Oxides
By far the most important oxide is cineol (or eucalyptol) which stands virtually in a class of its own. It has an expectorant effect, well known as the principal constituent of eucalyptus oil. It is also found in a wide range of other oils, especially those of a camphoraceous nature such as rosemary, laurel, tea tree and cajeput. Other oxides include linalol oxide found in hyssop (decumbent variety), ascaridol, bisabolol oxide and bisabolone oxide.
In general, the term ‘essential oil’ is rather loosely applied to all aromatic products or extracts derived from natural sources, including concretes, resinoids and absolutes which contain a mixture of volatile and non-volatile components, such as wax or resin. This is not strictly accurate, since they are only partially composed of essential oils and are obtained by different methods of production, which include the use of solvents or more recently, carbon dioxide extraction. However, it is always the essential oil content in a given product that accounts for its aromatic quality.
Some plant materials, especially flowers, are subject to deterioration and should be processed as soon as possible after harvesting; others, including seeds and roots, are either stored or transported for extraction, often to Europe or America. The method of extraction which is employed depends on the quality of the material which is being used, and the type of aromatic product that is required.
Essential Oils
An essential oil is extracted from the plant material by two main methods: by simple expression or pressure, as is the case with most of the citrus oils including lemon and bergamot, or by steam, water or dry distillation. The majority of oils such as lavender, myrrh, sandalwood and cinnamon are produced by steam distillation. This process only isolates the volatile and water-insoluble parts of a plant – any other (often valuable) constituents, such as tannins, mucilage and bitters, are consequently excluded from the essential oil. Sometimes the resulting oil is redistilled or rectified to get rid of any remaining non-volatile matter; some essential oils are redistilled at different temperatures to obtain certain constituents and exclude others – as with camphor which is split into three fractions, white, yellow and brown.
The process of steam distillation as it is practised today
Essential oils are usually liquid, but can also be solid (orris) or semi-solid according to temperature (rose). They dissolve in pure alcohol, fats and oils but not in water and, unlike the so-called ‘fixed’ plant oils (such as olive oil), they evaporate when exposed to air leaving no oily residue behind.
Concretes
Concretes are prepared almost exclusively from raw materials of vegetable origin, such as the bark, flower, leaf, herb or root. The aromatic plant material is subjected to extraction by hydrocarbon-type solvents, rather than distillation or expression. This is necessary when the essential oil is adversely affected by hot water and steam, as is the case with jasmine; it also produces a more trueto-nature fragrance. Some plants, such as lavender and clary sage, are either steam distilled to produce an essential oil or used to produce a concrete by solvent extraction. The remaining residue is usually solid and of a waxy non-crystalline consistency.
Most concretes contain about 50 per cent wax, 50 per cent volatile oil, such as jasmine; in rare cases, as with ylang ylang, the concrete is liquid and contains about 80 per cent essential oil, 20 per cent wax. The advantage of concretes is that they are more stable and concentrated than pure essential oils.
Resinoids
Resinoids are prepared from natural resinous material by extraction with a hydrocarbon solvent, such as petroleum ether or hexane. In contradistinction to concretes, the resinoids are prepared from dead organic material, whereas concretes are derived from previously live tissue. Typical resinous materials are balsams (Peru balsam or benzoin), resins (mastic and amber), oleoresins (copaiba balsam and turpentine) and oleo gum resins (frankincense and myrrh). Resinoids can be viscous liquids, semi-solid or solid, but are usually homogeneous masses of non-crystalline character. Occasionally the alcohol-soluble fraction of a resinoid is called an absolute.
Some resinous materials like frankincense and myrrh are used either to make an essential oil by steam distillation or a resin absolute by alcohol extraction directly from the crude oleo gum resin. Benzoin, on the other hand, is insufficiently volatile to produce an essential oil by distillation: liquid benzoin is often simply a benzoin resinoid dissolved in a suitable solvent or plasticizing diluent.
Like concretes, resinoids are employed in perfumery as fixatives to prolong the effect of the fragrance.
Absolutes
An absolute is obtained from the concrete by a second process of solvent extraction, using pure alcohol (ethanol) in which the unwanted wax is only slightly soluble. An absolute is usually subjected to repeated treatment with alcohol; even so, as is the case with orange flower absolute, a small proportion of the wax remains. Absolutes can be further processed by molecular distillation which removes every last trace of non-volatile matter. The alcohol is recovered by evaporation which requires a gentle vacuum towards the end of the process. Some absolutes, however, will still retain traces of ethyl alcohol, at about 2 per cent or less, and are not recommended for therapeutic work because of these impurities.
Absolutes are usually highly concentrated viscous liquids, but they can in some cases be solid or semi-solid (clary sage absolute). In recent years, much research has been devoted to the extraction of essential oils and aromatic materials using liquid carbon dioxide; oils produced in this manner are of excellent odour quality and are entirely free of unwanted solvent residues or non-volatile matter.
Pomades
True pomades are the products of a process known as enfleurage, which is virtually obsolete today. This was once the principal method for obtaining aromatic materials from flowers that continued to produce perfume long after they were cut. A glass plate was covered in a thin coating of specially prepared and odourless fat, called a chassis. The freshly cut flowers, such as jasmine or tuberose, were individually laid in the fat which became saturated with their volatile oils. The chassis would be frequently renewed with fresh material throughout the harvest. Eventually the fragrance-saturated fat, known as pomade, would be treated by extraction with alcohol to produce the pure absolute or perfume.
Periods of the year for the treatment of various flowers once grown in Provence, France
Natural versus ‘Nature Identical’
Many perfumes or oils, once obtained from flowers such as carnation, gardenia and lilac, are nowadays produced almost entirely synthetically. In the pharmaceutical industry these chemically constructed products are called ‘nature identical’. The perfumery and flavouring industries require continuity in their products and naturally occurring substances are always subject to change, due to seasonal conditions. However, the so-called ‘nature identical’ products and the naturally occurring essential oils are of an entirely different character, which is reflected in their relative costs