The term “essential oil” is a contraction of the original “quintessential oil.” This stems from the Aristotelian idea that matter is composed of four elements, namely, fire, air, earth, and water. The fifth element, or quintessence, was then considered to be spirit or life force. Distillation and evaporation were thought to be processes of removing the spirit from the plant and this is also reflected in our language since the term “spirits” is used to describe distilled alcoholic beverages such as brandy, whiskey, and eau de vie. The last of these again shows reference to the concept of removing the life force from the plant. Nowadays, of course, we know that, far from being spirit, essential oils are physical in nature and composed of complex mixtures of chemicals.1
The International Organization for Standardization (ISO) in their Vocabulary of Natural Materials (ISO/D1S9235.2) defines an essential oil as a product made by distillation with either water or steam or by mechanical processing of citrus rinds or by dry distillation of natural materials. Following the distillation, the essential oil is physically separated from the water phase.
According to Dr. Brian Lawrence “for an essential oil to be a true essential oil, it must be isolated by physical means only. The physical methods used are distillation (steam, steam/water and water) or expression (also known as cold pressing, a unique feature for citrus peel oils). There is one other method of oil isolation specific to a very limited number of essential oil plants. This is a maceration/distillation. In the process, the plant material is macerated in warm water to release the enzyme-bound essential oil. Examples of oils produced by maceration are onion, garlic, wintergreen, bitter almond, etc.”.2
For now, lets explore the biological role of essential oils within aromatic and medicinal plants.