An essential oil is simply the pure aromatic liquid that remains after the distillation or pressing of plant material. The word “essential” is thought to come from the ancient belief that the “essence” of the plant, or all that is essential about it, is preserved in this precious liquid. The term “oil” refers to the fact that it is hydrophobic, or easily separates from water. The volatility, or the property of readily dissipating into the air at room temperature, is what makes essential oils “aromatic”.
Modern biology and chemistry can help explain how this works. As plants evolve in nature, they face challenges such as drought, disease, predation and the need to attract pollinators. In order to respond to these challenges, plants continually alter their chemical composition in order to survive and thrive.
Every essential oil is made up of 100-300 chemical constituents, each having its own unique odor and other characteristics. These compounds are composed of various combinations of carbon, hydrogen and oxygen, which give rise to the terpenes, alcohols, ketones, and phenols, etc., which make them so biologically active.
These chemical compounds are stored in specialized cells, either internally or externally. The external structures, or glandular trichomes, are highly fragrant, readily releasing tiny amounts of biological material when touched. Lavender and Rosemary are good examples of this type of plant, where the fragrance is easily released into the air simply by brushing the plant lightly.
Alternatively, the internal secretory cavities can emit aromatic molecules when the skin, leaves or seeds are broken open, as in citrus or eucalyptus plants. In either case, in nature, these highly fragrant compounds are discharged to warn away or fight predators (such as mold, or herbivores) deter encroachment of other plants, or attract pollinators such as bees and butterflies (8).
It’s important to understand how essential oils work in nature. The plant’s chemical composition has been finely tuned over thousands of years in order to work in synergy with its environment. Even though it can be tempting to believe an EO’s primary constituent is responsible for an individual therapeutic response, in truth, it’s the combination and balance of all the molecules working together harmoniously that achieves the results. Research supports this theory, which forms the core of why using whole essential oils, properly sourced and produced, is so important.
Removing the Essential Oil from Plants
The process of removing essential oil from a plant most often involves some type of distillation. Because of the aromatic nature of essential oils, it is important to obtain them at temperatures low enough to preserve their benefits. Distillation does this by using just enough heat to allow the essential oils to vaporize, without destroying their inherent characteristics and beneficial properties.
To accomplish this, plant material is placed on a grid inside of a distillation device or still, which is then sealed. Steam and/or heated water is carefully released onto the plants, causing the breakdown of the cell walls. The released volatile molecules then rise as vapor and are captured through a pipe, which leads to a condenser. As the vapor rises and then cools, the vapor returns to liquid form as essential oil, and water containing trace amounts of EOs (called “floral water” or “hydrosol”). Since most essential oils are lighter than water, they rise to the top of the liquid collection chamber where they are easily siphoned off.
Another way to obtain EOs, specific to citrus plants (such as lemon, lime and grapefruit), is called expressing or cold pressing. This is a method where pressure or centrifugal force is exerted on the skin of the fruit to rupture the secretory cavities, thereby releasing the essential oils from the skin of the fruit.