Essential Oils: A Short Guide   Botanical Classification: A Short Guide


An essential oil is:

During the distillation process, the essential oil can be continually separated off in a purpose-built separating vessel (traditionally a Florentine flask) which can be modified to isolate oils lighter or heavier than water. Once tapped off, it is usually necessary to dry the oil over an inert material, such as anhydrous sodium sulphate.

Essential oils may be present in many different types of plant materials (wood, bark, leaves, stems, flowers, stigmas, reproductive parts etc.) at concentrations ranging from thousandths of a percent to one or several percent. Oil is often contained in specialised secretory structures which include secretory cells, ducts, cavities, glandular trichomes etc. The yield of essential oils from seeds can often be high - in the several tens of percentage - but for the majority of other materials, the main range is 0.1% to 1%.

An essential oil is not:

The term essential oil is therefore not applied to carbon dioxide extracted products. Unfortunately some products in the essential oil trade have been labeled "oils", when they are really other products e.g. resin-absolutes etc.

Myths relating to essential oils:

(a) essential oils derived from natural materials are not artificial.

Essential oils isolated by steam distillation are different in composition to those naturally occurring in the oil bearing glands of plants, since the steam distillation conditions cause chemical reactions to occur which result in the formation of certain artificial chemicals, called artifacts. Some of these are considered beneficial e.g. the formation of chamazulene during the steam distillation of Chamomile oil; whilst others may not be e.g. the hydrolysis of linalyl acetate during the distillation of clary sage. Few, if any, essential oils are unscathed by the thermal conditions of steam distillation, but some distillation techniques can, in certain instances, be a measure less damaging than others (e.g. hydrodiffusion – a sort of inverted steam distillation where steam is introduced at the top of the vegetable material-packed container, and oil and condensate issue from the bottom – can produce oils with higher ester contents i.e. less thermally induced hydrolysis).

(b) essential oils are unprocessed before we get them.

In fact, after field distillation, many crude oils are subsequently rectified to improve their odour and keeping qualities, and sometimes to improve also their colour and appearance and possibly also to reduce moisture loading. Examples here include Litsea cubeba, Eucalyptus oils etc. Sometimes a trading company might rectify an oil that would not normally be further processed, in order to achieve a specific distinctive odour effect. Certain oils may have other specific treatments e.g. dementholisation of solid crude peppermint oils is carried out, the natural menthol is isolated and sold separately, and the remaining (now liquid) peppermint oil is sold into the marketplace.

(c) essential oils are "better" from aromatherapy distillers because they are distilled longer and under milder conditions.

On the positive side, it is possible that longer distillation times may give a more complete oil. It is also possible however, that longer distillation time may lead to the accumulation of more artifacts than normal. This may have a curious effect of appearing to improving the odour, as sometimes when materials that have a larger number of components are sniffed, the perception is often of slightly increased sophistication, added fullness and character, and possibly, and extra pleasantness. The term "milder conditions" is pretty meaningless in this context -just how mild to vegetable tissue is any temperature above boiling-point? One important factor not to forget in direct-fired distillation, is that intimate contact of vegetable material with hot-spots on the still surface may cause burning-on the material via hot-spots on the inside of the still, thereby producing off odours.

To further illustrate the point that up to as limiting point, higher temperatures are not necessarily that damaging to essential oils per se, subcritical water extraction of vegetable materials has been demonstrated (Clifford et al 1999) which has been applied to clove buds, and rosemary oil. In the latter case the extraction technique when carried out at 150° C gave a slightly better yields than steam distillation, and gave an extract richer in oxygenated materials. The advantage of steam distillation is that it is a relatively cheap process to operate at a basic level, and the properties of oils produced by this method are well known. Newer methodology, such as subcritical water extraction, may well eventually replace steam distillation, but so far even contenders such as carbon dioxide extraction - although establishing a firm market niche - have not really threatened to take over as the major preparative technique.

(d) GC-Mass Spec analysis will tell you everything about the composition of an essential oil.

GC-MS is one of the best techniques we have to identify the constituents of an essential oil. When properly used it can easily detect and identify major components of essential oils, and give us some indications of the quality and authenticity of the oil. The technique does have limitations however. Many minor components of essential oils (<0.01%) do not register on GC detector systems, yet can be powerfully perceived by the nose – as indicated by aromagrams (these are constructed by dividing the output of the GC column between the detector and an odour port, where components can be individually smelled and identified or described by perfumers/odour analysts, as they progressively elute from the column). In certain cases these undetected materials can contribute profoundly to the odour profile, and may also be responsible for psycho-physiological effects of the oils.

Essential oils are products of the secondary metabolism of plants, and generally are fragrant volatile materials consisting of complex mixtures of mono- and sesqui-terpene hydrocarbons, and oxygenated materials biogenically derived from them. Other common constituents include phenyl propanoids from the Shikimic acid pathway, and their biotransformation products, and other compounds from the metabolism of fatty acids and amino acids. As well as these major groups of compounds, a large number of other types of chemical components are also found, including nitrogen & sulphur compounds.

In previous years, it was considered that essential oils were an end-point of metabolism, but now it is known that in times of stress the oils can be broken down to provide energy for the plant.

Essential oils are used in flavourings, perfumes, in Aromatherapy, as insect & animal repellents, in pharmaceutical preparations, as anti-microbial agents and in many other ways. Legislation is making progressive encroachments on the unrestricted use of essential oils; for example many are included in the EU’s register of flavouring substances. Safety is also becoming a key issue for the allowable use of essential oils in consumer products such as perfumes and cosmetics. Certain dangerous oils may be restricted or banned by national legislation i.e. Chenopodium oil from in the U.K.

The fact that essential oils are, traditionally, perfumery materials, and that perfumers do not necessarily require oils to be pure (just consistent in quality batch to batch, and to be able to achieve the desired effect in product use) has lead to some problems for Aromatherapists who require pure essential oils, 100% derived from the named source. Aromatherapists also use certain oils which they believe have efficacious properties and bring benefits to clients in application, these oils are not necessarily always those in the mainstream of popular use for example rosemary oil verbenone type, or Helichrysum italicum ssp. serotinum. This has lead to the establishment of a small industry of essential oil distillers supplying the Aromatherapy market.

*Regarding dry-distillation and essential oils: The International Standards Organisation (ISO) includes dry-distilled oils within their technical definition of essential oils. Dry distilled oils contain high levels of artifacts (phenols, tars) from the thermal destruction of vegetable tissue. So this inclusion cannot be within the spirit of what essential oils are ideally meant to represent to many end-users: the isolated fragrant principles of secondary metabolism of plants stored within specialised structures (see above) and isolated with minimum chemical changes from human intervention. I believe that I am not alone in challenging the ISO.essential oils definition, which, in my opinion, should be ignored.

Reference: Clifford A.A. (1999) "Extraction of Natural Products with superheated water" In : Proceedings of the GVC-Fachausschss ‘High Pressure Chemical Engineering’, Mar 3-5 1999, Karlsruhe, Germany.

Tony Burfield  Dec 2000
Copyright © 2000 by Tony Burfield, All Rights reserved.