Methods of Extraction of Essential Oils
Essential oils are volatile, and therefore the most logical method of extraction would be distillation. However, most essential oil constituents - terpenes, sesquiterpenes, and their oxygenated derivatives - are destroyed or charred before they reach boiling point.
Liquids boil when their vapor pressure becomes equal to the external (atmospheric) pressure. However, as mentioned earlier, an essential oil cannot be heated to a level where its vapor pressure becomes equal to atmospheric pressure due to decomposition. In such a case, there are two ways to proceed. One method is to reduce the external pressure, that is, through vacuum distillation. In many essential oil-containing materials, the amount of volatile matter is very low, and therefore resorting to vacuum distillation is very expensive and impractical. However, to remove fractions or compounds from an essential oil, vacuum distillation can be employed, such as in the deterpination of citrus oils. The other method is steam distillation (see later).