Carbon dioxide (CO2) extracted essential oils are pure and stable and have no residue of CO2 left in them. However, the therapeutic possibilities need to be verified for each oil on account of their different compositions and until there has been some research on this, it may be wise not to use them yet in aromatherapy.
At the moment the price is high; perhaps after a number of years, when the initial cost of the equipment has come down, the prices will be lower.
Hydro-Diffusion or Percolation
Percolation is even newer than CO2 extraction and in 1991 we visited a unit in France. It was extremely interesting; most of the resultant oil had an aroma nearer to the plant than a distilled oil. The equipment, unlike that for CO2 extraction, is very simple and the process quicker than distillation, the plant being in contact with the steam for a much shorter time.
This process works like a coffee percolator. The steam passes through the plant material from top to bottom of the container, which has a grid to hold the plant material. The oil and condensed steam is collected in a vessel in the same way as distillation. The colour of oils I have seen is much richer than that of distilled oils and time and tests alone will, as with the CO2 method, reveal their true value in aromatherapy – it is certainly exciting! Percolation is not suitable for all oils; there are still a few practical difficulties to overcome (sometimes an emulsion is produced), but I am tempted to do a research project on one or two myself!
Expression
This method of extraction is used exclusively with citrus fruits, where the essential oil, located in little sacs just under the surface of the rind, simply needs to be pressed out. You can do this for yourself on a small scale: squeeze a succulent section of orange peel within an inch of a candle flame or lighted match and the tiny droplets of essential oil will ignite like baby fireworks. This shows not only the volatility of essential oils, but also that one needs to keep them away from a naked flame, as they are highly inflammable.
Expression is usually carried out by a factory producing fruit juice, thus maximizing the profit from the whole fruit. Most essential oil of orange comes from the USA, where millions of oranges are processed for their juice. The best essential oil does not come from there, as in order to maximize the crop, the trees and fruit are sprayed with chemicals, and these toxins reach the essential oil glands. This would not be so important if citrus fruits were distilled, as most pesticides and fertilizers, being composed of larger molecules, do not come through in the distillation process. Nevertheless, as expressed oil is taken directly from the fresh peel without heat, it is best that citrus oils for therapeutic use be obtained from organically or naturally grown produce.
FIGURE 2.2: Hydrodiffusion
Cold-pressed citrus oils are special, in that they are acknowledged to be exactly the same composition as when in the plant itself. In many juice/ essential oil factories the peel is steam distilled after expression, which releases even more oil (though of a poorer quality). Sadly for the aromatherapist, this is often added to the expressed essential oil to ‘bulk’ it, so care is needed when purchasing.
Although the oil was extracted by hand in the past (and collected in sponges), the size of the industry nowadays necessitates expression by machinery, when it is known as ‘scarification’. In expression, both volatile and large molecules (such as waxes and other substances) are contained in the finished product, in contrast to distillation where only the tiny volatile molecules are collected.
The storage life of expressed oils is shorter than that of distilled oils, and although they have approximately 0.002 per cent antioxidant added to them, they are best kept in a cool, dark place (many people choose the refrigerator). In too cold an environment the dissolved waxes are precipitated, making the oil cloudy; this does not affect the therapeutic qualities (the wax has no therapeutic effect) so should this happen, simply strain your oil if you prefer it clear – warming the oil will not work, as the waxes will not go back into solution.
Solvent Extraction
Absolutes and resinoids are obtained by solvent extraction and are not classed as essential oils. They are highly concentrated perfume materials, containing those plant molecules which are soluble in the solvents used to produce them.
Resinoids
Resins are the solid or semi-solid substances exuded from the bark of trees or bushes when wounded (cut, as in a rubber tree). The gum-like substance produced does not exist in the tree beforehand but is produced pathologically solely as a result of the incision (poor tree!), and hardens on exposure to air. Various solvents can be used to extract the aromatic molecules from the resin, the most frequently used being hydrocarbons (e.g. benzene, hexane or alcohols – each extracting different molecules). The solvents are filtered off and afterwards removed by distillation to leave either resinoids (from hydrocarbon solvents) or absolute resins (from alcohol solvents).5
Concretes
The extraction of concretes is similar to that of resinoids (hydrocarbons are used as solvents). For concretes however, plant material (leaves, flowers, roots, etc.) is used instead of a resin – this is the main difference. Most concretes are solid, wax-like substances and are much used in food flavourings.
Absolutes
An absolute is prepared from a concrete, by adding an alcohol to extract the aromatic (alcohol-soluble) molecules. The alcohol is then evaporated off gently under vacuum, leaving the absolute, a thickish, coloured liquid. The total process is much more complicated than I have made it sound!
Absolutes and resins are much used in the perfumery world, and although they can be useful in some applications of aromatherapy, it must be appreciated that they always retain a small percentage of the solvents used in their production. Some solvents may cause a substance sensitivity (see chapter 6) on certain skins, depending on the quality and quantity of the retained solvent. Adulteration is also a factor in this respect. Jasmine absolute, a favourite aroma for many people and possibly the most important fragrance in the perfume industry (there is no essential oil of jasmine available), is extremely vulnerable to adulteration6 and available at a wide range of prices – reflecting the quality.
Enfleurage
Pommades were obtained from the enfleurage process used long ago (replaced by concretes), when petals or leaves were laid on trays of animal fat for many days, being replaced regularly until the fat used as a solvent was saturated with the plant extracts.
Adulteration
The perfume industry is far and away the biggest user of essential oils, followed by the food industry. These industries have to obtain an oil having the same chemical formula (therefore giving the same aroma and flavour) time after time, year after year. It has always been accepted in perfume and flavouring trades that essential oils are standardized and not always 100 per cent true, nor are they always from a named botanical species – they have no need to be. They are being used not to influence the health of the body, but as fragrances or flavourings – a totally different kettle of fish! When