Most essential oils available today are extracted using a steam distillation process. It's the oldest form of essential oil extraction and is believed by many to be the only way oils should be extracted. This distillation process really is quite simple and as long as this extraction process is closely monitored, the steam will remain at a temperature that won't damage the plants.
The desired plant material is placed onto a still. A still is a specialized piece of equipment that is used in the distillation process. It consists of a vessel into which heat is added and a device that is used for cooling.
The plant is first placed into the vessel. Next steam is added and passed through the plant. The heat from the steam helps to open the pockets of the plant that contain the plant's aromatic molecules or oils. Once open, the plant releases these aromatic molecules and in this state, the fragrant molecules are able to rise along with the steam.
The vapors carrying these molecules travel within a closed system towards the cooling device. Cold water is used to cool the vapors. As they cool, they condense and transform into a liquid state. The liquid is collected in a container and as with any type of oil/water mixture, it separates.
The oils float towards the top while the water settles below. From there, it's a simple matter of removing the oils that have been separated. These are the highly condensed, aromatic oils used in aromatherapy.
The water is not discarded, however. The water, which also contains the plant's aroma along with the other parts of the plant that are water soluble, are the hydrosols - a milder form of the essential oils. These, too are also used in aromatherapy.
When steam is used, it's created at a pressure higher than that of the atmosphere. The boiling point is above 100 degrees Celsius and creates an extraction process that is safe and fast. If the temperature is allowed to become too hot, however, the plant material as well as its essential oils can easily become damaged.
The water distillation process involves placing the desired plant material in a still and then submerging it in water. The water is then brought to a boil. The heat helps open the pockets containing the plant's aromatic molecules so they can be extracted. The vapors cool and condense, the essential oils separate from the water and they're collected.
The water in this case provides protection for the plant because it acts as a barrier. Less pressure is used as well as a lower temperature than that which is used in the steam distillation method. This extraction method works well with plants that cannot tolerate high heat.
Other distillation methods
The hydro distillation process is similar to steam distillation. The only difference is that instead of introducing the heat from the bottom and up through the still, as happens in steam distillation, the heat passes into the still from the top. It's cooled from below, which makes collection of the essential oils easier. This method actually results in a higher yield of essential oils because less steam and consequently less processing time are involved.
In a water/steam combination distillation method, plant material is submerged into heated water and steam is forced through the water, opening the pockets containing the aroma molecules. When cooled, the essential oils condense and are collected as described above.