Aromatherapy Essential Oils - Methods of Extraction of Aromatic Essential Oils

Essential Oil: Aromatherapy Essential Oils

There are several 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

Distillation is the most widely used and the most economical method of extracting essential oils. There is a great deal of skill involved in the process of distillation in the if the precious essential oil is not to be lost or changed 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.

In 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 freeing the essential oils. The essential oil molecules and steam are carried along a pipe and channelled through a cooling tank, where they return to the 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.

Expression

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 rind. This process was originally carried out using simple hand pressure. The citrus essence was squeezed from the rinds and then collected in a sponge which, once saturated, was squeezed into a bucket. Due to the labour costs involved the majority of citrus oil is now expressed using mechanical presses.

Solvent Extraction

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 for 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.

Absolutes

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 vaccum. The thick, viscous, coloured liquid known as the absolute is left behind.

Resinoids

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 with 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 (eg. benzoin resinoid). Resinoids are often employed by the perfume manufacturers as fixatives to prolong the aroma of a fragrance (as are concretes).

Enfleurage

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 ben 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 to be replaced by fresh flowers. The process is repeated many times - even beyond months - until the fat is saturated.

Carbon Dioxide Extraction

The relatively new method was introduced only in the 1980s. The price is high because the equipment used is expensive. The process has been designed for the perfume industry. Oils which are extracted utilising carbon dioxide are supposed to be superior, pure and very close to the natural essential oil as it exists in the plant - and they are completely free of residues of carbon dioxide.

Hydrodiffusion/percolation

Hydrodiffusion or percolation is the most modern method of extraction. This process is faster than distillation, and the equipment is much more simple than that used for carbon dioxide extraction. Steam spray is passed through the plant material (which is suspended on a grid) from above. The emerging liquid composed of oil and condensed steam is then cooled. The result is a mixture of essential oil and water (as in the distillation process) which can be easily separated.

Maceration

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.

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