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.