Essential oils are highly concentrated aromatic extracts that are distilled or expressed from a variety of aromatic plant material, including flowers, flowering tops, fruits/zests, grasses, leaves, needles and twigs, resins, roots, seeds, and woods.
Through the process of converting energy from the sun (photosynthesis) plants produce not only the food they need to survive but a host of secondary plant chemicals to support their health and survival. Some of these secondary chemicals are what make up essential oils.
Technically, while in the plant, essential oils are actually a collection of many volatile chemicals (components) stored within the plant. These volatile components are responsible for the aroma and part of the taste of many medicinal plants. While in the plant, they are constantly changing their composition, helping the plant to adapt to an ever-changing internal and external environment.
Why do plants produce essential oils?
Plants produce aromatic components to:
- Attract insects: A variety of insects, from bees to butterflies, are attracted to flowering plants due to its aroma, its color, or its physical appearance. Scent, however, is considered to be one of the most ancient sense used.
- Repel competition: Aromatic plants are capable of sending out volatile chemical messages to other plants to prevent them from growing and competing for limited resources within its area or zone.
- Protect against predators: Plant are able to release a complex mixture of volatile oils, or terpenes to deter insects and other animals from approaching them.
- Defend and protect: Plants can produce an array of antimicrobial, antifungal, and antibacterial agents to protect themselves from a wide range of organisms that may threaten their survival.
Where do plants store essential oils?
Plants store volatile components either in external secretory structures found on the surface of the plant, or internal secretory structures, found inside a part or parts of the plant.
For plants that have external secretory structures, all you have to do is rub your hand or fingers on the leaf and its aroma is imparted to your skin. But often you will need to break open the leaf or cut into the root or press the citrus peel, to release the ‘essential oil’. This represents internal secretory structures.
Examples of plants with external secretory structures include: Basil, lavender, sweet marjoram, melissa, oregano, peppermint, rosemary, and spearmint
Examples of plants with internal secretory structures include: The citrus fruit, eucalyptus, frankincense, angelica, most other essential oils
Extracting essential oils
The two main processes used to extract essential oil are: distillation and expression. Other processes such as enfleurage and solvent extraction produce aromatic products called ‘absolutes’. And CO2 extraction produces CO2 extracts.
Distillation: An Alchemical Process
Distillation appears to have been practiced throughout ancient times. From the early Middle Ages and beyond, a crude form of distillation was known and was used primarily to prepare floral waters or distilled aromatic waters. These appear to have been used in perfumery, as digestive tonics, in cooking, and for trading.
During distillation, the plant material is placed upon a grid inside the still. Once inside, the still is sealed, and, steam or water/steam slowly breaks through the plant material to remove its volatile components. These volatile components rise upward through a connecting pipe that leads them into a condenser. The condenser cools the rising vapor back into liquid form. The liquid is then collected in a container below the condenser. Since water and essential oil do not mix, the essential oil will be found on the surface of the water where it is siphoned off. Occasionally an essential oil is heavier than water and is found on the bottom rather than the top, such as clove essential oil.
Expression, also called cold pressing or ecuelle a piquer, is a method of extraction specific to citrus essential oils, such as tangerine, lemon, bergamot, sweet orange, and lime. It was once done by hand (picture above)!
Expressed citrus oils are produced by mechanical separation (cold pressing) of the oil from the peels of the various citrus fruits. Expressed citrus oils offer the advantage of cold process which results in an aroma which is identical to fresh citrus peels.
What is the difference between expressed and distilled citrus oils?
Some aromatherapy companies sell both a distilled and an expressed citrus essential oil from the same species. The main differences between a distilled and an expressed citrus essential oil have to do with their toxicity, volatility, and aroma. Distilled citrus oils tend to deteriorate more quickly and are considerably more unstable than the expressed oils.
Not all aromatic plant material can be distilled or expressed as it is too fragile to hold up to the distillation process. Plants such as jasmine, tuberose, carnation, gardenia, jonquil, violet leaf, narcissus, mimosa, and other delicate flowers are extracted using solvent extraction.
Solvent extraction is the use of solvents, such as petroleum ether, methanol, ethanol, or hexane, to extract the aromatic lipophilic (fat soluble) material from the plant. The solvent will also pull out the chlorophyll and other plant tissue, resulting in a highly colored or thick/viscous extract. After some processing, the final product is known as an absolute.
Absolutes are highly concentrated aromatic substances and will most often resemble the natural aroma of the plant and are normally more colored and viscous than essential oils. Absolutes are used extensively in the botanical perfume and cosmetics industry.
CO2 Hypercritical Extraction
Hypercritical carbon dioxide (CO2) extraction produces CO2 extracts which are becoming increasingly more popular. How does co2 work? CO2 under pressure will turn from a gas into a liquid that can then be used as an inert liquid solvent. This liquid solvent is able to diffuse throughout the plant material thus extracting its aromatic constituents. CO2 extracts contain most of the same constituents as their essential oil counterparts, although some may contain some chemical components not found in essential oils. For instance, the essential oil of ginger (Zingiber officinale) does not contain the bitter principles, however the CO2 extract does. CO2 extracts are known for their strong similarity in aroma to the actual plant aroma.
Note: Due to the potential of pesticide residue in co2 extracts, we recommend organic co2 extracts only.