The essential oils are produced by steam distillation from leaves, berries, flowers or rhizomes of plants. The steam distillation technology is based on the fact that the volatile constituents of plants can be separated from the plants by increasing the temperature of plants either in contact with heated water or saturated steam in a closed vessel. In the presence of steam or boiling water these substances are volatilized at a temperature close to 100 degree centigrade at atmospheric pressure. The mixture consisting of essential oil molecules and water vapour is allowed to pass through a cooling system called condenser, to become a liquid in which oil and water form two distinct layers. Most (but not all) essential oils are lighter than water. Therefore, they form the top layer and are easily collected and separated from the water. The steam that is used for the distillation is generated either within the steel vessel that contains the plant material (by boiling water at the base of the vessel) or by an external boiler.