What happens when essential oils oxidize?

Recently I saw a posting on Facebook where someone was saying that they knew that citrus oils could oxidize when they were old, but they were wondering whether that made them unsafe or just less potent.  An interesting question but to have a real understanding of what happens it helps to have a little bit of an understanding of the chemistry of essential oils.

As many of us know each essential oil contains many different chemical constituents, and each of these chemical constituents has its own properties and actions. When a particular chemical constituent is part of the chemical profile of an essential oil it can lend its unique properties and actions to that essential oil, but when something happens to cause that chemical constituent to change, the whole chemical profile of the essential oil containing it will change too. How susceptible a particular chemical constituent will be to change, very much depends on its actual structure and whether or not its valences are saturated or not. Without going into too much detail:

  • Chemistry is a branch of physical science that studies the composition, structure, properties and change of matter. It is generally divided into Organic and inorganic chemistry. Essential oils fall into the category of Organic Chemistry.
  • Organic Chemistry is the branch of chemistry concerned with compounds of carbon.
  • The structure of all the constituents found in essential oils contain carbon and hydrogen. Monoterpenes and Sesquiterpenes just contain carbon and hydrogen molecules. Alcohols, Sesquiterpenols, Phenols, Aldehydes, Acids, Esters, Ketones, Ethers, Oxides, Lactones and Coumarins, all start with the carbon, hydrogen structure to which is added the defining functional group.
  • Each atom has its own valence (how easily the atom or radical can combine with other chemical species. This is determined based on the number of electrons that would be added, lost or share if it reacts with other atoms.) Hydrogen has a valence of 1, Carbon has a valence of 4, and Oxygen a valence of 2. In order for a compound to be stable all the valences should be filled.
  • Because of their carbon-hydrogen structure, monoterpenes have empty valences and in the presence of oxygen will quickly take on oxygen to fill those valences and the oil becomes oxidized. Once oxygen is added to the monoterpenes carbon-hydrogen structure, the constituent changes from a monoterpene (only carbon-hydrogen) to one of the other Functional Groups.


As examples, the monoterpene limonene can, in the presence of moist air, quite easily oxidize into an alcohol, carveol ; a ketone, carvone; or an oxide, limonene oxide. While pinene can oxidize into an alcohol, verbenol; a ketone, verbenone; or an oxide pinene oxide.

Monoterpenes are considered to have the general properties of being: antiseptic, possible skin irritants, aggressive to skin, mucous membrane irritants, air antiseptics, slightly analgesic, antiviral, bactericidal and decongestant.

Alcohols have the general properties of being: antiviral, antiseptic, strong bactericidal, stimulant, diuretic. Ketones have the general properties of being: analgesic, antiviral, anti-fungal, good for scar tissue, sedative, mucolytic, expectorant, abortifacient. With the caution to use in low doses and for short periods of time.

Oxides have the general properties of being: mucolytic, expectorant, immuno-stimulant, mucolytic, can irritate skin, diuretic.

Of course, when oxidization takes place we have no way of knowing which chemical constituent has oxidized, nor without proper testing, can we know what it has oxidized into, nor can be know how much change has taken place. But what seems clear, to me, is that we now have an oil that has different properties to the ones we would expect it to have, and probably different cautions too.

It is also important to keep in mind that essential oils are complex, usually containing a large number of different chemical constituents, so they are always much more than the ‘sum of their parts’ and we can never base our knowledge of an essential oil on just one or two chemical components.

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