Essential oils can be extracted via two key methods: Distillation (includes hydrodistillation) and Expression. Absolutes, other the other hand, can be extracted via Solvent extraction or Enfleurage, although enfleurage is rarely performed in the modern day. Another type of aromatic product available on the market are CO2 extracts. They differ in chemistry from their related distilled essential oils but are becoming increasing available on the market.
The Practice of Distillation
Distillation appears to have been practiced throughout ancient times. Based upon the current interpretation Paolo Rovesti’s discovery of an earthenware distillation apparatus, the production or extraction of aromatic oils by means of steam distillation, has been known for 5000 years.During the fifth century AD, the famed writer, Zosimus of Panopolis, refers to the distilling of a divine water and panacea. Throughout 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.
Although an extensive trade of odoriferous material has been shown to have occurred in the ancient Orient and ancient Greece and Rome, the oils used were not essential oils per se, “rather they were obtained by placing flowers, roots, and other plant material into a fatty oil of best quality, submitting the glass bottles containing these mixtures to the warming influence of the sun and finally separating odoriferous oil from the solid constituents”.
In 900 AD, Avicenna, the famous child prodigy from Persia who wrotemany documents on plants and their uses and also instructions for massage, was accredited with refining the process of distillation by improving the cooling system.
Today distillation is still the most common process of extracting essential oils from plants. The advantage of distillation is that the volatile components can be distilled at temperatures lower than the boiling points of their individual constituents and are easily separated from the condensed water.
The Distillation Process
During distillation the plant material is placed upon a grid inside the still. Once inside, the still is sealed, and, depending upon the above methods, steam or water/steam slowly breaks through the plant material to remove its volatile constituents. These volatile constituents 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 vehicle 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 with clove essential oil.
- Water Distillation
The plant material comes into direct contact with the water. This method is most often employed with flowers (rose and orange blossoms), as direct steam causes these flowers to clump together making it difficult for steam to pass through.
- Water and Steam
This method can be employed with herb and leaf material. During this process, the water remains below the plant material, which has been placed on a grate while the steam is introduced from outside the main still (indirect steam).
- Steam Distillation
This method is the most commonly used. During this process, steam is injected into the still, usually at slightly higher pressures and temperatures than the above two methods.
- Percolation or Hydro-diffusion
This is a relatively recent method and is very similar to steam distillation except that the steam comes in through the top rather than the bottom, and there is a shorter distillation time. It is useful in extracting essential oils from woody or tough material or seeds such as fennel and dill.
Note on Boiling Point: The boiling point represents the temperature at which a liquid is converted to a gas at a specified pressure. The fundamental nature of steam distillation is that it enables a compound or mixture of compounds to be distilled (and subsequently recovered) at a temperature substantially below that of the boiling point(s) of the individual constituent(s). Essential oils contain substances with boiling points up to 200°C or higher, including some that are solids at normal temperatures. In the presence of steam or boiling water, however, these substances are volatilized at a temperature close to 100°C at atmospheric pressure.
Hydrosols: A By-product of Distillation
Hydrosols, also known as hydrolats, are the by-product or product (depending on the distiller purpose) of the distillation process. Hydrosols contain the water-soluble constituents of the aromatic plant and retain a small amount of essential oil. Every liter of hydrosol contains between 0.05 and 0.2 milliliter of dissolved essential oil, depending on the water solubility of the plant’s components and the distillation parameters.
*Please Note: The addition of essential oils to water is not at all the same as true hydrosols, and it is recommended that you read the ingredients label on products to ascertain whether or not you are getting a true hydrosol. When water and essential oils are mixed together with or without a dispersant, this is called a “spritzer” or “aromatic spritzer,” and this product should not be confused with a true hydrosol.
Expression, also referred to as cold pressing, is a method of extraction specific to citrus essential oils, such as tangerine, lemon, bergamot, sweet orange, and lime. In older times, expression was done in the form of sponge pressing, which was literally accomplished by hand. The zest or rind of the citrus would first be soaked in warm water to make the rind more receptive to the pressing process. A sponge would then be used to press the rind, thus breaking the essential oil cavities, and absorb the essential oil. Once the sponge was filled with the extraction, it would then be pressed over a collecting container, and there it would stand to allow for the separation of the essential oil and water/juice. The essential oil would finally be siphoned off.
A more modern method of extraction, and less labor-intensive, has been termed the Écuelle à piquer process that involves a prodding, pricking, sticking action to release the essential oil. During this process, the rind of the fruit is placed in a container having spikes that will puncture the peel while the device is rotated. The puncturing of the rind will release the essential oil that is then collected in a small area below the container. The end process is the same as above. The majority of modern expression techniques are accomplished by using machines using centrifugal force. The spinning in a centrifuge separates the majority of essential oil from the fruit juice.
What is the difference between expressed and distilled citrus oils?
Expressed citrus oils are produced by mechanical separation (cold pressing) of the oil from the peels of various citrus fruits such as orange, grapefruit, tangerine, lemon, lime, petitgrain and bergamot. Expressed citrus oils contain small amounts of naturally occurring nonvolatile residues such as waxes. Expressed citrus oils offer the advantage of cold process which results in an aroma which is identical to fresh citrus peels.
Citrus oils can also be distilled from either the peels or whole fruits. While the aroma differs from expressed citrus oils, the distilled versions offer certain advantages:
1) Distilled citrus oils do not contain nonvolatile residues which can clog diffusers, stain fabric, and shorten the shelf life.
2) Distilled citrus oils don’t contain nonvolatile furocoumarins and are generally less photosensitizing.
Extraction Techniques for Absolutes & CO2 Extracts
Flowers were being processed via enfleurage in the Grasse region of Southern France long before the modern method of solvent extraction. In the early days of perfumery, many flower scents were extracted via enfleurage, now considered an ancient art that is passed down from father to son or from generation to generation.
Enfleurage is a cold-fat extraction process that is based upon the principles that fat possesses a high power of absorption, particularly animal fat. The fat used must be relatively stable against rancidity. It is a method used for flowers that continue developing and giving off their aroma even after harvesting (e.g., jasmine and tuberose).
Today, Grasse continues to be one of the few areas in the world that continues to employ enfleurage as a method of extraction, although it is rare in the aromatherapy market due to the expense. If one finds a jasmine enfleurage on the market, this would typically be considered an absolute.
Some plant material is too fragile to be distilled and an alternative method must be employed. Solvent extraction is the use of solvents, such as petroleum ether, methanol, ethanol, or hexane, to extract the odoriferous lipophilic 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. The first product made via solvent extraction is known as a concrete. A concrete is the concentrated extract that contains the waxes and/or fats as well as the odoriferous material from the plant. The concrete is then mixed with alcohol, which serves to extract the aromatic principle of the material. The final product is known as an absolute
Solvent extraction is used for jasmine, tuberose, carnation, gardenia, jonquil, violet leaf, narcissus, mimosa, and other delicate flowers. Neroli and rose can be distilled or solvent-extracted. The name neroli typically implies the essential oil, whereas the name orange blossom is commonly used for the absolute or hydrosol of neroli. The name rose is used to describe either the essential oil or the absolute. Companies selling essential oils should clarify whether the product you are purchasing is an essential oil or absolute. This information should be on the label and in the product catalog.
After the solvent extraction process has been completed, the resulting absolute will have an extremely low concentration of solvent residue, approximately 5 to 10ppm (parts per million). The current European Union standards are for less than 10 parts per million solvent residues in a finished absolute.7 However, even with such a potentially small residue (less than .0001%), many aromatherapists disagree with the use of absolutes for individuals with a compromised immune system due to the potential effect of the residual pesticide.
However, absolutes do have therapeutic value and are often used for psychological purposes and for animals, particularly horses. Many therapists incorporate absolutes, such as rose absolute, jasmine, and tuberose, as a valuable part of their therapeutic applications of aromatherapy. Ultimately the decision to use absolutes is up to the practitioner and his/her own personal preferences.
Absolutes are highly concentrated aromatic substances and are obtained from delicate flowers by either enfleurage or solvent extraction. Absolutes 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 cosmetic and perfume industries due to their strong aromas. There are also different grades of absolutes. The top grade is the uncut, which can be a thick or semisolid substance, making them difficult to work with. Less expensive grades are diluted with alcohol to make them more user friendly, although often the strength of aroma is slightly diminished.
CO2 Hypercritical Extraction
Hypercritical carbon dioxide (CO2) extraction is a relatively new process used for the extraction of aromatic products. The basic concept is that 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 they can contain some elements 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. Also, the CO2 extract of frankincense (Boswellia carterii) has immune enhancing and anti-inflammatory activity not found in the essential oil. CO2 extracts are known for their strong similarity in aroma to the actual plant aroma. Other common CO2 extracts on the market include German chamomile (Matricaria recutita) and Calendula (Calendula officinalis).
The three main disadvantages for this process are cost, potential pesticide residue, and the lack of information regarding their safety and therapeutic benefits.7 With regard to pesticide residue, Guba comments that “carbon dioxide extraction has been demonstrated to concentrate from 7 to 53 times more pesticide residues in the final extract.” Therefore, it seems pertinent to only use organic plant material for CO2 extraction. Perhaps as more CO2 extracts become available and more practitioners use them, further details regarding their applications will become apparent. Two of the most common essential oils available via CO2 extraction include frankincense and ginger.