Hypercritical Carbon Dioxide (CO2)
The use of hypercritical carbon dioxide extraction is a fairly new way to extract essential oils from botanical material and although a bit on the expensive side, does yield good quality oils.
Carbon dioxide becomes hypercritical at 33 degrees celsius, which is a state in which it is not really gas or liquid, but has qualities of both, and is an excellent solvent to use in the extraction of essential oils since low temperature is required and the fact that the process is near to instantaneous.
The carbon dioxide is inert and therefore does not chemically react with the essence that is being extracted and hence, essential oils can be extracted in a similar way as other solvent extraction methods. To remove the carbon dioxide solvent, you simply need to remove the pressure applied.
This process has to take place in a closed chamber since the hypercritical pressure required for carbon dioxide is 200 atmospheres. To achieve this pressure a specific equipment is required which is very expensive.
The advantage of this method, of course, is that no solvent residue remains, since at normal pressure and temperature, the carbon dioxide simply reverts to a gas and evaporates. Hypercritical carbon dioxide extraction has given us essences of some aromatics that don't yield essential oils, for example Rose Hip Seed and Calendula.
Many carbon dioxide extractions have fresher, cleaner, and crisper aromas than steam-distilled essential oils, and they smell more similar to the living plants. Scientific studies show that carbon dioxide extraction produces essential oils that are very potent and have great therapeutic benefits.