Essential oils are called “essential” because they are the “essence” of a plant.
In the Middle Ages, it was believed that essential oils were essential for life, giving them the moniker that has stuck even to the present day. Modern references define an essential oil as the essence or extract, that is the source of a plant’s aroma and flavor. For example, peppermint plants smell like peppermint because of the essential oil contained in their leaves and stems. Oranges smell like oranges because of the essential oil contained in the peel.
Why do plants produce essential oils?
Essential oils are a critical part of a plant’s immune system. Plants produce essential oils to protect themselves against environmental threats. The parts of a plant with the greatest amount of essential oil are usually the parts with the greatest risk for invasion by microorganisms: the bark, sap, leaves, seeds, and fruit rinds. The compounds found in essential oils have all sorts of biological activities. They are known to protect against environmental threats, soothe the body, and even calm the mind.
Essential oils are made of volatile aromatic compounds.
Volatile aromatic compounds are small organic molecules that tend to change from the liquid state to the gas state at room temperature. These molecules are so incredibly small that a single drop of essential oil contains around 40,000,000,000,000,000,000 (40 million trillion) of them. The word “volatile” emphasizes their tendency to evaporate quickly at room temperature. This property is what makes them smell so potent. When you first open a bottle of essential oil, you instantly notice the aroma, and you can smell it even from a distance. The physical and chemical properties of volatile aromatic compounds allow them to quickly enter the gas state, move through the air, and directly interact with olfactory sensors in the nose.
Essential oils can be made up of anywhere between 1 and 1000 different compounds with different chemical identities. For example, Birch oil is almost entirely composed of one compound: methyl salicylate. Spikenard, on the other hand, contains hundreds of compounds. Most oils fall somewhere in between these two extremes. For instance, Frankincense essential oil contains over 65 distinct chemical compounds in various quantities. The different compounds in essential oil are known as constituents. Each constituent has its distinct structure, meaning that the shape, size, and arrangement of chemical bonds in that molecule are unique. The different constituents in an essential oil determine both the oil’s aroma and the benefits it offers.
The exact composition of an essential oil varies between plant species.
When speaking of essential oils, the word “composition” refers to the oil’s constituent makeup or, in other words, what chemical constituents it contains and how much of those constituents are present. For instance, Bergamot essential contains over 35 different compounds, but it has especially high levels of two constituents called limonene and linalyl acetate. Blue Tansy essential oil, on the other hand, contains over 50 compounds, with the two most abundant constituents being chamazulene and sabinene.