CO2 Extraction (sometimes called Hypercritical or Supercritical carbon dioxide extraction) is a relatively new process used to extract the very purest essence of a plant.
Whilst essential oils from plants are usually extracted using heat distillation which is, in itself, a destructive process, CO2 Extraction uses only low or high pressure which means the complexities of the oil's constituents are preserved.
When high pressure is applied to CO2 (carbon dioxide) the gas acts like a liquid. This liquid CO2 can be used as a quite inert and very safe solvent which, when pumped into a chamber containing plant material, will act like a solvent attracting the plant's volatile elements into it.
Essential oils can be extracted in this way and have the added benefit of avoiding the high temperatures of steam distillation which, in itself, can destroy some volatile compounds, especially those of an extreme delicate nature.
Not all CO2 extracts are essential oils however as that term typically refers to steam distillation. Many CO2 extractions, whilst more efficient perhaps, result in an extraction not dissimilar to the steam distilled oil but some are much more complex and rounded and, of course, the offerings can be very much more varied.
Plants whose scent would easily be destroyed by the heat in the steam distillation process might more easily be captured by the Supercritical CO2 extraction process.
Essential Oils or CO2 Selects:
These "Selects" are obtained at relatively low CO2 pressure and contain only volatile, CO2 soluble components. These tend to resemble the classic steam distillate essential oils but with the advantage of no temperature degradation and the potential for additional volatile substances that may not be distilled out of the plant under normal steam distillation.
The consistency of the essential oils extracted with the CO2 method will vary from batch to batch just as the plants themselves vary and just as steam distilled oils vary. Naturally, there is also the extraction process itself and the actual processor to consider.
Most CO2 Select extractions are alcohol soluble.
Extracts Called “Totals:”
These "Totals" are obtained at higher CO2 pressures and contain all CO2 soluble components, including waxes, resins, colorants, resembling a classical hexane extract, with the advantage of no solvent residue. Therefore the resultant oils are like absolute oils.
These "Totals" are very very much like the herb itself. CO2 totals are usually thick and pasty due to the beneficial fats, resins, and waxes they contain that come from the plant material itself. These totals are soluble in essential oils and vegetate oils but seldom in water or alcohol and are considered Absolute Oils.