The term “Essential Oil” comes from the original “quintessential oil.” This is an 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.
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).
While essential oils are in the plant, they are constantly changing their chemical composition, helping the plant to adapt to the ever-changing internal and external environment. Recent scientific research has shown that these are some of the ways plants produce essential oils for a variety of purposes including:
To attract pollinators and dispersal agents
Insects have been pollinating flowers for over 200 million years. Insects, like humans, are attracted to specific plants for one of three possible reasons: its aroma, its color, or its morphology or physical structure. Scent appears to be more ancient than flower color as an attractant to insects.3 Various insects, including bees, butterflies, and even beetles, are known to be attracted by the aroma of a plant.4
A type of plant-to-plant competition – to play a role in allelopathy,
Allelopathy occurs when a plant releases chemicals to prevent competing vegetation from growing within its area or zone. An often-cited example is in southern California, home to the dominant shrubs Salvia leucophylla (sage bush) and Artemisia californica (a type of sage). Both species release allelopathic terpenoids, eucalyptol and camphor, into the surrounding area, which effectively prevents other plant species from growing around them. This is allelopathy. Chemicals that deter competing growth (terpenes, for example) are referred to as allelochemics.
To serve as defense compounds against insects and other animals
Plants, like other living things, need to protect themselves from various types of predators. Plants use terpenoid compounds to deter insects and other animals from approaching them. Shawe pointed out that “insects are very rarely found on peppermint plants and the presence of linalol in the peel of citrus fruits confers resistance to attack by the Caribbean fruit fly.”5 The Douglas fir tree releases a complex mixture of volatile oils, or terpenes, from their needles to defend against the spruce budworm. Even more fascinating is that the Douglas fir trees “will vary the composition and production of terpenes each year thus decreasing the ability of the budworm to develop widespread immunity to specific compounds.6
To protect the plant by their antifungal and antibacterial nature
Resins and complex combinations of terpenes are released by some plants and trees, such as evergreens, to act as antimicrobial, antifungal, and antibacterial agents against a wide range of organisms that may threaten the survival of the plants. Compounds such as sesquiterpene lactones found in plants such as feverfew, yarrow, and blessed thistle, have been found to play a strong antimicrobial role as well as a protective role from herbivores.
All of these actions produce a secondary metabolite action that can be quite complex, and when one oil is added to another, then added to the biochemistry of a person, there is much to be considered.