How are essential oils isolated from odoriferous plants?

Essential oils are isolated from odoriferous plants by first crushing or grinding the plant material to reduce particle size and rupture the oil-bearing glands. Several methods are then used to extract the oils:1

• Steam Distillation: The most common and important method. It involves heating plant material—either directly with water, by suspending it above water, or through dry distillation—to vaporize the volatile oil, which is then recovered using a condenser.1
• Solvent Extraction: Used for materials that cannot be processed by steam distillation. Volatile solvents like petroleum naphthas, benzene, or alcohol are used to recover the oils.1
• Enfleurage and Maceration: Historical methods involving the use of fats. In enfleurage, flowers are placed on purified fat to absorb the oil, while maceration uses molten fat at higher temperatures to extract the oils.12
• Expression: A mechanical process used specifically for citrus oils, where the outer peel is squeezed in presses and the resulting mixture is decanted or centrifuged to separate the oil.1