Strictly speaking, the term ‘essential oil’ describes an aromatic oil captured by distillation and composed entirely of volatile molecules. Nevertheless, the term is often stretched to include aromatic extractions of every conceivable origin, including those captured by volatile solvents, liquid carbon dioxide and the recent phytol method.
Although the roots of this method can be traced back to the Mesopotamians nearly 5,000 years ago, distillation as we know it today was initially developed by the Arabian medieval alchemists and perfected in Grasse, the home of the perfume industry in the south of France. Generally, soft plant material such as leaves and flowers may be placed in the vessel of a still without prior treatment; fibrous material such as wood, bark, seeds and roots must be cut up, crushed or grated to help release the volatile oil.
Home Distillation of Essential Oils
Although many old herbals (and a few aromatherapy books) extol the virtues of distilling essential oils at home, it’s doubtful that this would be a practicable option. For a start, to obtain a ‘kitchen table’ still would be an amazing accomplishment in itself since they are so hard to come by. Modestly sized distillation apparatus can be bought from laboratory equipment suppliers 0nly if you have a special commercial license. For the determined kitchen alchemist, it is possible to rig up your own equipment for producing usable floral waters such as lavender water or rosewater which can be employed as skin tonics – or even as a base for home made eau de cologne. But home-distillation is extremely time consuming with little return for one’s efforts.
The oils of citrus fruits such as bergamot, lemon and lime are much easier to obtain. Indeed, the essence (the zest of the fruit) is found in such profusion that it sprays the surrounding air when the fruit is peeled. If the zest comes into contact with a candle flame or lighted match, the oil droplets will ignite into a cascade of tiny sparks, albeit for a fraction of a second. The highest quality citrus essences are captured by a simple process known as expression. Although this was once carried out by hand (by squeezing the rind and collecting the oil in sponge), machines using centrifugal force are now used instead. Unlike distilled oils, expressed essences also contain non-volatile substances such as waxes. The drawback with expressed oils is their relatively short shelf-lite even though producers usually add tiny amounts of a preservative at source, the oils will still deteriorate within six to nine months, whereas most distilled essences will keep for upwards of two years.
In the USA, the production of citrus essences is a profitable side-line of the fruit juice industry. There are many low-grade citrus oils with an inferior aroma obtained by steam distillation of the citrus skins after the best oil has been expressed. Unscrupulous suppliers may extend an expressed oil by ‘bulking’ it with such a product.
The enfleurage method of extraction was once widely used in the perfume industry to capture the fragrances of flowers such as jasmine, orange blossom and tuberose, whose exquisite fragrances would be spoiled by the intense heat of distillation. Except for the tiny amount of enflurage oils produced for tourists visiting the distilleries in Grasse, this method of extraction is no longer commercially viable.
Volatile Solvent Extraction
The high cost of the labor-intensive and time-consuming enflurage method has led to the wide use or volatile solvents such as petroleum ether, hexane and benzene as a means of capturing the essences of certain plants and resins – yet the enflurage process gives a higher yield. Solvent extraction is employed a great deal in the perfume industry because it produces superb fragrances which are closer to the aromas contained within the plant. Nevertheless, these oils may not be suitable for aromatherapy as they often contain solvent residues and other non-volatile substances which can cause skin reactions in sensitive individuals when the oil is applied directly to the skin. They are also extremely vulnerable to adulteration. Having been subjected to the volatile solvent (usually hexane) the aromatic plant material produces a waxy substance known as a concrete.
Although concretes are used in the perfume and fragrance industries, as they still contain appreciable amounts of non-volatile substances and residues of solvent, they are more likely to provoke allergic reactions in those with sensitive skin. To obtain the aromatic liquid the concrete undergoes repeated treatment with alcohol which is finally recovered by evaporation under a gentle vacuum. The resulting viscous oil is known as an absolute.
With the exception of benzoin, the essential oils found in resins, such as frankincense, myrrh and galbanum, can be obtained by steam distillation of the dried resinous exudations. These are to be preferred for aromatherapy. The solvent extracted versions commonly employed by the perfume industry are known as resinoids. Unfortunately, benzene (a potentially carcinogenic volatile solvent) is, on occasions, used in the extraction of benzoin and other resins.
Liquid Carbon Dioxide Extraction
This process was hailed as nothing less than revolutionary when it was introduced at the beginning of the 1980s. Although a potentially excellent method of extraction, producing oils whose aromas arc closer to those of the living plant, the apparatus required for this operation is not only massive, but also extremely costly. It will also take years for the equipment to pay for itself; until such time, the cost of carbon dioxide extracted oils will remain very high.
Although the method is much more complicated than described here, suffice it to say that carbon dioxide gas at exceedingly high pressure (pressures which are only found three miles under the sea) is employed to dissolve the aromatic oils from the plant material. When the pressure is allowed to fall (to one hundred times atmospheric pressure) the oils form a mist and can be collected. The resultant oils are free of the potentially harmful residues associated with solvent extraction, but there are those who would argue that carbon dioxide is an acidic gas and therefore detrimental to the chemical structure of essential oils.
Generally, carbon dioxide extracted oils have fewer terpenes, a higher proportion of esters and contain other substances whose molecules are too large to come through the distillation process. Since a great many terpens are formed as a result of distillation, the lower terpenes content of carbon dioxide oils (and the related difference in ratio of other constituents) means their chemistry is a little closer to nature. Surprisingly, this is not always beneficial. Take ginger, for example; the carbon dioxide extract has a superior aroma to that of the distilled oil, but it is also more likely to irritate sensitive skin. A major drawback with carbon dioxide extracts in general, apart from their high price, is that very few suppliers stock these oils.
Hydrodiffusion or Percolation
Although introduced more recently than carbon dioxide extraction, hydrodiffusion is similar to steam distillation, except that the steam is produced above the plant material and percolates down through it. The advantage of hydrodiffusion over distillation is that the process is quicker, especially for fibrous material such as woods and barks whose cells do not give up their oils easily. The resultant oils are reported to have a superior aroma and a richer color than those obtained by ordinary distillation.
The Phytonic Process
The Phytonic Process was recently developed by British microbiologist Dr Peter Wilde in collaboration with the multinational chemical company, ICI. Proponents of phytonics technology believe that it heralds the biggest breakthrough in aromatic oil extraction since the discovery of distillation.
The process has been developed around a family of new solvents collectively known as phytosols, whose unique character ensures that the aromatic oils of plants can be captured at (or below) room temperature. This means that the exceptionally fragile, heat-sensitive components of an aromatic oil are not lost, or radically altered, in the extraction process.
Oils extracted using the Phytonic Process cannot, strictly speaking, be called essential oils, but neither can they accurately be called absolutes. Indeed, chemical analysis reveals phytonic – extracted oils to be somewhat different from other aromatic oils. English rose phytol oil, for instance, contains over 290 separate, identifiable components (including the water soluble phenylethyl alcohol), whereas the best available rose absolutes contain only 210 components.
A few oils are not quite what they seem. for example it is extremely difficult to obtain the true essence of melissa oil (distilled from lemon balm) because so little is produced. When it is available, the oil is costly, for the plant yields such a tiny amount of oil. Most of the so-called melissa oils on the market are actually blends of infinitely cheaper essences such as lemongrass, lemon and citronella. Only when you have smelled true melissa essence and compared it to the falsified versions will you be better able to tell the difference. Honest suppliers will highlight the difference, perhaps by labeling the blended version ‘Melissa Type’ and the genuine oil, ‘Melissa True’. A word of warning: unfortunately, there are also a number of ‘Melissa Type’ blends on the market which are partly or wholly compounded using aroma chemicals.
If you come across an oil labeled ‘Amber’, be suspicious. It is certain to be a synthetic compound or a blend of clary sage and benzoin (a resinoid with a vanilla-like aroma). True amber oil, from the fossilised resin, is unobtainable. Ambergris (a substance excreted by the sperm whale and sometimes found floating on the sea) is also known as ‘amber’, an extremely costly fragrance material used in high-class perfumes. Ambergris is not generally available to the public. However, there is a genuine essential oil of ambrette seed, sometimes labeled ‘ambre’ (note the spelling). Ambrette essence is a popular, though very expensive, perfume material which is also occasionally used in aromatherapy.