Essential oils are the volatile, organic constituents of fragrant plant matter that contribute to both their fragrance and flavour. These volatile oils (also known as essences) are found in specialised glandular cells located in various anatomical parts of the plants such as the flowers, leaves, fruit, seeds, wood, bark, twigs, stems and roots. Only a relatively small proportion of the plant population contain volatile oils which they mainly produce for defence or survival purposes, for example to prevent damage from insects or plant pathogens or to attract pollinators for reproduction.

How are aromatic oils extracted from plants?

The method of extraction ultimately depends on the type of plant material being used, where the oil is located in the plant, and the type of aromatic product required (e.g. essential oil, concrete, absolute).

Pure essential oils are extracted either via distillation or by cold-pressing (expression). Other aromatic oils such as absolutes or resinoids are produced via solvent extraction. 

Although oils such as Rose absolute or Benzoin resinoid are often referred to as essential oils, this is not technically accurate as they contain both the volatile essential oil and other non-volatile components such as plant dyes, wax or resin. It is always the volatile essence however that gives the absolute or resinoid its aromatic qualities.

Distillation

The historical and most widely used method of obtaining essential oil from plants is by steam distillation. This involves heating the plant material, either by boiling it in water (direct distillation) or placing it on a rack and heating water beneath to allow steam to rise up through it (steam distillation).  This process causes the specialised cells that contain the volatile components to break down and release their essence in the form of vapour. 

The vapour and steam produced is then cooled which turns it back into liquid form. The steam condenses into a watery distillate and the volatile essence turns into essential oil. As the oil is lighter than the watery distillate it collects at the top and can easily be separated off. In some cases, the watery distillate is sold as a hydrolat or flower water.

Note: This process extracts only the volatile and water-soluble parts of the plant and excludes many other (often valuable) components. Some oils can be redistilled or rectified to remove any remaining non-volatile matter, or to collect certain constituents or exclude others (e.g. Camphor).

Expression / Cold pressing

This method is used for citrus oils such as Bergamot, Grapefruit, Lemon, Lime, Orange, Mandarin, Tangerine and Yuzu. As the essence is contained close to the surface in the peel, it can easily be extracted via simple pressure, which helps retain all of its aromatic goodness. The process involves puncturing the skins of either the whole fruit or just the fruit peel and pressing the essential oil out. A small amount of juice is also extracted, which is then separated off from the oil. 

Citrus oils may also be produced by steam distillation and Lime is often extracted this way in order to produce a non-phototoxic oil. Cold-pressed oils however are generally preferred for use in aromatherapy and perfumery.

Solvent Extraction

Solvent extraction is used to produce absolutes and resinoids from plant material that is either too delicate for distillation, has a very low yield of essential oil, or is a resinous material that has insufficient volatile components to produce an oil via distillation.

The absolute is then obtained from the concrete by a second process of solvent extraction, which uses pure alcohol (ethanol) to remove the unwanted wax and dyes. Even after repeated treatments a small proportion of these elements may remain. Absolutes may be further processed by molecular distillation which can remove all final traces of non-volatile components. The alcohol is then evaporated off via gentle vacuum distillation.

The resulting absolute is an extremely concentrated, viscous liquid, that can in some cases be solid or semi-solid. As they contain more components of the plant than essential oils produced by steam distillation they are considered to be ‘absolutely’ complete in terms of their aromatic complexity – smelling almost exactly like the flower from which they came, which is not always the case with distilled oils. Their superior and highly concentrated fragrance makes them popular for use in perfumery.

In aromatherapy, the purist (but outdated view) is that they should not be used as they may contain traces of solvent. This tended to be the case back in the 1950s when benzene was used as the standard solvent, as this could leave a residue of 6-20% solvent in the finished product. Today however, with hexane, the residue is thought to reduce to about 10ppm (parts per million) which is extremely low and considered fine for use in aromatherapy. Once the absolute is correctly diluted to 1% this further reduces to parts per trillion. Any alcohol residue is also low, and many cosmetics in fact contain far higher levels of alcohol. It is also worth noting that absolutes are used for flavouring in the food industry which is of course very tightly regulated. If deemed safe for internal consumption you can therefore be reassured that they are perfectly safe for use on the skin.

While our Jasmine and Violet Leaf absolutes are extracted with hexane as described above, our Rose centifolia and Rose damascena absolutes are produced via alcoholic extraction only. The rose petals are put straight into ethanol which produces the absolute without needing to create the concrete first.

Note: Rose otto and Neroli (Orange Flower) essential oils are also extracted via steam distillation. 

Resinoids

These are extracted from resinous material such as balsams (Benzoin, Peru Balsam), resins (amber), oleoresins (Copaiba balsam) and oleo gum resins (Frankincense, Myrrh) via a hydrocarbon solvent such as hexane. The resulting resinoid can be a viscous liquid, solid or semi-solid substance, making them difficult to pour. For this reason, our Benzoin Resinoid is diluted at 50% with Benzyl benzoate for ease of use. On occasion the alcohol-soluble fraction of a resinoid is called an absolute. 

Note: Some resinous materials such as Copaiba, Frankincense, Elemi, Myrrh and Peru Balsam can be steam distilled to produce an essential oil. 

Why do you see absolutes and resinoids classed as essential oils?