Small scale extraction methods for essential oils

Estimated reading time: 4 minutes

When you peel an orange, you become aware of a special aroma that is released by the fruit. This is the essential oil in the orange peel, and these are the compounds that give herbs, spices, fruits and flowers their flavour and perfume. Essential oils are found in the seeds, bark, leaves, roots, flowers, balsam, wood, and resin of plants.

There are four basic methods of essential oil distillation:

• Hydro distillation.
• Water-and-steam distillation.
• Steam distillation.
• Vacuum distillation.

Of these, hydro distillation and water-and-steam distillation are most used for the small-scale extraction of essential oils.

The choice of distillation method is dictated by:

• The sensitivity of the essential oil to the action of heat and water.
• The volatility of the essential oil that will be extracted.
• The water solubility of the essential oil that will be extracted.

Hydro distillation

In the hydro distillation process, plant material is completely submerged in water and heated to boiling in order to release the essential oil. A copper still is mounted on a brick furnace and filled with the prepared plant material, after which water is added until the material is fully covered. The still is connected to a condenser that is placed in a water tank, and all joints are carefully sealed to prevent vapour loss. As heating continues, a mixture of steam and essential oil vapour is produced, which is then condensed in the condenser and collected in the copper container, where the oil is separated from the water.

Hydro distillation produces oils of a lower quality and has some drawbacks:

• The plant material closest to the bottom wall of the still is in close contact with the furnace fire, which could cause the material to char and impart an undesirable odour, known as a burning note, to the essential oil.
• Oxygenated substances tend to partially dissolve in the water in the still and cannot be completely removed.
• Because the stills used for hydro distillation are small, it takes time to extract enough essential oil, which often leads to high grade oil being mixed with oil of a lower quality. This in turn reduces the grade of the final product.
• Hydro distillation is a slow process, requiring a long distillation time, which means it consumes a lot of firewood/fuel.

It is also important to note that the quality of the oil obtained through this method is directly related to the skill of the processor in:

• Managing the still.
• Selecting the raw plant material.
• Preparing the raw plant material.

Water-and-steam distillation

The water-and-steam distillation method is an improvement on hydro distillation whereby a perforated grid is placed in the still just above the bottom. The main equipment used in water-and-steam is:

• The distillation still, which is usually made of stainless steel.
• Condenser.
• Oil separator and receiver.
• Brick furnace.

During operation, the still is fitted with a perforated grid and placed directly on the furnace. The furnace is fuelled with agro-waste and/or firewood. The plant material to be extracted is placed in the perforated grid, after which water is added to the still to a level just below the grid. The still is then connected to the condenser via a vapour line. As distillation proceeds, the condensed mixture of water and essential oil is collected and separated in the oil separator.

Water-and-steam distillation is extremely popular due to the simple construction of the equipment, its low cost, and easy operation.

Essential oils commonly used in South Africa

– Theresa Siebert, ARC-Natural Resources and Engineering

For more information, email Theresa Siebert at SiebertT@arc.agric.za