Extraction Methods for Plant Oils
Plant oils are obtained from their plant hosts through a variety of processes commonly called Extraction Methods.
Figure 1. Standard Extraction Processes Used.
Extraction Methods are a major characteristic of oil specifications because of the potential adverse effects if misemployed. Thus, an oil's specifications are incomplete without this information.
Key Control Parameters
Each extraction method must be appropriately controlled for the most optimum quality per unit cost achievable. These parameters include:
The fragile aromatic molecules of an essential oil are easily destroyed or altered by high temperatures; and so, the Extraction Method must use low-temperatures. High temperatures seem to cause a harshness in the oil. Even the oil's pH and the electropositive and electronegative balance are greatly affected. As examples, extraction of lavender oil must be < 245 °F while cypress should be extracted at 245 °F. Vegetable oils are also adversely affected by exposure to excessive heat.
Likewise, high pressure during distillation can also damage the delicate volatile aromatic molecules resulting in a loss of critical elements or transforming them causing harshness and other undesirable results. For example, pressure during lavender extraction should not exceed 3 psi, and cypress at about 5 psi.
• Exposure Time
The length of time it takes to process botanical material through the chosen extraction method can be critical to the result. As examples, lavender oil requires about 1-1/2 hours for distillation. Cypress oil requires 24 hours of distillation to extract all of its active ingredients. However, if distillation is shortened by only two hours, the resulting cypress oil will be missing 18-to-20 of it's constituent chemical elements!
• Equipment and accessories
The equipment and accessories used for any extraction method should be chosen carefully and be fully compatible with the requirements of the resulting oil. Distillation pots, siphon tubing, filtration screens and all equipment that is to come into direct contact with the botanical material or oil (in any part of the extraction process), should not be made of chemically-reactive metals (copper and aluminum) nor any material that might contaminate the oil. Therapeutic-grade or other food-grade oils shall only be handled using food-grade stainless-steel cooking chambers and carrying vessels.
• Batch size
Some essential and plant oils require smaller batches of plant material in relatively small distillation units to facilitate extraction of higher-quality oils or when an oil requires more delicate, careful handling to prevent loss of critical aromatic and/or therapeutic elements. By contrast, commercial-grade essential and plant oils are produced more quickly in larger units, and at lower cost, but the resulting oil quality will be compromised. However, such oils are perfectly suitable for many uses such as mass-produced food and beverages, soap manufacturing, and similar uses.
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The following is a list of the extraction methods, in alphabetical order, currently in employ around the world.
The vast majority of true essential oils are produced by distillation. Distillation converts the volatile liquid (the essential oils) into a vapor and then condenses back into a liquid. It is the most popular and cost-effective Extraction Method in use today. However, due to use of heat in this Method, it may not be used on very fragile plant material, because major therapeutic characteristics would be adversely effected, or where the Method is employed with great difficulty.
Cohobation, sometimes referred to as redistillation or folding, is a process whereby previously distilled oil (see Figure 3) is added back to the remaining botanical material producing an increasingly more concentrated, "purified" form. "Purified" means that some of the oil's chemical constituents (anywhere from 200 to 800 in a given botanical material) are removed with each distillation. Thus, this process can be used to remove certain non-skin-safe chemicals from the oil. The resulting product is also more concentrated, requiring less for a given effect. Cohobated oils cannot be of a therapeutic grade.
Some essential oils require cohobation. For example, when Rose oil is extracted during Water Distillation, the one main constituent, Phenyl Ethyl Alcohol, cannot be extracted as this Method dissolves it into the water of the distillation still. The requisite missing constituent is captured with redistillation of this still water, the exact missing proportion of which is added back to the previously extracted oil. The resultant "complete" oil is called Rose Otto oil.
Hydro-diffusion differs from normal steam distillation (see Figure 3) only in that the steam is introduced from the top, rather than bottom, of the botanical material. Consequently, the steam condenses in a bottom chamber where the resultant oil benefits from less steam exposure for a shorter duration and, thus, a higher yield.
In Hydro-distillation, the botanical materials are immersed in water and brought to a boil. Refer to Figure 2 below.
Figure 2. Hydro-distillation Diagram.
This Method provides limited overheating protection since the surrounding water acts as a heatsink to reduce the maximum temperature. Great care is also taken in the processing to exactly control temperature and length of exposure to preclude damage to the oils' character.
Hydro-distillation is used when the plant material has been dried and will not be damaged by boiling. It is also used for powdered materials such as powdered almond, and flowers, such as orange and rose, that need to float freely as they tend to lump together when just steam is passed through them.
Botanical material containing high amounts of esters are precluded from this Method since extended exposure to heat will start to break down the esters into it's constituent alcohols and carboxylic acids.
Hydro-distillation can also be employed under vacuum which reduces the temperature to < 100 °. This is beneficial in protecting both botanical materials and oils; e.g., Neroli oil, which is sensitive to heat, can be successfully extracted using this method.
The water from this process is also used and marketed as “floral waters” (also called “hydrosol” or “sweet water”), items such as rosewater, lavender water and orange water.
Rectification is the process by which essential oil impurities are removed via redistillation, either by steam or vacuum.
⇒ Steam Distillation
Most common essential oils, including lavender, peppermint, and eucalyptus, are distilled. Distillation is used for fresh plant material, especially those that have a high boiling point such as seeds, roots and woods. Most oils are distilled in a single process. One exception is Ylang-ylang (Cananga odorata) which takes 22 hours to complete through fractional distillation.
In Steam Distillation, raw plant material (consisting of the flowers, leaves, wood, bark, roots, seeds, or peel) is placed in an alembic (distillation apparatus) over water which is heated and steam forced through the material.
Figure 3. Typical Steam Distillation Process
The steam releases and vaporizes the aromatic molecules from the botanical material, while being very carefully regulated to impart only sufficient heat to force expulsion of the aromatic molecules while not exceeding burn temperature. These vapors are then passed through a cooling coil where they condense back into liquid which is then collected and separated into it's constituent components (see Hydrosol).
The pressurized steam increases rates of condensed liquid separation while, simultaneously, raising it's boiling point and, thus, reducing the requisite processing temperature sufficiently to prevent damage to the resulting oil. Some thermolabile oils, like Lavender, are protected using this Extraction Method. Optimum oil characteristics are maintained while it's key compound (Linalyl Acetate) is not separated into it's constituent components of Linalool and Acetic Acid.
⇒ Water-and-Steam Distillation
Figure 4. Typical Water-and-Steam Distillation Process
Basically a combination of Water Distillation and Steam Distillation, this Method immerses the botanical material in the water of a still, which also has a heat source, plus steam is applied to the mixture. This is used for either fresh or dried plant material that would be damaged by boiling.
Enfleurage is an old method which was used in the production of perfumes and pomade extracts for perfumery.
Using a cold fat, cold wax or warm oil as a base to extract the volatile aromatic molecules, the flower petals of the plant (rose, jasmine, neroli, violet and others) are layered onto the base material between multiple stacks of glass sheets within a wooden frame (see Figure 5) which are heated to failitate infusion. The volatile aromatic compounds are then separated from the resulting saturated oil, paste or wax (as applicable) using a volatile solvent such as ethyl alcohol. In the final step the solvent is evaporated at low temperatures and reduced pressure leaving the pure, fairly thick oil known as an “absolute.”
Figure 5. Typical Enfleurage Set-up
Cold enfleurage has the advantage that even the most delicate components of the flower oils are preserved. However, the process is not very efficient — the resulting “absolute” is a maximum of one percent (1%) yield — and is very expensive. Flower oils prepared with this method do not contain terpene-hydrocarbons, which indicates that these compounds are not present as such in the flower, but form during extraction.
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These Methods utilize high mechanical pressure to squeeze oil from botanical material. Most nut and seed oils are extracted using one of these Methods. However, some manufacturers refine the resulting oil using chemicals or high heat and, thereby damage the oil's therapeutic characteristics.
Similar to Écuelle à Piquer, this expression method is mostly used in the processing of citrus essential oils. A machine strips off the fruit's outer peel which is washed into a centrifugal separator and spun at high-speed to release the oil contained therein. This process can be subject to oil-altering enzymatic action if the oil is allowed to remain in contact with other cell contents for an extended period.
⇒ Cold Pressing
Cold Pressing is an expeller pressing process in which the environment is heat-controlled to keep temperatures below 120 °F. Keeping temperatures low is especially important with delicate oils in preserving their inherent therapeutic characteristics. Most nut and seed oils, as well as many fixed oils are extracted using Cold Pressing.
Figure 6. Typical Cold Press Set-up
Citrus essential oils are also extracted using a variant of this Method. After the fruit has absorbed the water and become more elastic, it is inverted to help rupture the oil cells. A sponge is located adjacent to the rind and the material squeezed to release the volatile oil into the sponge. Once saturated the essential oil is collected from the sponge and decanted.
⇒ Écuelle à Piquer
This form of expression extraction is used mainly to obtain citrus essential oils, and is a little less labor-intensive than that of the sponge method.
Figure 7. Typical EaP">Écuelle à Piquer
As the name implies, the fruit is placed in a high-speed rotating bowl-shaped container which has inwardly-facing spikes with which the fruit's skin is punctured. As the fruit's skin cells rupture, they release the oil contained within which is siphoned off through a tube at the center of the container. Thereafter the oil is separated from the water-based parts of the initial siphoning and decanted.
⇒ Expeller Pressing
Expeller Pressing is a chemical-free mechanical process that extracts oil from seeds and nuts through crushing. The material is exposed to friction-generated temperatures which is unregulated. Thus, harder materials create higher temperatures. No external heat sources are applied during this process.
Figure 8. Typical Expeller Process
Expeller Pressed oil is usually more expensive because the process is lower-yielding and is usually able to extract only about 2/3 of the available oil.
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This method is named for this new type of benign gaseous solvents used for extraction of aromatics oils and biologically-active components from botanical materials.
Developed in the late 1980's by Dr. Peter Wilde, the Florasol R134a solvent, used at or below ambient temperatures and, thus, no thermal degradation of the resulting oils, employs the unique selectivity of the solvent to produce a free-flowing clear oil free of waxes.
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• Fractional Distillation
Fractional Distillation is a distillation process by which oil is extracted in discrete batches, each referred to as a "fraction," rather than continuously.
Figure 9. Typical Fractional Distillation Process
Each resulting batch has less and less of certain chemical constituents. Ylang Ylang is a good example where this process is employed.
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Similar to enfleurage, maceration involves extracting the volatile aromatic compounds using a vegetable oil. Sometimes referred to as “infusion,” plant material is bruised and soaked in the oil which is heated to accelerate infusion of the oil with the volatile aromatic compounds. The final, saturated oil is then filtered to remove any plant particulate matter.
The resulting infused oil, containing the therapeutic properties of both the vegetable oil and the botanicals that were infused into the oil, is used widely for massage in aromatherapy.
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RBD is an acronym meaning refined, bleached and deodorized and describes those oils that have been subjected to all three processes.
Some oils undergo a refinement process in order to remove impurities; improve the color or texture; or stabilize the shelf life of the oil. The oil is reacted with a weak base solution to saponify the free fatty acids into soap. The oil is then spun in a centrifuge and washed with water until the pure oil remains. The oil may also be degummed to remove the sticky phospholipids, color pigments and odor-lending portions.
Some lipids are bleached in order to improve the color and clarity of the oil. Bleaching is generally done by passing the oil through Fuller's earth and then filtering the oil.
Some lipids undergo a deodorization process in order to remove compounds that lend an unappealing or overpowering aroma to the oil. This is generally done by blowing high-temperature steam through the oil to vaporize the aromatic components. This process is made more effective by heating the oil to high temperatures and performing this process under a vacuum to help remove all of the volatile odorous substances. Due to the high temperatures used, deodorization is clearly the most damaging process of refinement.
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• Solvent Extraction
Solvent extraction is an oil-extraction process using organic solvents such as hexane, ethanol, petroleum ether, or methanol amongst others. This Method, which is performed at low temperatures, is particularly suitable for botanical materials that have very-low yields of essential oils, or where they are made up of mostly resinous components and, as such, delivers a far finer fragrance than from distillation. Solvent extraction is used for very delicate aromatics, like hyacinth, jasmine, linden blossoms, narcissus, tuberose and others that cannot survive the rigors of steam distillation.
This method is fast and inexpensive but it produces a non-pure oil and is, therefore, mostly used in mass-produced perfumery. Also, due to the faster process and lower cost, solvent extraction is also used to produce higher yields of vegetable oils (including coconut, corn, grapeseed, rice bran and soybean oils) and sometimes for seed oils (including seeds, nuts and kernals).
Figure 10. Typical Solvent Extraction Process
During the process, the solvent dissolves all extractable matter from the botanicals including non-aromatic waxes, pigments and highly volatile aromatic molecules. The resulting solution, containing both solvent and dissolved botanical -material, is filtered and the filtrate subjected to vacuum distillation to recover the solvent for further use, leaving a waxy mass with an extremely low solvent-residue level of about 10 parts per million (i.e., 0.001%). This remaining compound is what is called a "concrete."
The concrete is further processed to remove the waxy material — through warming and stirring with alcohol (usually ethanol), and subsequent agitation, freezing and cold-filtering — yielding a highly-concentrated oil called an "absolute." The extracted waxy material, known as "floral waxes" are used for candles and thickening cream & lotions as a softly floral-scented alternative to beeswax.
• Supercritical Carbon Dioxide Extraction
Supercritical carbon dioxide (“CO2”) extraction combines the best aspects of steam distillation and solvent extraction. When carbon dioxide is put under high-pressure of at least 72.9 atmospheres of pressure at a temperature as low as 31.1°C (the “critical point”), it becomes a supercritical fluid with the permeation properties of a gas but the solvation properties of a liquid.
Consequently, the process extracts a wider range of volatile aromatic elements; is unaltered by heat or the CO2 solvent which is inert and will not chemically interact with the aromatic compounds; is easily and completely removed by returning the CO2 to normal atmospheric pressure where it return to it's natural gaseous form; and, thus, yields a richer, more intensely-scented oil with greater therapeutic qualities than those produced by any other extraction method to -date. However, the equipment required is expensive, and, thus, reflected in the price of essential oils obtained by this process.
Two types of oils are obtained using this method of extraction:
⇒ CO2 Selects
Selects are oils which are extracted at lower pressures and contain only the volatile, CO2 soluble components. The heavier waxes, resins, and color compounds are left behind. The viscosity of a CO2 Select oil is generally thick but pourable. These oils closely resemble steam-distilled oils by composition but they may contain more volatile components that are not extracted during steam distillation.
⇒ CO2 Totals
Totals are oils that are extracted at higher pressures and contain both the volatile and non-volatile CO2-soluble components of a plant. These oils contain the essential oil of the plant plus the heavier waxes, resins, and color compounds making them very similar to the raw plant itself. CO2 Totals are generally thick and pasty but are soluble in essential oils and vegetable oils (although some may need gentle warming).
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