The Life Cycle Of An Essential Oil
Only certain plants contain essential oils, and they are known as aromatic plants. Many fruits and vegetable also contain essential oils, though most are not extracted. The oil is formed in very small structures in the leaves, flowers, or other parts of the plant, known as secretory cells. Sometimes these protrude from the surface of the plant. The process of essential oil creation is complex, and involves many enzymes within the plant, because each one of a hundred or more chemicals needs to be "manufactured".
Why they are so complex is not clear, but many of the single chemicals they contain are used as odorous communication signals, either between insects (which can also produce the same chemicals) or between plants and insects. Seen in the context of evolution, essential oils are a means of communication – they carry meaning.
Both the quantity and the composition (mix of ingredients) of an essential oil are constantly changing within the plant – with time of day, time of year, maturity of plant, weather, and many other factors. Consequently there are optimum seasons, and times of day, to harvest a plant for its essential oil.
Harvesting is either done either by hand, or mechanically, with a mobile device similar to a small grain harvester. Some plants, such as rose flowers, can only be picked by hand, but low-growing herbs such as lavender are usually cut mechanically. The oil extraction process is often carried out on the same day as harvesting, to minimize essential oil loss through evaporation, though this is not important with hard materials such as seeds, roots and woods.
Extraction may be done by cold pressing (citrus fruit oils), steam or water distillation, or by use of a solvent. There is currently much interest in CO² extraction, a process that captures more of the plant's fragrant molecules than distillation. Other solvent-extracted products, such as concretes, absolutes, resinoids and attars can also be made. Hydrosols, the waters from distillation, are also becoming increasingly popular as natural ingredients in their own right. Both hydrosols and essential oils have been used for their fragrance and medicinal properties since the invention of distillation 1,000 years ago.
Following extraction, an essential oil is stored and transported in metal drums, and is often exported. It may then be blended with other oils and fragrant materials to make a flavour or fragrance, compounded into a medicine, or sold in its own right for aromatherapy.
Essential oils very slowly degrade, as do all plant-derived materials, and as this progresses they lose their fresh fragrance, and also their therapeutic potency. For this reason, they should always be used when reasonably fresh – certainly within one year of opening the bottle. Essential oils will keep fresh for longer if kept cold – either a refrigerator or freezer is fine for storing oils.
Once applied to the body, whether by inhalation, application to the skin or other means, an essential oil enters the bloodstream. The essential oil is metabolised – chemically changed – quite rapidly, but during this period a multitude of therapeutic effects are possible. Some of these are due to constituents before they are metabolized, and some are due to metabolites – new chemicals formed in the body. The essential oil constituents often work synergistically, though they don't always do so. They act in a variety of ways, such as interacting with receptor sites on the surface of cells. Menthol, in peppermint oil, for example, lowers blood pressure marginally by altering the way calcium is exchanged at the surface of blood vessel cells.
Another way in which essential oils operate is by interacting with the cells of the 'olfactory epithelium' – a membrane inside and at the very top of the nose – after inhalation. The interaction of fragrant molecules and olfactory membrane initiates a nerve signal that may quickly trigger various parts of the brain, invoking a memory, a feeling, or simply the perception of a fragrance.
As metabolism progresses, it makes less and less sense to talk about the essential oil as a single substance, since each of its constituents is metabolised and excreted separately. With the help of human enzymes, the constituents are chemically changed to make sure they do not cause any toxicity, and also to make them more water-soluble. The bulk of the constituents are excreted in the urine. They then enter the waste water system, where they readily biodegrade.