Résumé : |
The purpose of this series of articles is to briefly review the theory and practice of distillation of essential oils. Although many producers use some form of distillation to extract essential oils, very few thoroughly understand the theory behind the practice that lays the basis for the practices they utilise.
This first article will first briefly outline the scientific and engineering principles behind distillation, the second article will outline the various stages and types of distillation, and the final article will conclude with a brief discussion about applying these principles. The practice of distillation goes back to ancient times, perhaps as early as 484 BC, when Herodotus recorded the production of turpentine oil in his writings.1 Strong evidence also exists that the Arabs understood the distillation process, where the words chemistry, alcohol and alembic have their origins. It is most likely the Arabs inherited their knowledge of distillation techniques from the Syrian Empire.2 However, almost all distillation until midway through the nineteenth century was water distillation. Water distillation is where plant material is totally immersed in water, which is brought to a boil by a direct fire. Once the water is at boiling point steam begins to pass through a cooling coil (usually made of copper) to condense the distillate. Oil would then be collected from the top of the collection vessel upon separation with the water. Oils distilled within the geographically-centred and artisan-based perfumery industry at the time included rose, lavender, lavandin, rosemary, and herbs like thyme. Midway through the nineteenth century, the Germans and French in Grasse began experimenting to improve the distillation process. Equipment and techniques for watersteam and vacuum distillation were developed, greatly improving upon yields that were achieved through simple hydro or water distillation. Pre-distillation techniques, like comminution, were enhanced and fertilisers were applied to aromatic crops with dramatic results.3 However, it was only in the beginning of the twentieth century that steam from an external source to the charge bin was introduced, bringing in the method of steam distillation. The equipment and skills used for the distillation of aromatic materials from plant material is still very basic in many parts of the world. Many production centres, like the Australian eucalyptus and tea tree industries in the first half of the twentieth century, utilised available items like ship water tanks as charge bins to hold foliage during distillations.4 Even today in remote parts of the world, many stills adopt primitive designs and utilise very basic techniques in production, sourcing steam from direct fires. |