Distillation Technology for Essential Oils or fragrance

• Distillation of aromatic plants simply implies vaporizing or liberating the oils from the plant cellular membranes in the presence of moisture, by applying high temperature and then cooling the vapor mixture to separate the oil from the water on the basis of the immiscibility and density of the essential oil with respect to water.
• Choice of distillation process is based on following considerations:

I. Sensitivity of the essential oil to the action of heat and water

ii. Volatility of the essential oil

iii. Water solubility of the essential oil

Methods of distillation

1. Water distillation (or hydro distillation):

• Simplest and oldest process available for obtaining essential oils from plants.
• Hydro distillation differs from steam distillation mainly in that the plant material is almost entirely covered with water in the still which is placed on a furnace.
• Care should be taken during distillation of powdered herbs, as they tend to settle on the bottom of the still and get thermally degraded.
• Also, for plant material that tends to form mucilage and increase the viscosity of the water, the chances of charring are greater.

Drawbacks:

• As the plant material near the bottom of the still comes in direct contact with the fire from the furnace, it may char and thus impart an objectionable odor to the essential oil.
• The prolonged action of hot water can cause hydrolysis of some constituents of the essential oil, such as esters.
• Heat control is difficult, which may lead to variable rates of distillation.
• The process is slow and distillation times are much longer than those of steam distillation.

2. Water and Steam Distillation

• To eliminate some of the drawbacks of water distillation, some modifications were made to the distillation units.
• A perforated grid was introduced in the still, to support the plant material and to avoid its direct contact with the hot furnace bottom.
• When the water level is kept below the grid, the essential oil is distilled by the rising steam from the boiling water. This mode of distillation is generally termed water and steam distillation.

3. Direct steam distillation

• In this method of distillation, steam is generated separately in a steam boiler and is passed through the distillation tank through a steam coil.
• The plant material is tightly packed above the perforated grid.
• Steam, containing the oil vapor, is condensed in a tube condenser and is separated in the oil receiver.
• Steam distillation is preferred when a lot of area is under cultivation and more than one unit is to be installed. Also, for distillation of high boiling oils and hardy materials such as roots and woods, steam distillation is more efficient.
• Steam distillation also reduces the time required for the extraction of oils.

4. Distillation with Cohobation

• Cohobation is a technique that can be used for water distillation or for water and steam distillation. It uses the process of returning the distillate water to the still after the oil has been separated from it so that it can be re-boiled.
• This is basically an improvised methodology of the directly fired type steam and water distillation units for oils which have partial solubility in water.
• Although most of the essential oils have finite solubility in water, some oils like those of rose, geranium etc have comparatively higher solubility.
• In such extractions, the loss of oil with the outgoing water of distillation can become alarmingly high.
• This problem can be solved by returning the condensate water from the separator back to the still; this is known as cohobation.