Essential oils are natural aromatic extracts from plant material including grasses, leaves, flowers, needles, twigs, peels of fruit, seeds, bark, and roots. For example, rose essential oil comes from the flowers, basil from the leaves, lime from the rind, anise from the seeds, sandalwood from the wood, frankincense from the resin of its tree, and so on.

Essential oils are liquid mixtures of organic compounds that constitute the specific scent of a plant or its parts. Large amounts of plant material are needed to gain a few millilitres of essential oil. Rose petals e. g. contain very little oil. Most of the rose essence used in the perfume industry is supplied by Bulgaria. Rose petals have to be harvested in the morning and are processed immediately afterwards. There are several methods to extract the essential oils from the plant tissue.

(1)Extraction with cold solvent

The simplest method is suspending the chopped material in a suitable solvent – e. g. ethanol or pentane. You collect the suspension and let it rest in a closed bottle for some time before filtering it with the aid of a funnel and a paper filter. Fill the solution into a round bottomed flask and remove the solvent with the help of a liquid jet vacuum pump while heating the solution gently in a water bath.

(2)Extraction with hot solvent

This is best done with a Soxhlet apparatus. This ingenious device helps to extract the plant material repeatedly with boiling liquid in a continuous process. You need: a round bottomed flask (still pot) with a heating mantle, a Soxhlet extractor fitting to the flask, a reflux condenser (with rubber tubes) fitting to the extractor, a Soxhlet thimble; the plant material that is to be extracted (e. g. orange peels) and a solvent (e. g. pentane).

How to proceed:

Join the parts of the apparatus as shown in the image. Fill the orange peels into the thimble and insert it into the extractor. Pentane is filled into the flask. The amount should be four to five times the volume that fits into the extractor. Heat the solvent so that it boils and keep it refluxing at the condenser until the extractor has discharged the solution that forms at least three times into the flask. Disassemble the equipment and remove the solvent as described above (1) – preferably with a rotary evaporator. The remaining oily liquid is the essential oil. Do you like its fragrance?


Schematic representation of a Soxhlet extractor

1: Stirrer bar (magnet) – not necessary 2: Still pot 3: Distillation path 4: Thimble 5: Material to be extracted 6: Siphon top 7: Siphon exit (When the level of the solvent in the thimble rises to the siphon top a suction effect occurs and the liquid pours into the flask below) 8: Expansion adapter (necessary if the Soxhlet and the condenser don’t match) 9: Condenser 10: Cooling water in 11: Cooling water out

The rotary evaporator rotates the flask with the liquid in a hot water bath while applying a vacuum so that the boiling point of the solvent is lowered. It evaporates, is re-condensed by the cooler and collets in the flask below the cooler. After removing the solvent the essential oil remains in the rotating flask.

(3)Steam distillation

This is another complicated equipment suited to extract essential oils from plant materials. It is based on the fact that water and essential oils do not mix. This is why the pressures of their vapours above the liquid mixture add: Pvapour-total = Poil-vapour + Pwater-vapour

As a result the boiling point of the mixture is lower than that of its components, because the boiling point is reached as soon as the vapour pressure equals the air pressure that prevails. The method is especially suited for essential oils that would decompose at higher temperatures.

Steam distillation apparatus

In the Erlenmayer flask to the left of the image above steamis generated and flows into the round bottomed flask next toit. This flask contains the plant material (e. g. carnationpetals) in water. This water has also to be heated. Thesteam will carry the essential oil with it. The vapours liquefyin the condenser and the water oil mixture is collected in theflask to the right. Pour the mixture into a separatory funnelin order to isolate the essential oil. Whether the oil floats ontop of the water or accumulates at the bottom of the funneldepends on whether it is lighter than water or heavier. If theamount of essential oil is very low you can add pentane. Theoil will dissolve and form the top layer. After separating itfrom water the solution has to be dried and then the solvent is removedas described in the first and second example.

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