Methods of Essential Oil Extraction
Various Methods for Extracting Essential Oils
There are various methods for extracting essential oils. Some of these methods include steam distillation, hydro diffusion, water distillation, pressing (or expelling / expression), supercritical carbon dioxide (CO2 extraction), and various chemical extraction methods like ethanol, hexane and butane. Other terms you should know that are associated with essential oil extraction are cohabitation, otto and rectification. Back To Top
Steam Distillation and Hydro Diffusion
Steam distillation and hydro diffusion are very similar. The only main difference is that with steam distillation the steam rises up through the plant material and with hydro diffusion the steam is forced down through the plant material. In both cases the steam lifts the volatile organic compounds (VOCs) out of the plant material and carries them away until they condense. After the steam has left the plant material it moves into a tube where it cools, condenses and drips into a container. There the water and other compounds collected in the steam separate into layers where they can be easily collected.
Both steam distillation and hydro diffusion are generally good for obtaining high quality and therapeutic essential oils, and steam distillation is extremely common. The only main possible issues with these two methods is they extract less of the whole plant essence than other methods and it is possible to severely denature or damage the essential oil if too much heat is used. Back To Top
Water distillation is similar to steam distillation and hydro diffusion some ways. The main difference is that the plant material is boiled instead of steamed and the essential oil is distilled from the steam that comes off the boil. In some cases the essential oil binds to the water vapor and stays with it when it condenses, like when rose essential oil is obtained by water distillation. This is called cohabitaion. When this happens the essential oil that is collected at the end of the initial process is not complete, and the constituents that are bound to the water must be re-distilled and then added back into the rest of the essential oil. When this cohabitated portion of the essential oil is re-distilled and then added back into the rest of the essential oil, the essential oil is called otto. Rose oil in particular is often obtained in this way and so you will see it as “rose otto.”
The drawback to water distillation is that the essential oil may be subjected to heat for a longer period of time, which is not ideal. However, boiling the material within a vacuum can allow for processing temperatures less than 100 degrees which is better for the oil. Back To Top
Pressing/expressing is a fairly strait forward process–essentially the material is put through a press. Sometimes it is heated to some extent to help the process and sometimes the material is simply allowed to heat with the friction in the pressing process. Expeller pressing is when there is nothing done to manage heat—the material is simply pressed to release the oil. Depending on the material, if much pressure is required to press out the oil, the process may inherently raise the temperature higher than what is ideal for the oil due to the friction that is created. True “cold pressing” is when heat is not allowed to rise above about 122 degrees Fahrenheit. Additionally, sources of oils obtained through pressing or expressing should be known to be free of pesticides and chemicals before extraction, as pressing methods do not inherently remove these and other chemicals. Back To Top
Supercritical CO2 Extraction
Supercritical CO2 extraction has the ability to produce exceptionally high quality essential oils. Since CO2 is the same stuff we exhale when we breathe there are no worries about toxic chemicals. Supercritical CO2 extraction is the process of using liquid CO2 as a solvent to extract components of plant material. It is a gas at room temperature and can be super cooled to a solid that we know of as dry ice. But when CO2 is maintained at a temperature and pressure above its critical gas/fluid change point it becomes a supercritical fluid. As a supercritical fluid CO2 has good solvent properties and can extract various components from plant matter.
CO2 essential oils come in two types: CO2 Select and CO2 Total. CO2 Selects are obtained using less pressure and generally have a consistency closer to steam distilled oils. CO2 Totals are obtained using more pressure, so they are usually a bit thicker due to the presence of heaver molecules in the oil. Steam distilled essential oils can be very good oils, but CO2 extracts will include more of the plant properties than steam distilled type essential oils. Both types of CO2 extracts are high quality extracts and are good for therapeutic uses among other things.
Some of the benefits of CO2 extraction are that it uses low temperatures (so there’s no heat denaturation to the oil and other components extracted), it’s non-toxic and safe to use, it’s inert—meaning that it doesn’t bind with the material extracted, and it simply evaporates when the essential oil and other components are brought back to room temperature and leaves no residue. The down side is that CO2 extracted essential oils are more expensive. Even so, CO2 essential oils are some of the best available. Back To Top
Rectification is a process used for oils that contain compounds not desired in the essential oil. Often these are particles that are still from the plant the oil was taken from, they just aren’t desired in the essential oil for whatever reason. The process of rectification is simply redistilling the oil, whether with steam or in a vacuum. Sometimes the oil will be labeled “double distilled.” Eucalyptus is one oil that may at times be put through a second distillation process. Rectified essential oils are not necessarily ideal for therapeutic uses. Back To Top
Hexane and Butane
Hexane and butane are chemical compounds that are used to extract essences from plants. These chemicals can leave residues and heavy metals and are hence not good for therapeutic or culinary uses. For therapeutic or culinary uses, steam methods, pressing methods and CO2 extraction are the best. Back To Top