Figure 4. Steam distillation of essential oils [23].
Hydro-diffusion extraction is a type of steam distillation that the only
difference with the steam distillation is inlet way of steam into the
steel. Steam is entered from the top of plant material, whereas is
entered from the bottom in steam distillation. This method is used when
the plant material has been dried and is not damaged at boiling
temperature [36]. This process can also work under low pressure or
vacuum and reduce the steam temperature below 100 °C. This method is
superior to steam distillation due to advantages such as shorter
isolation times, higher oil yield and less steam used. Hydro-diffusion
is also known as down hydro-diffusion or hydro-diffusion and gravity.