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How Do We Obtain Essential Oils and Aromatic Plant Constituents?
There are several different ways to obtain the volatile aromatics from plants. The method used will depend on the plant material and the most effective way of obtaining the essential oils or aromatics from that plant. There are many delicate flowers that will be destroyed by water or steam distillation; however, their aromatic molecules can be obtained by solvent extraction. There are also volatile oils that can only be obtained by expeller pressing, such as Citrus peel oils.
Extracting essential oils using water is carried out by the following methods:
Water or Steam Distillation
This is the most widely used and most economical method of extracting essential oils. The process requires a great deal of skill and knowledge as there are many variables involving time, temperature and type of plant being distilled. Also, some plants are distilled immediately upon harvest, some are partially dried and some may require to be completely dry before distillation. Basically, the plant material is placed above boiling water (water distillation) or in a separate chamber where steam passes through (steam distillation). The latter is sometimes called dry distillation if done under pressure. The advantage of steam distillation over water distillation is that raw material does not come in contact with boiling water. The heat and steam cause the structure of the plant cells to break down, releasing the essential oil. The steam and essential oil are then channeled through a cooling coil, where they condense and return to a liquid state, comprised of both water and essential oil. Most essential oils will float to the top; heavier oils such as clove will sink to the bottom. The two are then separated, using a separatory funnel or by siphoning. The water remaining after this process is called a hydrosol and contains minute amounts of the essential oil and other water-soluble plant constituents.
In this method, plant material is charged in the still, water is added to immerse the charge, leaving sufficient vapor space. The quantity of water should be adequate for the plant material to move freely in boiling water and avoiding localized overheating. The water is heated under direct fire or steam jacket, or in some cases a closed steam coil. It may be necessary to add water during the distillation process to prevent dry material from being exposed to direct heating. The steam vapor produced is condensed and oil is separated from the water as in steam distillation. Water distillation is used when the plant material may form large compact lumps through which steam cannot penetrate. Water distillation is a slower process than steam, requiring more stills, more space, more fuel and is the least economical of the methods using water. Certain components (esters and aldehydes) can deteriorate under prolonged contact with boiling water, and some water-soluble components cannot be completely recovered.
Expression or Cold Pressing
Citrus contains small essential oil sacs, located just beneath the surface of the peel. Originally, pressing by hand was required, obviously a labor-intensive process. Now, the majority of Citrus peel oil is expeller pressed using efficient mechanical presses.
Solvent Extraction is a method that does not yield a 'true' essential oil, which can only be obtained by steam distillation. This method is employed to extract aromatics from delicate flowers, gums and resins that are adversely affected by hot water and steam and, therefore, cannot be distilled. There are several different stages for solvent extraction to produce an absolute.
To obtain an absolute, the first step is to prepare a concrete. Preparing a floral concrete requires thorough knowledge of the particular plant material being processed. Experts have developed their own methods based on an individual understanding of the specific botanical, which is as important as any technical consideration. They also will have worked extensively with the particular hydrocarbon solvent used, which has its own qualities. Hexane is probably the most used solvent and, in the case of delicate flowers, should be purified to a point so as to only dissolve pigments, waxes and volatile oil, leaving the water content behind. Benzene is an alternative solvent used. There is a lot of water in fresh flower material, so this is a delicate process, and the water content in fresh flower material could dilute the solvent to the point of being ineffective. At this stage of extraction, alcohol cannot be used as the solvent must be volatile enough to come out of solution. The process of making concretes involves layering the flowers, insuring that they are perfect and not bruised, thinly so that the hexane or solvent can easily touch every surface. The flowers are 'washed' over and over with the hexane to draw the aromatics into solution.
Once a concrete is prepared, it is kept in a cool place where, with its high concentration of waxes, will keep naturally preserved up to a number of years.The professional extractor will then proceed to make the conversion from the concrete to an absolute. This process involves stirring the warmed (about 115-125 degrees F) waxy concrete with alcohol into which the aromatic molecules will transfer. During this process, the concrete will break up into very tiny globules, increasing the surface area of the original mass. The aromatic molecules are more soluble in alcohol than the wax. Some wax may dissolve, which is removed by freezing the solution at very low temperatures (30 degrees F) and agitating to precipitate out most of the wax. The absolute can be used as is, leaving a minute amount of wax that can act as a natural fixative. The attars from India are made at this point in the process by adding sandalwood as a fixative, which imparts a very different scent.
Supercritical Fluid and High Pressure Solvent Extraction
The simplest way to explain CO2 extraction or high-pressure solvent extraction is that you place plant material in a pressure vessel and pump liquefied gas or liquid solvent through at a specific pressure and temperature. The pressure will then force into the cell walls, separating the desired constituents rapidly. CO2 is now considered a superior solvent for botanicals; it can be widely manipulated with subtle changes in pressure and temperature and leaves no toxic residue behind. There are (1) low-pressure, cold extraction which involves chilling the CO2 to between 35-55 degrees and pumping through the plant material at between 800 and 1500 psi and (2) Supercritical Fluid extraction which involves heating the CO2 to above 87 degrees F and pumping above 1,100 psi. Low pressure CO2 is the preferred method for producing high-quality botanical extracts. Supercritical CO2 is seen as a promising green solvent because it is non-toxic and separation of reaction components from the starting plant material is simpler than with traditional organic solvents. Advantages of using CO2 extracts include:
- Closest organopleptic profile, flavor and fragrance to the raw botanicals
- No loss of top notes and back notes
- No chemical and solvent residue, thus pollution free
- High concentration of minor active ingredients means better efficacy
- Better shelf life due to co-extraction of antioxidants
With the re-emergence of natural perfumery, hobbyists and artisan perfumers are creating small amounts of aromatic perfume materials (especially those that cannot be obtained by steam distillation and may not be commercially available) by tincturing in high quality perfumers alcohol (ethanol). Note: this is NOT denatured alcohol. This process is somewhat similar to maceration (below), and fresh aromatic material is placed into 190-200 proof alcohol in a lidded glass vessel.The vessel is agitated daily and plant material is removed as it is 'spent', indicated by lack of color and depleted, limp appearance. Leaving the plant material in the alcohol too long will ruin the odor as the plant material decomposes. To increase the fragrance strength of the tincture, plant material is removed and replaced over and over again with fresh material, much like as is done in enfleurage (below). With some materials, it is useful to use the washing technique obtained with a Soxhlet. It is sometimes necessary to refrigerate or freeze the filtered alcohol from season to season to keep increasing the fragrance intensity with a succession of fresh flowers.This is the only way to obtain true natural fragrance of flowers such as Lilac and Buddleia. Natural perfumers are now experimenting with aromatic materials from goat hair to lichens and moss, and various wild mushrooms.
Another method of obtaining aromatic components and other plant constituents from plants is carried out by combining with animal fats or vegetal (fixed oil).
Maceration, or infused oils have been used since antiquity and was most likely the first process used to capture scent. Not only the aromatics are recovered in this process, but other plant constituents useful in herbalism. In this process, plant material is chopped or crushed to open up the cell walls, placed firmly (but not too tight) into a vessel, and covered with vegetable oil to infuse plant constituents into the fixed oil. The oil can be 'cold infused' and left at room temperature for several weeks (up to a month or so), or heated to a very low temperature (not to exceed 100-105 degrees F) for a shorter duration, sometimes in a crock pot, 5-7 days. Many different fixed oils are used, including fractionated coconut oil which will not go rancid and greatly extends the shelf life of your finished product. Olive oil is probably the best oil for maceration, is relatively stable and, if refrigerated, has a reasonable shelf life. After the maceration period, remove all plant material. You may want to use a tincture press when infusing with dried plant material. Some plants are infused fresh, producing the most efficacious product, such as Calendula flowers, Chickweed, St. Johns Wort and Arnica. Fresh plants contain water, so care must be used to insure all water is removed from the finished product or it will mold and go rancid. One should 'rest' or partially dry fresh plant material for 24-48 hours prior to infusing. After the maceration is complete, remove fresh plant material without squeezing or pressing so as to not leave any more water behind than necessary. Leave the infused oil open for a day or two to evaporate off some water. It is best to siphon the oil off the top and leave a half inch or so of oil in the bottom, which will contain the water that settles. Do not agitate your vessel during evaporation period as you want all water to remain at the bottom. Dried plants can be infused in the same manner and the oil will not contain water.
Before the advent of hexane and benzene, all absolutes were made by the process of enfleurage, an ancient method similar to maceration. This labor-intensive, time consuming process used to be the primary method to extract volatile constituents from delicate flowers such as Jasmine and Frangipani. It involved daily picking of the blooms at exactly the right time when they were emitting fragrance at the prime opportunity to capture the volatile aromatics. The blooms were carefully lain upon a layer of odorless, purified cold fat (usually animal), spread over glass sheets mounted in large wooden frames. The fat would absorb the aromatic molecules from the flowers, which would be removed and fresh flowers continually added until the fat was saturated with heady scent. The finished product could then be washed with alcohol to produce an absolute. This process is little practiced today, however, the results obtain an exquisite, if costly aromatic ingredient.