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Essential Oils Chemistry
The known chemical constituents of plants are of two kinds: products of primary metabolism and products of secondary metabolism. The first are chiefly carbohydrates, amino acids and fixed oils, produced by light-absorbing (photosynthetic) processes. The latter grouPof chemicals arise from the primary metabolites. They include glycosides, terpenoids, alkaloids and essential oils.
The chemistry of essential oils is elaborate. An individual oil may have hundreds of ingredients, the principle components being a grouPof complex substances known as terpenes and their compounds or derivatives. This explains why a single essence has a wide range of therapeutic actions.
Although essential oils have been credited with possessing almost all of the plant's therapeutic properties in a concentrated form, the word 'almost' needs some clarification. Essential oils lack the water-soluble constituents such as tannins, bitter compounds, sugars, mucilages and pectins, all of which play an important role in the medicinal action of the plant. With the exception of cold expressed citrus essences, the heat of distillation (and other warm extraction methods) causes changes in the natural chemical composition of the essential oil, resulting in substances not found in the plant. Having taken these things into account, some herbalists believe that the term 'natural' is something of a misnomer when applied to an essential oil.
While it is true that the essential oil lacks certain chemical compounds found in the raw plant material, it is still a highly complex substance, so complex that it cannot be replicated in the laboratory. Even though high-tech processes such as gas-liquid chromatography (GLC) can
Separate out main components of essential oils by looking at the chemical 'fingerprint ' produced, it is impossible to isolate the numerous trace elements including number of unidentified compounds which make uPthe whole. For this reason, synthetic or nature identical aromatic oils never quite the same as organic essences.
Main Chemical components
While it is interesting to take a closer look at the individual components of essential oils, it is important not to lose sight of the whole. Assigning actions to an essential oil based on its chemical composition can be misleading. As we have seen, the therapeutic properties of the whole oil is the result of synergism: an interaction of all its chemical constituents working harmoniously together so that the whole becomes more potent than the sum of its individual parts. Bearing this in mind, here is a brief survey of the main chemical compounds found in essential oils.
Terpenes
This is a vast grouPof chemicals with widely varying properties, so it is impossible to generalize about their therapeutic actions. However, common terpenes include limonene (an antiviral agent found in 90 per cent of citrus oils), and pinene (an antiseptic found in high concentrations in pine and turpentine oils). Others, such as chamazulene and farnesol (found in chamomile essence), possess remarkable anti-inflammatory and bactericidal properties.
Esters
The most widespread grouPfound in plant essences, which includes linalyl acetate (found in clary sage and lavender), and geranyl acetate (found in sweet marjoram). Esters are fungicidal and sedative, usually with a fruity odor.
Aldehydes
These substances are found notably in lemon-scented essences, such as lemongrass and citronella. Aldehydes generally have a sedative, though uplifting, quality.
Ketones
Certain ketones are known to be toxic, so this chemical grouPis regarded with a degree of caution. However, it is misleading to generalize about the toxicity of individual chemical components without knowing the exact ratio of the substance in relation to other chemicals in the whole oil. Certain essences, however, do contain appreciable quantities of toxic ketones, so should be avoided by lay people. Mugwort, tansy, wormwood and common sage contain the potentially risky thujone, while pennyroyal contains pulegone. Non-toxic ketones include jasmone, found in jasmine, and fenchone in sweet fennel. Ketones ease congestion and aid the flow of mucus, which is why plants and essences containing relatively large quantities of these substances are usually helpful for upper respira-tory complaints.
Alcohols
Some of the most common alcohols include linalol (found in abundance in lavender), citronellol (rose, lemon, eucalyptus and geranium) and geraniol (geranium and palmarosa). These substances tend to have good antiseptic and antiviral properties and an uplifting quality.
Phenols
These are bactericidal with a strong, stimulating effect on the central nervous system. Essential oils containing relatively large quantities of certain phenols are potentially irritant to skin and mucous membranes. Common caustic phenols include eugenol (found in clove essence), thymol (thyme) and carvacrol (oregano). However, anethole (from fennel) and estragole (tarragon) are not at all caustic.
Oxides
These are found in a wide range of essences, especially those of a camphoraceous nature, such as rosemary, eucalyptus, tea tree and cajuput. Oxides tend to have an expectorant effect; for example, eucalyptol (eucalyptus). |
