Depending on the types of the botanical plants, physical extraction methods can range from distillation, to expression to solvent extraction. Outlined below three of the more commonly used methods of extraction.
Steam distillation involves packing containers with plant parts that are being subjected to steam at high pressure. The hot steam evaporates the essential oils from the plants. Both steam and essential oils vapour are then condensed through a condenser which the distillate (mixture of water and essential oil) are collected. As water and essential oil are not miscible, two layers will be formed in the liquid. Essential oil being typically less dense will be on the top layer and water will be at the bottom layer. The essential oil is then separated from the water.
Citrus fruit oils are often expressed using the cold-pressed method. During this process, trough with projections penetrate the peel of the fruits that are rolled over. The projection penetrate the tiny pouches of the fruits. The fruits then are subject to mechanical pressing to squeeze the juice from the pulp to release the essential oil. Through centrifuging, the essential oil forms the top layer of the liquid and is separated from the juice. Usually heat is not required in this expressing process, explaining the term “cold-pressed”.
Essential oil extracted using this method has a relatively short shelf life, typically of 6 months.
Solvent extraction method is useful for botanics that give low yield. This process can involve the use of organic solvents (like hexane) or carbon dioxide. Waxes and pigments are also extracted as by-products, which can be separated by another process.
Solvent extraction can be divided further into these processes (but not limited to): maceration, and solvent extraction by carbon dioxide.
In maceration, flowers or herbs are soaked in carrier oil. The carrier oil will then absorb the aroma of the flowers or herbs, which will be removed physically, and decanted to produce infused oil.
Solvent extraction by using carbon dioxide is a new extraction method that is fairly expensive yet able to produce good quality oils.
Carbon dioxide exists naturally as a gas. Under pressure, carbon dioxide undergoes change in state to liquid (just like how dry ice is produced when the gas becomes solid). Carbon Dioxide is a safe liquid solvent used in this process since it forms part of human or plant’s biological activity. Furthermore, at liquid state, it is unreactive and does not react chemically with the essence that is being extracted.
The removal process of carbon dioxide solvent is also simple as once the pressure is released, carbon dioxide revert to its natural state – gas.
As the pressure required is hundred-folds higher than our atmospheric pressure, heavy-duty stainless steel equipment is required, hence incurring high investment for this extraction method.