Essential oils: definition and quality

Plants have accompanied man since the very genesis of our history. Over time, we have learnt to recognise them, cultivate them and use them, even deciphering them and determining their exact compositions. Now we know how to extract their very best, and it is in essential oils that we find the quintessence of the plant kingdom.

A look back on these powerful fragrance essences from aromatic plants …

Essential oils: concentrated extracts of fragrant metabolites

Thanks to the long process of evolution and the resulting selection, plants have been able to adapt to defend themselves and withstand various environmental conditions. Some have transformed morphologically, while others have evolved their metabolism to develop molecules of interest, involved in various functions such as insect control, reproduction or even communication⁽¹⁾.  It is from this last large group of aromatic plants that we today derive essential oils, concentrated extracts of these fragrant volatile metabolites.

Want to find out more? Download the guide to essential oils in cosmetics!

To recover this precious liquid, it must be carefully separated from the plant, with the art of extraction primarily based on the know-how and mastery of the process used⁽²⁾, which, depending on the plant in question, may be:

• Steam distillation: steam passes through the plant to extract volatile compounds. This process is suitable for almost all types of plants or parts of plants and results in less hydrolysis than hydrodistillation.
  (Essential oils of Lavender, Lavandin, Chamomile, Immortelle, Geranium and Eucalyptus, etc.)
• Hydrodistillation: this consists of steam distillation while the plant remains immersed in water. It is suitable for all products, even those most difficult to extract.
  (Essential Oils of Rose, Frankincense and Myrrh)
• Cold expression: a mechanical process that avoids deterioration of the essential oil, allowing us to extract more molecules than steam distillation, though it is reserved for the rinds of citrus fruits.
• Dry distillation: this technique involves pyrolysis of wood – it is specifically used for softwoods.

Finally, it is sometimes necessary to resort to secondary processes (rectification/fractionation/redistillation) in order to select olfactory notes or to remove certain molecules according to their desired use.

The essential oils obtained in this way can be classified according to various typologies: though they may be associated with the botanical family of the plant from which they are derived or according to their chemical family, perfumers prefer a classification according to their olfactory properties.

It is for this reason that we find the charm and poetry of terminology used in the world of perfumes to describe these families. As a result, we speak of floral (ylang, geranium, rose), hesperidic (lemon, orange and bergamot), woody (vetiver, patchouli, cedar, pine), aniseed (fennel, tarragon, star anise, aniseed), spicy (clove, carrot, parsley) and aromatic (lavender, sage, rosemary) notes, among others.

Purity and naturalness: the quality criterion par excellence for essential oils

Essential oils are complex natural mixtures, which may, unfortunately, be modified by the deliberate or accidental addition of substances of lesser value (synthetic or natural) or contaminants.⁽⁵⁾.

However, the term "essential oil" may only refer to a pure and natural product: it is the criterion of quality par excellence. To meet these requirements, the regulations impose standards represented by an analytical triptych: organoleptics, physicochemistry and chromatography.

• The organoleptic: among the organoleptic criteria controlled, smell is certainly the most important as it is decisive in the perfume and cosmetics industry⁽³⁾. Then, colour and appearance will be evaluated.
• Physicochemistry: as essential oils are a mixture of molecules, each contributing to its properties, they has their own physicochemical values (density, refractive index and rotatory power) which will form part of an initial evaluation of their good quality.
• Chromatography: this technique makes it possible to qualify and quantify each volatile molecule present in an essential oil, up to thresholds of 0.01%.

Each essential oil has typical organoleptic, physicochemical and chromatographic characteristics, guided by standards. They are defined by bodies such as AFNOR⁽⁶⁾, ISO⁽⁷⁾ or the European Pharmacopoeia⁽⁸⁾. These characteristics are dependent on the variety of plant used, the means of production, seasonality and the geographical area of cultivation. In addition to the panel of typical molecules, details of oxidation, ageing and contamination are investigated⁽⁴⁾ in order to confirm the quality of the essential oil.

How can we ensure "pure and natural" claims?

To ensure the purity of the essential oil, we must increase our standards and the technicality of our analyses. Chiral chromatography is therefore used to verify the spatial configuration of molecules or to evaluate the amount of stable and unstable carbon and/or hydrogen isotopes. These techniques allow us to validate the product's naturalness, or even to assume its geographical origin.