Classical methods of extraction, such as hydrodistillation and solid–liquid extraction with organic solvents, are traditionally used for the extraction of lipid fractions and essential oils (EOs) from medicinal and aromatic plants (Mamidipally, P. K., & Liu, S. X., 2004). Essential oils are concentrated hydrophobic liquids containing volatile aromatic compounds derived from plants.
There are several apparatuses for determination of EOs content in various plant material, but the most frequently used is Clevenger type. This is also the official standard method for extracting EOs for quality control. During hydrodistillation the EOs components form an azeotropic mixture with water. Most of the EOs do not mix well with water in the liquid phase so after condensation, they are separated by decantation.
Extraction by hydrodistillation has been efficiently applied for isolation of wide variety of EOs. As an example, the coriander seeds contained up to 1% of EO, where monoterpenoid linalool was the main compound (>50%), and limonene, camphor and geraniol were present in significant quantity (Bajpai et al., 2005). The content of basil EO determined by Ph. Jug. IV procedure was 0.565%. The predominant compounds were linalool (49.79%), δ-cadinene (8.07%) and germacrene (5.73%) (Filip et al., 2014).
Hydrodistillation can be achieved by one of the two methods:
- Clevenger distillation – the material to be extracted is immersed in boiled water
- Steam distillation – steam passes through a bed of the material to be extracted
A typical lab scale Clevenger hydrodistillation apparatus reported in the Ph. Eur.
In both methods the vapours of the volatile components are carried by the steam to a condenser. On condensation oil-rich and water-rich layers are formed. These are separated by decantation.
Industrial plant for isolation of essential oils uses steam distillation extraction process. Saturated water steam (0.5-3 bar), generated in separate boiler, passes through a bed of herbal material. After that, steam combined with essential oils vapors enters the condensator. The separator separates oil from hydrosol. The capacity of plant for distillation is usually from 0.2 to 4 m3.
A typical industrial scale hydrodistillation plant
Scheme of steam distillation system
Drawbacks:
- According to Pourmortazavi and Hajimirsadeghi (2007), during the hydrodistillation besides huge energy consumption, elevated temperature could cause chemical alteration and decomposition of heat-sensitive compounds (hydrolysis of esters, polymerisation of aldehydes, etc.).
- The hydrodistillation is usually time-consuming, which should be taken into account during scale-up on industrial level.