What Is An Essential Oil?

I thought it would be useful to start from definitions: when discussing essential oils definitions are useful and important because they can help non-professionals to disentangle the many pieces of information they read and hear about the various “aromatic” products present on the market, and also because definitions help us to delimit a field of discussion and research.

Definitions of essential oils come in different shapes and forms. For example, we may say that essential oils are fat-loving substances (in that they usually float on water, or do not mix with it) obtained from aromatic plants, with an aroma that resembles the plant of origin, and that are volatile and flammable. We could say that in most cases they are dominated by chemical compounds belonging to the terpene and terpenoid class, but they sometimes contain other chemical classes. We could also say that the plant produces and stores them in different tissues, in some cases in superficial hair on leaves, flowers, stems, etc. In other cases, essential oils are collected in bags that may be superficial (as in citrus peel) or deep (such as in ginger rhizome), or else that they are mixed with other substances, such as resins (oleoresins and oleogumresins) from both conifers and exotic plants such as myrrh and incense.

All this would be correct, but it is more a description than an exhaustive definition. And definitions are important. The one that I prefer is the following:

An essential oil is the hydrophobic product of steam distillation of aromatic plants or, but only in the special case of citrus peels, the hydrophobic cold pressed product. Usually the term refers to the product obtained by extracting a single drug, where by drug we intend an organ or tissue of a single botanical species.

Let’s explain it better; this is a procedural definition, that is, it does not speak of the content, of the nature of the essential oil, but it describes the procedures suitable for producing it.

In particular it tells us that:

  1. There are two accepted procedures, steam distillation and cold pressing of the citrus peels.
  2. These procedures must be carried out on plant materials and not, for example, animals or minerals.
  3. Essential oils are hydrophobic, i.e. they do not mix with water.
  4. Essential oils are usually extracted from a single part of a specific plant, such as orange blossoms, or cinnamon bark, or anise fruit. From this point of view, cinnamon leaf oil is distinct from that of cinnamon bark, and leaves and barks are not distilled together.

The definition is similar but does not coincide with that drafted by the International Organization for Standardization (ISO) and its French branch (AFNOR). Their definition of essential oil is in fact:

A product obtained from a raw vegetable raw material either by steam or water distillation or by mechanical processes starting from the epicarp of Citrus fruits and by dry distillation

In this definition you can notice the presence of a third production method, namely dry distillation, also called destructive, pyrogenic or empyreumatic distillation, which is used to “distill” woody material, in particular birch (Betula pendula), and juniper woods, such as Juniperus oxycedrus, Juniperus phoenicea and Juniperus sabina.

Despite the name, this distillation is very different from steam distillation. While in steam distillation the plant material is permeated by a water vapor stream that “strips” the essential oils from the plant and carries them upward, in dry distillation the material to be distilled is burned in absence of oxygen at very high temperatures and the vapors and combustion fumes are collected in descending distillation, to obtain a dense substance, called tar. This tar contains products derived from the degradation of essential oil, and phenols (particularly polycyclic aromatic hydrocarbons), some of which, for example benzo-alpha-pyrene, are carcinogens. That is why the aromatherapy community generally does not accept the last part of the ISO definition.

By simplifying the definition given earlier, we could say then that:

An essential oil is that water-soluble substance that is obtained when we distill an aromatic plant into steam stream (or when we cold-press the citrus skins)

To conclude, all this means that “essential oils” obtained through the use of solvents (CO2, benzene, toluene, acetone, ethanol, hexane etc.) or by dry or destructive distillation, or by molecular distillation are not to be considered as genuine essential oils.

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