ITDG 
Interdev Agroalimentaire  

Small-scale production of essential oils

	Type de fiche	Techniques/Technologies
	Mise en œuvre	ITDG(Royaume Uni)
	
	Zone géographique	Global; 
	Mots clés	GIROFLE; DISTILLATION; HUILE ESSENTIELLE; AGRUME

	Echelle d’action	Groupe
	Public(s) cible(s)	Small- and medium-scale food processors

Contexte
Essential oils are the volatile oils distilled from aromatic plant materials. The odour and flavour of these oils is usually dependent upon these oxygenated compounds.  Many oils are terpenoids and a few oils are benzene derivatives. 

Essential oils can be divided into two broad categories:

  Large volume oils, which are usually distilled from leafy material, eg lemon grass, citronella and cinnamon leaves.
  Small volume oils, which are usually distilled from fruits, seeds, buds and, to a lesser extent, flowers, e.g. cloves, nutmeg and coriander.

Essential oils can be distilled from the waste products of other fruit processing operations, such as citrus fruit peels and fruit seeds.

Problématique / Objectifs
Harvesting
Correct harvesting is very important.  The essential oil content varies considerably during the development of the plant.  If the plant is harvested at the wrong time, the oil yield can be severely reduced.

The oil is usually contained in oil glands, veins or hairs, which are often very fragile.  Handling will break these structures and release the oils. Therefore great care is required when handling the plants to prevent loss of the valuable oils. 

Drying 
Drying the material prior to distillation considerably increases the oil yield.  However, oil can also be is lost during drying if the procedure is too harsh (the volatile components are driven off).  Great care is needed to avoid excessive temperatures during drying.  Drying in direct sunlight should be avoided as this can result in oil losses as high as 50%. Drying in the shade or partial shade will reduce the amount of oil lost.  The dried product should be distilled as quickly as possible.  If it does have to be stored, it should be stored in the shade and should not be allowed to get wet.  Before the leaves are placed in the distillation unit they can be cut into smaller pieces.  This increases the percentage of oil collected.  

Flowers
These should be distilled as quickly as possible after harvest as the volatile compounds deteriorate rapidly.

Description/Mise en oeuvre
Distillation
There are three methods of distilling essential oils that are suitable for small-scale processors:

  Water distillation - This is the simplest and usually cheapest distillation method.  The plant material is immersed in water and boiled.  The steam and oil vapour is condensed and the oil is separated from the water.  This method is suitable for flower blossoms and finely powdered plant material

  Water and steam distillation - This is the same as the water distillation, except that the plant material is not immersed in water but held above the boiling water on a grid.  This is slightly more expensive than water distillation but better for herb and leaf material.

  Steam distillation - In this process the steam is prepared in a separate chamber and piped into the distillation tank..  This is more expensive than the other methods, but is especially good for plant materials with high boiling point oils.

In addition, oils can be distilled by solvent extraction, but this is costly and dangerous due to the high volumes of solvents required. For these reasons it is not re commended for small-scale processors.

Water distillation

The distillation temperature should be about 100°C.  Care needs to be taken to prevent the plant material being damaged by contacting the overheated still walls.  The pressure in the still should be atmospheric.  The distillation time depends on the individual plant materials, but is usually three hours.  Prolonged distillation produces only a small amount of extra oil, but does add unwanted high boiling compounds and oxidation products to the essential oil, thus reducing the purity and quality.

The rate of distillation and yield of oil are low, as water soluble oils are retained by the water remaining in the distillation unit.  The quality of the oil can be poor due to ester hydrolysis and possible damage from the plant material coming into contact with the wall of an overheated tank.

The distillation unit

The distillation unit consists of three parts:

Figure 1
Retort or tank (see Figure 1)
This is the container that the plant material is placed in and covered with water.  A direct heat source or steam jacket heats up the chamber and boils the water.  The plant material needs to be kept covered with water, therefore vaporised water must be replaced.  A removable cover is needed so that the plant material can be placed in the chamber and spent material removed.

A pipe should lead from the top of the tank to the condenser. The size of the tank depends on the size of the operation.  The diameter of the tank should be slightly less than the height.

Iron and copper react with essential oils and therefore should not be used for the construction of the tank.  Aluminium reacts with phenol-containing oils, so should be avoided in those cases.  Stainless steel and glass are the best materials to use.  A cheap alternative that can be used is heavily tinned copper.

Condenser (see Figure 1)
The condenser converts the steam and oil vapour to liquid.  This is very important and the rate of distillation depends on this process.  If the condenser is too small, the distillation rate will be reduced.

The condenser is usually a coiled pipe in a large tank of cooled water.  The water should be kept cool by changing it regularly.

As with the tank, the condenser should be made from stainless steel, glass or, in some cases, aluminium.  All joints should be soldered, as even a small hole will result in large oil losses. 

Figure 2
 
Figure 3. Separators
Separator (see Figures 1 and 2)
This part of the equipment separates the oil from the water. It is usually achieved by letting the mixture settle in a large container made of glass.  If the oil is heavier than water, the oil is collected from the bottom of the container, and if lighter from the top, see Figure 2.

If the water is cloudy after separation, it should be used in the distillation unit with the next load of plant material.  This is called 'cohabitation'.

 Figure 3
 
Water and steam distillation
This is the same as the water distillation, except that the plant material is not immersed in water but held above the boiling water on a grid, see Figure 3.  This is slightly more expensive than water distillation but better for herb and leaf material.

The plant material does not need to be finely chopped or powdered.  The distillation temperature should be about 100°C and at atmospheric pressure.

Steam distillation 
In this process the steam is prepared in a separate chamber and piped into the tank.  This is more expensive than the other methods.  This is especially good for plant materials with high boiling point oils.

In this method the temperature and pressure can be increased for certain oils.
The rate of distillation and yield of oil are high and the quality of the oil is good.

Storage
The oils should be stored in darkened glass bottles, galvanized iron or stainless steel containers.  The head-space in the containers should be minimal to avoid deterioration through oxidation.

Résultats/Eléments chiffrés
Distillation of essential oils from citronella and lemongrass
The first harvest can take place 6-9 months after planting. The grass can then be harvested up to four times a year.  If harvested too often, the productivity of the plant will be reduced and the plant may even die.  If the plant is allowed to grow too large, the oil yield is reduced.  For lemongrass it should be 1.2m high with 4-5 leaves.  The grass should be harvested early in the morning as long as it is not raining.  Harvesting can be done with machetes or simple knives.

Cinnamon leaves
Cinnamon leaves are harvested during the wet season since the rains facilitate the peeling of the bark.  Harvesting involves the removal of the stems measuring 1.2-5 cm in diameter.  This takes place early in the morning.

Flowers
These should be picked very carefully and processed as soon as possible.

Grading/Quality
The criteria for essential oil quality are:

  Colour - most oils should be clear, colourless and clean.  A murky oil is a sign of water being present.
  Odour - often the odours are specific to the areas in which the plant is grown.  This makes it very difficult for new producers to enter the market.
  Relative density
  Refractive density
  Optical rotation
  Solubility in ethanol
  Content of specific chemicals

Enseignements
  Distillation of essential oils is a fairly complicated procedure.  It can be carried out at the small-scale, but it does require some knowledge of the procedure and specialised equipment, which may be difficult to obtain and/or expensive.

  Essential oils are high-value products.  This is a very specialised market and standards are high.  Before starting to produce essential oils, processors should carry out a market survey to determine local demand for the various products.

  While essential oils can be distilled from citrus peels that are removed during processing of citrus, this is an expensive 'sideline' due to the equipment required.

  The market is very uncertain.  The prices for citronella and lemongrass oils varied by over 300% in the early 1980s.  This makes it very difficult for the small producer.

Auteur : Sue Azam Ali
Saisie le : 05/12/2001
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