it produces 'absolutes' and 'resinoids'. The technique is used to obtain a higher yield or to extract oils that cannot be obtained by any other process. Jasmine, for example, is adversely affected by hot water and steam.
To yield an absolute the aromatic plant material (flowers, leaves, etc.) is extracted by hydrocarbon solvents such as benzene or hexane.
The plant material is covered with the solvent and slowly heated to dissolve the aromatic molecules. The solvent extracts the odour and then the solvent is filtered off to produce a 'concrete'.
A concrete is a solid, wax-like substance containing about 50 per cent wax and 50 per cent volatile oil such as jasmine. To obtain the absolute the concrete is mixed with pure alcohol to dissolve out the aromatic molecules, and then chilled.
This mixture is filtered to eliminate waste products and to separate out insoluble waxes. The alcohol is evaporated off gently under vacuum.
The thick, viscous, coloured liquid known as the absolute is left behind. This method is widely used for rose, jasmine and neroli.
A trace of the solvent, however, will always remain. Therefore an absolute can never be as pure as an essential oil which has been extracted via the process of distillation
Solvent extraction can also be used for gums and resins to produce resinoids. Resins are the solid/semi-solid substances which exude naturally from a tree or plant that has been damaged. Commercially, resins are obtained by cutting into the bark or stem, and the gum-like substance hardens once it is exposed to the air. The natural resinous material is extracted via a hydrocarbon solvent such as petroleum ether, hexane or alcohol.
These solvents are then filtered off and subsequently removed by distillation. A resinoid remains where a hydrocarbon solvent has been used (e.g. benzoin resinoid).
If an alcohol solvent has been used then an absolute resin is produced (e.g. frankincense and myrrh resin absolutes may be extracted from the crude oleo resin gum - both, however, may be extracted by steam distillation to produce an essential oil).
Resinoids are often employed by the perfume manufacturers as fixatives to prolong the aroma of a fragrance (as are concretes).
The process of enfleurage also yields an absolute, although this method is virtually obsolete nowadays. It is very time consuming and labour intensive and, therefore, highly expensive. Formerly this was the main method of extraction for delicate flowers such as jasmine which continue to produce perfume even after they have been picked.
It involves the use of purified, odourless, cold fat which is spread over sheets of glass mounted in large rectangular wooden frames. Flowers are strewn upon this layer of fat which absorbs the essential oil. After approximately a day the flowers are removed and replaced by fresh flowers. This process is repeated many times - even beyond two months - until the fat is saturated. This fragrance-saturated fat is known as a 'pomade'.
The pomade is washed in alcohol and then treated. The alcohol evaporates first and the pure absolute is produced.
Carbon Dioxide Extraction
This relatively new method was only introduced in the 1980s. The price is high because the equipment used is expensive. The process was designed for use by the perfume industry. Oils extracted using carbon dioxide are considered to be superior, purer and very close to the natural essential oil as it exists in the plant and they are completely free from residues of carbon dioxide.
Research would be necessary to evaluate their therapeutic benefits since the composition of the essential oil is different.
Hydrodiffusion or percolation is the most modern method of extraction. This process is faster than distillation and the equipment is considerably simpler than that used for carbon dioxide extraction. Steam spray is passed through the plant material from above. The emerging liquid composed of oil and condensed steam is then cooled. The result is a mixture of essential oil and water which can be easily separated.
Research is necessary to evaluate the place of these oils in aromatherapy.
For this process plants are placed into a vat of warm vegetable oil which causes the plant cells to rupture, causing the absorption of the essential oils. The vat is then agitated for several days.
The resulting oil is filtered and bottled and is ready for use as a massage medium. Examples of macerated oils are calendula, carrot and hypericum.