The Difference Between Essential Oils and Fatty Oils

Ever wondered what an essential oil really is? Knowing the science behind essential oils certainly helps. 

Aromatherapy has been around for centuries and essential oils have been too (though they weren’t always known as such). It’s hard to know exactly how essential oils got their name but most sources suggest “essential” was derived from essence, or the essence of a plant, and oil because when distilled, the aromatic “essence” that was extracted from the plant had a similar property to fats and oils. It was insoluble—did not dissolve—in water. But, essential oils are not fats, lipids, or fatty oils.

How can we tell the difference? Simply, fats and lipids have different molecular structures than essential oils. Essential oils are hydrocarbons and can contain as many as 300 different compounds, depending on the source. Chemically, essential oils can be classified as alcohols, ethers, esters, aldehydes, ketones, amines, amides, phenols, and mainly, terpenes. They also have a relatively low molecular weight, with most essential oils weighing in at less than 300g/mol. That’s all a bit technical. Simply, essential oils lack the building blocks to make them fats or lipids.

Conversely, there are fats and fatty oils, which are types of lipids. Fats and fatty oils have a distinguishing molecular structure, called a triglyceride. Triglycerides contain three fatty acid chains (hydrocarbon chains) connected by glycerol. These molecules are typically heavier than essential oils, ranging in molecular weight from 300 g/mol for a sparse few to approximately 1000g/mol (and higher). If the triglyceride is a solid at 25℃, it is called a fat; if it is liquid, it is called an oil. Fats are typically derived from animals whereas oils are most often derived from plants. Pure fats and oils are colorless and odorless—it’s only the compounds that they are attached to that give them a colored appearance or smell.  Like essential oils, lipids are insoluble in water. So despite having some similar properties and sources, it is the structure of the molecule that truly distinguishes essential oils from fats and fatty oils.  

Other types of lipids include phospholipids, which also contain glycerol, and steroids, which have a very specific molecular ring structure. 

Why is this important? Because even though their name may suggest it, essential oils are not fats. In fact, they are quite different than fats and even, what we traditionally think of as oils. Distinguishing essential oils from fats also helps us understand that certain ailments that may affect our lungs—such as lipoid pneumonia—are not caused by essential oils. As the name suggests, lipoid pneumonia is caused when lipids, or fats, enter or accumulate in the lungs. Knowing that essential oils are not lipids gives us a bit more peace of mind that the aromatherapy we’ve evolved to breathe over the span of human life is, in fact, as healthy as it ever was.