Chemical constituents of essential oils
Essential oils are extracted from a variety of plants including trees, shrubs, flowers, grasses, herbs and fruits. The oils extracted are concentrated in different parts of the plant. For example, mandarin, lemon, lime, grapefruit and bergamot are squeezed from the peel of the fruits, geranium oil is extracted from the leaves and stalks, cumin oil comes from the seeds, myrrh, frankincense and benzoin are extracted from the resin of their respective trees and ginger oil comes from the stems which grow under or along the ground.
Research has discovered over 1,000 different chemical components in essential oils with many more yet to be discovered and analysed. However, listing and naming these components to recreate the essential oil would be ineffective as it is the synergistic effect of all the components that makes an essential oil what it is. Take Tea Tree as an example, it is powerfully antimicrobial, but its' 2 main components (terpinen-4-ol and gamma terpinene) which make up 68 per cent, are much less so. Rose oil alone contains over 300 components and the properties of these components can change. For example, the components from the oils extracted from plants can change according to how, when and where these plants are grown and harvested. There is also the issue of how the oils are extracted. The process of distillation releases only lighter aromatic molecules, while absolutes, concretes and resinoids contain both the lighter and heavier molecules, thus creating a more powerful scent.
However, these essential oils are complex and much more than an aroma as they perform strong actions on our physical and mental mechanisms. In fact, we know that essential oils are non-invasive to the human body as chemically we are both similar, having descended, like all living organisms from the same single-cell line. We cannot live without plants for our sustenance; we eat plants and the animals which eat those plants, we breathe air which is cleaned by plants. For thousands of years humans have learned to utilise plants and develop an understanding of their chemicals, often without releasing it. We have learned that certain plants are at their optimum best at particular times and essential oils are a form of medicine and shamanism in certain cultures, where many more are used and whose components (and properties) we have yet to discover.
An average essential oil contains around 100 chemical components, but some are many times more complex. Essential oils consist of chemical compounds which have hydrogen, carbon and oxygen as their building blocks. These are then subdivided into 2 groups: the hydrocarbons which are made up of mostly terpenes and the oxygenated compounds which are mainly alcohols, aldehydes: esters, ketones, phenols and oxides. However, although technology is advancing, there are many more yet to be discovered. Current methods of analysing depend on the sensitivity of a technique called GC-Mass Spec (Gas Chromatography Mass Spectrometry) analysis. However, this technique is limited as it relies on the accurate interpretation of the operator and the fact that some components are so minor they do not register on GS detectors. Instead these minor components can be individually smelled and identified by experienced perfumers. In some cases, these undetected compounds can contribute hugely to an odour profile and may also be responsible for an oils particular effect on our physiological and psychological state.
Alcohols are generally considered safe and have a very low or totally absent toxic reaction in the body or on the skin and so can be used on children. They are extremely useful due to their antiviral, antibacterial and antiseptic properties. Alcohols are present either as a free compound or combined with a terpene or ester and are found in ylang ylang and lavender as linalool, geraniol in geranium and palmarosa and citronellol found in rose, lemon and eucalyptus. Other alcohols include menthol, nerol and benzyl alcohol.
Alderhydes are found in lemon-scented oils such as melissa, lemon verbena, citronella etc and include citral, citronellal and neral. They generally have sedative qualities with specific antiseptic properties. Other known alderhydes include benzaldehyde, cinnamic alderhyde and perillaldehyde. Essential oils containing alderhydes are helpful in treating inflammation, Candida and viral infections.
Esters are the most widespread group found in essential oils. They are formed through the reaction of alcohols with acids and include linalyl acetate found in bergamot and lavender, geranyl acetate found in sweet marjoram and geranyl formate found in geranium. Oils containing esters are often used for their soothing and antifungal qualities and for helping to balance the nervous system. Esters are generally safe with the exception of methyl salicylate found in sweet birch, which is not toxic, but harmful in concentration.
Some of the most toxic constituents are ketones, such has thujone found in mugwort, sage and wormwood oils, while pulegone is found in pennyroyal and pinocamphone in hyssops. However some are non-toxic such as jasmone found in jasmine, fenchone in fennel and carvone in spearmint and dill and menthone found in peppermint. Ketones are often found in plants that are used for upper respiratory ailments so help ease congestion and aid the flow of mucus.
The most important oxide is cineol (or eucalyptol) and it stands almost in a class of its own. It is an effective expectorant and is the principal constituent (70-85 per cent) of eucalyptus oil (from the blue gum tree). However it is found in a number of other essential oils such as bay, tea tree and rosemary. Common features of this oxide are its analgesic, antiviral, deodorising and balsamic properties. Other oxides include linalool oxide, bisabolol oxide and ascaridol.
Phenols have an antibacterial and very stimulating effect which can cause skin irritation. Due to their antiseptic and disinfectant properties they are very cleansing but may cause toxicity in the liver if used over a very prolonged period. Common phenols include eugenol found in clove, basil, rose and cinnamon, carvacrol found in oregano, spearmint and savory, and thymol found in thyme. Other phenols include methyl eugenol, anethole, safrole and apiol among others.
Terpene hydrocarbons tend to be antiviral and antibacterial but depending on their chemistry, they can also be analgesic, stimulating, antiallergic, anti-inflammatory and hormonal balancers. Terpenes are categorised as monoterpenes, sesquiterpenes and diterpenes and include antiviral limonene (found in 90 per cent of citrus oils), antiseptic pinene found in pine and terpentine and camphene in cypress oil. As noted above, depending on extraction can determine what constituents are present. Diterpenes are usually only present in absolutes or tinctures as the molecule is too heavy to allow for evaporation with steam during the distillation process. Specific research has been done on chamazulene and farnesol (sesquiterpenes found in chamomile oil) due to their amazing anti-inflammatory and bactericidal properties.
By Cherylin Nicholson