Essential oils are volatile substances – due to their small molecules, they evaporate quickly. This is the reason we immediately feel their smell. All of them are extracted from plant material – flowers, buds, wood, leaves, roots, seeds – using various methods. In this article, we will describe the most common four methods of extracting essential oils and explain the differences between them.
1. Steam distillation
This is the most common extraction method. The principle behind it is simple – if you pass steam over plant material, it will remove the volatile compounds from the rest of the plant material. Further, the mixture of steam and essential oils is cooled down in a condenser. This changes them back into a liquid state and allows them to be collected in a vessel. Two layers will form:
- the essential oil separates on the top (in most cases)
- the water mixed with water soluble compounds extracted from the plant material and small amounts of essential oils remains on the bottom of the collecting vessel
The two layers are separated by siphoning them.
The essential oils are then tested for their quality and bottled. There are lots of different factors affecting the distillation process (from the steam pressure used to the temperature and the materials from which the vessels are made). In many cases high temperatures and high pressure are used – but this affects the quality of the oil, as some of the compounds could be destroyed in the process. Therapeutic grade oils are obtained at a lower pressure and temperature so that the active compounds remain intact and the essential oil keeps all of its healing properties.
The second layer, the mixture of water, water soluble compounds and essential oils is called a hydrosol or hydrolat. It is usually sold as flower water. Keep in mind that just mixing essential oils with water is not the same thing as an actual floral water – so if you plan on buying floral water, check the label!
2. Cold pressing (Expression)
This method is used especially for citrus fruits essential oils (Lemon, Lime, Orange, Tangerine, Bergamot etc.). The rind of the fruits is literally squeezed using high pressure. This is a similar process to obtaining cooking oils from sunflower seeds, olives, almonds etc. Due to the high pressure, the essential oil is released from the peel and then separated from the rest of the material.
Cold pressing is preferred for citrus fruits as high temperatures affect some of the compounds found in the oil. Still, given the fact that the oils are only cold pressed, they usually contain small amounts of waxes that are naturally found in the citrus fruits peel. They can stain and also shorten the shelf life of the oil.
Steam distilled citrus oils smell different than the actual fruit (because some of the compounds are destroyed). Still, they do not contain nonvolatile residues (like the waxes) – so they do not stain and the chance of clogging a diffuser are lower. Also, they are less photosensitizing.
3. Resin tapping
Some oils are actually extracted from the resins produced by trees. Some examples include Frankincense, Myrrh, Cistus, Copaiba.
A small incision is made in the tree bark. As a result, the trees start producing resin to cover the wound and speed up the healing. This resin can be then collected and steam distilled.
The exception is the Copaiba oil – it is steam distilled directly from the liquid sap of the tree.
4. Solvent extraction
Some plant material is just too delicate for steam distillation. In this case, the essential oils are extracted using a solvent. The solvent – essential oil is then separated. Oils extracted using solvents are actually called absolutes. Examples of absolutes include Rose, Jasmine, Neroli. In most cases, they are not recommended for internal use, as they can still contain traces of solvents. Also, the cheaper the oil, the more likely it is that a cheap solvent was used – and many solvents are toxic. So be vigilant and make sure you are using only the best absolutes!