What are essential oils?

What are essential oils?

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I thought I’d start the publications on this blog speaking directly of the material we cover: essential oils. This post will also be the first in a series intended to clarify some concepts and definitions essential to navigate the world of essential oils.

It could seem easy enough to define an essential oil, but in fact this is where  important details and difficulties tend to hide.

Butt definitions are useful and important. They can help a non-specialist to discriminate between the various products on the market, and help define a field of discussion, research etc., help to create a common language.

I will give straight away a plain English definition of essential oil, saving the discussion for later:

An essential oil is the product of the distillation of steam of aromatic plants, or of cold pressing in the case of the peels of citrus fruits.

That said, let’s step back and see where this definition comes from, and with what reasons.

Let’s start by saying that at the moment there are no legally-binding definitions of an essential oil. However, there have been various attempts at a meaningful definition: some are based on a description of the chemical types that make up essential oils, others are based on botanical grounds, while still others are based on descriptions of the industrial process.

Let’s see these definitions and try to understand their strengths and weaknesses.

Chemical definition

A OE is a complex mixture of organic compounds extracted from aromatic plants, chemically characterized by terpenoids, but which also includes other groups of compounds, in particular phenol ethers, nitrogen compounds, sulfur-containing, etc. The terpenoids present in the essential oils are normally monoterpenoids and sesquiterpenoids, and in rare cases diterpenoids.

It is immediately apparent that this is not a definition but a description of a complex mixture, a description which is at the same time too “narrow” and too “large”. It is too narrow because it is just not possible to describe essential oils comprehensively as we can do, for example, for the chemical class of flavonoids, or alkaloids. It is too large because it admits into the category of essential oils aromatic materials that have little in common with them (absolutes, concretes, solvent-extracted materials, etc.). In fact the essential oils are not a”natural” cluster  but a product, the product of a historically and commercially determined process.

Botanical definition

We could then go for the botanical definition, and define the essential oils as:

an odorous product of metabolism of the plant, depending on the characteristics of the species and the individual, with specific metabolic functions and ecological relationship (attraction, defense etc.) and always changing, never defined in its chemical composition in a fixed manner.

This definition brings us closer to the complexity of the essential oils, but it still isn’t enough. In fact, essential oils as a product are not the same thing, in terms of chemical profile,  as the odorous molecules contained in the living plant (the essence); they for instance do not contain odorous molecules which are too heavy to be extracted by distillation, or those excessively light, that can be lost by evaporation during the distillation, or those soluble in water, that solubilize in the hydrosol, or those that are not in a free form (for example terpenoids in a glycosidic form, etc.). Moreover, essential oils contain molecules that are not present in the plant, artifacts of the production process such as chamazulene of Matricaria recutita. That is, as specified above, the composition of the essential oil as a final product, also depends on the parameters of the industrial process.

“Industrial” definition

So it seems that aprocedural definition is best, because it takes into account the inherent variability of the essence of the plant, avoids the problem of a stringent chemical definition  and takes into account the selective role of the extraction process.

We could briefly say that an essential oil is:

any product of the process of steam distillation, or, only in the case of citrus peel, the process of cold pressing.

Let’s look at this definition in more detail:

the essential oil can then be defined as a selective phytochemical extract which does not select a pure, chemically defined,product with a characteristic formula, but a mixture of isolated products present in widely differing proportions that share a similar physical behavior in the given conditions. Which means, in the case of OE by steam distillation, volatile under normal conditions, or at least with a vapor pressure significantly below 150 ° C, and which are at the same time insoluble or poorly soluble in water.

In the case of OE obtained by citrus peel pressing, they will be characterized only by their lipid solubility. It is therefore the extraction operation, the filter, to define the essential oil.

In many cases this definition is made more specific by saying that an essential oil must not have any added substance and any component removed. While the purpose of this addition it is understandable, the fight against fraud and adulteration, it clashes with the realities of production. In fact in many cases essential oils present on the market have undergone processes of clarification, cleaning and sometimes rectification which are now the norm, and that in some cases are useful. If I did not extract a percentage of the menthol in various Mentha essential oils, these would be solid or semi-solid at room temperature, given that menthol crystallizes at room temperature. Or, as in the case of certain oils of eucalyptus, certain oils are slightly adjusted to eliminate an excess of linear aldehydes that make the oils irritant and toxigenic. These practices cannot be put into the same basket with adulteration or fraud, although they should always be made explicit.

This definition is similar but different from that prepared by the ISO and AFNOR, according to which an OE is:

a product obtained from a vegetable raw material, either by distillation with steam, or with mechanical processes low Phoma of Citrus, both for dry distillation. The essential oil is then separated from the aqueous phase by means of physical processes.

This definition accepts dry or destructive distillation as an acceptable process. Many authors would take issue with this inclusion. Oils obtained by this method contain very high levels of artifacts (phenols, benzo-pyrenes, tars) originated by thermal destruction of plant tissues. Indeed this inclusion does not seem to reflect what ideally an EO should represent for end users: aromatic principles isolated from the secondary metabolism of plants and stored in specific strucures, isolated from the plant mass with minimal alterations caused by human intervention.

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