With the new labels of CO2 and SCO2 next to the traditional steam and hydro-distillations, solvent-extracted absolutes, and cold-pressed, a little education for the aromatherapy enthusiast is in order. Does one produce a nicer smelling oil, or one with more therapeutic value? Is one process better than another? Essential oil production, similar to the making of fine wines, is an art form as well as a science. The value of each process depends greatly on the experience and care of the distiller. Each method is important, and has it’s place in the making of aromatherapy-grade essential oils.

Steam and Hydro-Distillation

Steam distillation, the most common method of essential oil production, involves the flow of steam into a chamber holding the raw plant material. The steam causes small sacs containing essential oil to burst. The oil is then carried by the steam out of the chamber and into a chilled condenser, where the steam once again becomes water. (Hydro-distillation is a similar process where the plant material is boiled, with the resultant steam being captured and condensed). The oil and water are then separated; the water, referred to as a hydrosol, can be retained as it will have some of the plant essence. Rose hydrosol, for example, is commonly used for it’s mild antiseptic and soothing properties, as well as it’s pleasing floral aroma.

A number of factors determine the final quality of a steam distilled essential oil. Aside from the plant material itself, most important are time, temperature and pressure, and the quality of the distillation equipment. Essential oils are very complex products; each is made up of many, sometimes hundreds, of distinct molecules which come together to form the oil’s aroma and therapeutic properties. Some of these molecules are fairly delicate structures which can be altered or destroyed by adverse environmental conditions. So, much like a fine meal is more flavorful when made with patience, most oils benefit from a long, slow ‘cooking’ process.

The temperature of the extraction chamber should be maintained within a reasonable range, lest some components of the oil be altered or destroyed. This also applies to the chamber pressure. The essential oil of Lavender flowers, for example, should not be processed at over 245 degrees and three pounds per square inch. Tougher conditions will result in a harsh scent – more chemical than floral.

High temperatures and pressures are often used to produces large quantities of oil in a short period of time. The oils are often destined for use in cosmetic and food products, but can sold as essential oils for use in aromatherapy. These oils will be less expensive, though they are of limited therapeutic value.

What are Absolutes?

Some plants, and their flowers cannot be processed through steam distilling. They are too delicate, or their fragrance and therapeutic essences wil not be totally released by water. These oils will be produced as ‘absolutes – and while not technically considered essential oils they can still be of therapeutic value (the technical description of an essential oil is the aromatic compounds of a plant produced by steam distillation). Jasmine oil and Rose oil in particular are delicate flowers who’s oils are often found in absolute form. Jasmine oil is only found as an absolute, whereas rose is available in both a a steam distilled form and as an absolute.

The processing of an absolute first involves the hydrocarbon solvent extraction of a ‘concrete’ from the plant material, a semi-solid mixture of typically 50% wax and 50% volatile oil. The concrete is again processed using ethyl alcohol (the same alcohol found in beer, wine, etc.) in which the wax is only slightly soluble. The volatile plant oil separates into the alcohol and this mixture is removed. The alcohol is then evaporated and the result is an almost pure plant extract – depending on the care taken in the evaporation process, sometimes 2% or less of the ethyl alcohol may remain. The use of solvents in the extraction process notwithstanding, absolutes can have incredibly deep and complex aromas.

CO2’s and SCO2’s – Using Carbon Dioxide as a Solvent

The most modern technologies are Carbon Dioxide and Supercritical Carbon Dioxide distillation. Both methods involve the use of carbon dioxide as the solvent which carries the essential oil away from the raw plant material – though the gas essentially acts as the steam in steam distillation. The only difference between the two are the temperatures and pressures used – though neither uses a temperature greater than 87 degrees. With release of the pressure in either process, the carbon dioxide escapes in its gaseous form, leaving the essential oil behind.

These carbon dioxide methods have noteworthy benefits: Like steam distillation, there are no solvent residues left behind, and the resultant product is quite pure. Like solvent extraction, there is no heat applied to the plant material or essential oil to alter it in any way. The oil produced is very accurate with respect to the original state of the plant. The CO2 methods also are the most efficient, producing the most oil per amount of plant (one of the reasons for the high cost of essential oils is the low yield of oil from most plants – one ton of Rose petals produces less than 1 pound of oil, for example). The efficiency of CO2 extraction is particularly important when rare or endangered plant species are involved, such as Indian Sandalwood oil – less of the precious plant is needed to produce an equivalent amount of oil.

The Simple Cold Press

Cold pressing from the rinds of citrus fruit proudces oils such as Bergamot, Sweet Orange, Lemon, Lime, and the like. This involves the simple pressing of the peels at about 120 degrees. Little, if any, alteration from the oil’s original makup occurs – these oils retain their bright and uplifting aromas, like that of smelling a wonderfully ripe fruit.

Is there a Best Method?

CO2’s, with some obvious advantages, are not always the best choice for a particular need. They still are the most expensive, despite their higher yields. The resultant product differs slightly compared to one produced another way – the oils produced by steam distillation of some plants may sometimes be found to have a more agreeable aroma. Patchouli oil, for example, seems to benefit from the steam distillation process by becoming a little warmer and richer. Many other essential oils are quite effectively produced via steam distillation, with little alteration from the original plant state. Oils from other plant species do seem more ‘complete’ with CO2 processing, with Frankincense oil and most of the ’spice’ oils being good examples where a little something special is present in the aroma. This likely translates to greater therapeautic properties.

Producing essential oils of aromatherapeutic grade is skill requiring years of experience. It takes the work of a dedicated artesian at every step, from growing and harvesting to fine-tuning the distillation process, to produce a truly fine oil. The making of a fine essential oil relies far more on knowledge and experience than it does on the particular extraction method. There are, however, legitimate reasons to select one distillation method over another – some plants simply require a particular process to produce a fine oil, and the oil needed for a particular application may only be made by one process. In the end, as is often the case in aromatherapy, your own sense of smell can tell you which oil will work best for you.