The vast majority of true essential oils are produced by distillation. There are different processes used, however, in all of them, water is heated to produce steam, which carries the most volatile chemicals of the aromatic material with it. The steam is then chilled (in a condenser) and the resulting distillate is collected. The essential oil will normally float on top of the hydrosol (the distilled water component) and may then be separated off. To read more about hydrosols, please click here.
Steam Distillation: True steam distillation uses an outside source of steam which pipes the steam into the distillation unit, sometimes at high pressure. The steam passes through the aromatic material, and exits into the condenser.
Hydrodistillation: The botanicals are fully submerged in water, producing a “soup,” the steam of which contains the aromatic plant molecules. This is the most ancient method of distillation and the most versatile. It's the method most often used in primitive countries. The risk, of course, is that the still can run dry, or be overheated, burning the aromatics and resulting in an essential oil with a burnt smell. Hydrodistillation seems to work best for powders (ie, spice powders, ground wood, etc.) and very tough materials like roots, wood, or nuts.
Water and Steam Distillation: A water and steam distillation arrangement can be compared to a kitchen steamer basket, with the botanicals supported in a “basket” over boiling water, thus exposing the plant material only to the rising steam vapors. This is the best method for distilling leafy materials, but doesn't work well for woods, roots, seeds, etc.
Absolutes and Concretes: Solvent Extraction
Very delicate aromatics, Jasmine, Linden Blossom, etc. can not survive the process of distillation. To capture their magical aromas, a process of solvent extraction is used. An extracting unit is loaded with perforated trays of blossoms. The blossoms are washed repeatedly with a solvent (usually hexane). The solvent dissolves all extractable matter from the plant whch includes non-aromatic waxes, pigments, and highly volatile aromatic molecules. The solution containing both solvent and dissolvable plant material is filtered and the filterate subjected to low pressure distillation to recover the solvent for further use. The remaining waxy mass is what is called the concrete and it contains in the case of J. grandiflorum as much as 55% of the volatile oil.
The concentrated concretes are processed further to remove the waxy materials which dilute the pure essential oil. To separate the absolute from the concrete, the waxy concrete is warmed and stirred with alcohol (usually ethanol). During the heating and stirring process the concrete breaks up into minute globules. Since the aromatic molecules are more soluble in alcohol than in the wax an efficient separation of the two takes place. But along with the aromatic molecules a certain amount of wax also becomes dissolved and this can only be removed by agitating and freezing the solution at very low temperatures (around -30 degrees F). In this way most of the wax precipates out. As a final precaution the purified solution is cold filtered leaving only the wax-free material (the absolute).
This solvent extraction actually yields three usable products; first the concrete (as in rose concrete, my favorite solid perfume), the precious absolutes, and the floral waxes (for addition to candles, thickening creams and lotions as a softly floral scented alternative to beeswax).
Carbon Dioxide Extraction
When CO2 (carbon dioxide) is subjected to high pressure, the gas turns into liquid. This liquid CO2 can be used as a very inert, safe, “liquid solvent” which will extract the aromatic molecules in a process similar to that used to extract absolutes (above). The advantage, of course, is that no solvent residue remains, since at normal pressure and temperature, the CO2 simply reverts to a gas and evaporates. CO2 extraction has given us essences of some aromatics that don't yield essential oils, Rose Hip Seed, and Calendula, for examples. In my experience (or opinion!) if the same essential oil is available both as a steam distilled essential oil and a CO2 extracted essence, the CO2 seems to have a richer, more intense scent, since more of the aromatic chemicals are released through this process. For more details of this process and the different types of CO2 extracts please click here.
We are all familiar with the spray of orange essential oil that can be released by scoring or zesting the skin of the fruit. The cold pressed citrus oils are commercial produced just this way, by machines which score the rind and capture the resulting oil. Although many citrus oils are also produced by steam distillation, they seem to lack the vibrancy of the cold pressed oils.
This extraction method uses a new type of benign gaseous solvents. In the late 1980s Dr. Peter Wilde first recognized the unique properties of these solvents for the extraction of aromatic oils and biologically active components from plant materials, for use in the food, pharmaceutical, aromatherapy, and perfume industries. “Florasol” (R134a), is the solvent upon which the process is based. Extraction occurs at or below ambient temperatures, hence there is no thermal degradation of the products. The extraction process utilizes the selectivity of the solvent and produces a free-flowing clear oil free of waxes.