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There are three main extraction processes for essential oils

• Expression processes

  • Compression – sponge
  • écuelle à piquer
  • Machine abrasion

• Distillation processes

  • Water
  • Steam
  • Water and steam
  • Hydro-diffusion

• Extraction with volatile solvents

  • Enfleurage
  • Solvent
  • Hypercritical CO2

Expression processes

The expression process is a cold pressed method of extraction, which is generally used in the extraction of citrus essential oils.

Compression – sponge

The fruit is cut transversely into halves and the pulp is removed.  This is then soaked in warm water with the rind and the pith to absorb some of the water and make the rind more pliable.  Once the fruit has absorbed the water and become more pliable, it is inverted to rupture the oil cells.  A sponge is placed next to the rind and the fruit is squeezed to release the oil which is collected into the sponge.  The sponge is regularly squeezed and the essential oil is collected in a vessel and then decanted. 

écuelle à piquer

The écuelle à piquer process involves the fruit being placed in a device and rotated with the spike on the side rupturing the oil cells.  The essential oil and other materials, such as pigment, then runs down the centre of the device into a collection area.  The fluid is then separated and the essential oil.

Machine abrasion

The machine abrasion process involves a machine stripping off the outer peel which is separated by running water and is then put into a centrifugal separator which separates the essential oil from the aqueous matter.

Distillation processes

The distillation process heats the volatile liquid (the essential oils) at a high temperature that turns it into a vapour and then cools the vapour causing it to condense back into liquid form.  

Water

The water distillation process involves the plant matter being completely immersed in water and the still is then brought to the boil.  The water acts as a barrier to prevent it from overheating.  Once the material has condensed, it is cooled down and the water and the essential oil are separated and the essential oil is decanted.  (The water that is separated from the essential oil, “Floral waters”, can also be used such as rose water, lavender water and orange water.) To protect the botanical material as well as the essential oil, water distillation can be done under vacuum to reduce the temperature to under 100 degrees.

Steam

The Steam distillation process involves the plant material being placed in a still and steam forced over the material.  The stream releases the aromatic molecules from the plant by forcing open the cells in which the oils are kept.  The molecules released from the plant material evaporate into the steam.  The steam then contains the essential oil which is then passed through a cooling system which condenses the steam to form a liquid.  The liquid is separated into the essential oil and water.  The essential oil is then decanted.  

Water and steam

The water and steam distillation process involves the plant material being placed in a still and completed immersed in water.  The still has a heat source and steam is fed into the water and the plant material mixture.

Hydro-diffusion

The hydro-diffusion process is similar to the steam distillation process however; it differs in the way the steam is introduced into the still.  In the steam distillation process, the steam is fed in from the bottom where as in the hydro-diffusion process the steam is fed in from the top into the plant matter.  The condensation of the oil and steam mixture occurs below the area where the plant matter is held in place by a grille. 

Extraction with volatile solvents

In the extraction processes using volatile solvents, solvents are used to entice the essential oils out of the botanical material.

Enfleurage

In the Enfleurage extraction process, a chassis (glass plates in a frame) is covered with highly purified and odourless solid fat or oil.  The petals of freshly picked flowers are laid across it and pressed in.  The petals remain in this composite for a few days to allow the essence to disperse into the compound.  The depleted petals are then removed and replaced with a fresh harvest of petals.  This process is repeated until the mix is saturated with the essence, and should be repeated a numerous times until saturation is achieved.  When the mix has reached saturation point the flowers are removed and the enfleurage pomade (the fat and fragrant oil) are then washed with alcohol to separate the extract from the remaining fat.  This fat is used to make soap. Once the alcohol evaporates from the mixture the essential oil is left.  

Solvent

The process of extracting essential oils by using solvents such as petroleum ether, methanol, ethanol or hexane is used on fragile material such as jasmine, hyacinth, narcissus and tuberose.  These materials are unable to be extracted by steam distillation due to the heat.  The solvent extraction process involves the substances extracted being placed in a distillation vessel and gently heated. This heat is adequate to recover the solvent remaining in the mixture whilst not destroying any volatile constituents of the oil.  Once the solvent has been removed a solid wax-like substance is left.  This is called a “concrete”.  An absolute essential oil is made by melting a concrete by warming it with some alcohol and stirring it.  The essential oil, some of the wax, fixed oils and fats then dissolve into the alcohol.  This mixture is then distilled in a vacuum to remove the alcohol.  This resultant substance is an absolute, the most concentrated form of fragrance.

Hypercritical CO2

Carbon dioxide becomes hypercritical at 33 degrees Celsius.  In this state, it has qualities of both gas and liquid and is an exceptional solvent for the extraction of essential oils.  This is due to the low temperature required and that the process is close to instantaneous.  The carbon dioxide is inert and consequently does not chemically interact with the essence that is being extracted.  To remove the carbon dioxide solvent, you merely need to remove the pressure it is kept under.  To do this, the process has to take place in a closed chamber.  This is due to the hypercritical pressure required for carbon dioxide being 200 atmospheres.  200 atmospheres is 200 times the pressure of normal atmosphere. This is an expensive process due to the equipment required.