Essential oils are isolated from plant materials by steam and hydrodistillation and by hydrodiffusion. Steam distillation is performed with indirect (dry) steam. Hydrodistillations are featured by the fact that the plant material is heated with water and the steam is generated within the still. Hydrodiffusion is carried out with low pressure steam (0.1 bar), replacing the volatiles from the intact plant material by osmotic action.
Comparison with Hydrodistillstion
It will be clear, that the ease of isolation of a particular oil is dependent on the ease in which the plant cells release the oils. Essential oils can be present in isolated cells--organelles or idioblasts-(e.g. in Lauraceae, Rutaceae) or more at the surface of the plant material, e.g. in glandular hairs (as in Labiatae leaves), or in cavities, ducts or canals (like in UmbelIiferous fruits).
The thermodynamical aspects of steam distillation of essential oils from herb are well known. The hydrodistillation and hydrodiffusion of essential oils from plant material, however, is more complicated. The oil has to diffuse from the inside of the material to the surface. Moreover the plant material also contains non-volatiles, e.g. fats (fatty oils), which retain the volatiles. Thus during the isolation of volatiles from plant material by means of steam, the following physical parameters play a role: partition coefficients, diffusion rates, water solubilities, partial vapour pressures, times and velocities of heat transfer.