Essential Oils - The Whole Story

Essential oils are increasingly popular, but awareness of their benefits in aromatherapy, use in diffusers and inclusion in natural products should be balanced with some knowledge of their potential toxicity if misused.

These volatile, fragrant plant fractions serve multiple useful purposes for the plants that produce them. They play important roles in attracting pollinators and defending the plant against microorganisms, insects and herbivores. Essential oils may occur in different parts of a plant, from the flowers and leaves to the fruits, seeds, barks, wood and roots.

Essential oil content varies considerably among different species, making up from 0.05% to 7% of the plant. Even within a species, EO content can be effected by weather, growing conditions, developmental cycle of the plant and harvesting techniques. For example, lavender is best if hand harvested, as machine harvesting is more traumatizing to the tender blossoms, and full bloom vs. bud stage can produce widely different yields.

Essential oils are highly complex chemical mixtures that can consist of over 100 specific compounds. Each compound has a distinct chemical structure, but the same compounds are sometimes derived from different plants. Usually one or two major compounds shape the pharmacology of the essential oil, such as menthol, which comprises 40% of peppermint oil.

A plant species can produce multiple chemotypes, with different major components, usually tied to growing conditions in the region where it was grown. For example, rosemary grown in the Mediterranean region has different chemotypes than the same plant grown in northern or central Europe. These chemotypes have specific effects and actions, and one that is good for one purpose may not be good for another. If you want to use an essential oil topically or for aromatherapy, it’s best to research and find out the best chemotype for that particular use (e.g., thymus vulgaris ct. linalool vs. one of the many other chemotypes of thyme).

Essential Oil Extraction

There are two main methods of extracting essential oils: distillation and expression. Expression, or cold pressing, is used specifically for citrus. Previously done laboriously by hand, essential oils are now separated from citrus peels mechanically, using continual puncturing and centrifugal force.

Distillation involves water, steam or a combination of the two. Water distillation is used most often with flowers (including rose and orange blossoms), as this method results in a more fragrant product. Using steam with delicate blossoms tends to clump them together, and water can’t get to all the petals to release all of the fragrance. In water and steam distillation, herb and leaf material is placed above the water, and in steam distillation, the most common method, steam is injected into a still at slightly higher pressure and temperatures. The waters left over from distillation, called floral waters or hydrosols, are often used in cosmetics.

Sometimes a solvent is used for extraction. This method is not very popular, but it is the best option for some very delicate flowers, such as jasmine, which would be damaged by high heat in other extraction methods. Solvent extraction of jasmine blossoms produces what is known as Jasmine Absolute (your cue that the oil was solvent extracted).

Degradation of Essential Oils

The three major factors responsible for the degradation of essential oils are: oxygen, heat and light. Oxygen leads to oxidation, and heat and light speed up the process. Chemical changes in degraded essential oils can increase hazardous compounds.

It’s best to use an essential oil within one year of opening (within six months for some very volatile oils) and to store it in a dark glass container