Steam distillation is a very old technique, like hydrodistillation. It entails recovering the essential oil contained in the plant cells through use of hot water vapor. The natural aromatic raw material is put into a still into which is injected steam formed by a boiler or generator. The steam destroys the botanic cellular structure to release the odorant molecules. The essential oil-laden vapor is condensed through cooling in a condenser before being collected in an essencier (essential oil separator). The hydrosol and essential oil, having different densities, separate naturally in the essencier.
This technique has the advantage of being more efficient than hydrodistillation. It also limits the occurrence of hydrolysis in the essential oil. Examples of essential oils obtained using this method include iris, rosemary, tea tree, and cajeput.