Large quantities of essential oils are used in food industries as well as in perfume industries. The aroma and flavour substances often differ substantially in their chemical composition and especially in their sensory properties depending on their production procedures.
In steam distillation which is still the most often used process, the material is exposed to a relatively high temperature stress which can lead to artefacts of the essential oil components, especially due to the hydrolytic influence of the water. Extraction with low boiling organic solvents is used, too. Here, a very important step in the process is the complete removal of the solvents from extracts and its residue as they are in part toxicologically objectionable. In addition, the organic solvents have only a low selectivity. Apart from the desired substances, higher molecular, non-volatile components are also extracted, such as fatty oils, resins, waxes and colouring matters.
The extraction with dense gas proves to be a true alternative to the conventional procedures. To obtain essential oils or aroma substances which are used in perfume industries, in the pharmaceutical sector or in the food and human nutrition field, carbon dioxide offers itself as an extractant which is physiologically unobjectionable and leaves the obtained products free of solvents. Another advantage is the mild operational process. Under exclusion of oxygen and at temperatures around 40°C, even thermally unstable substances can be won undecomposed with dense carbon dioxide; thus, essential oils genuinely present in the plant material are obtained. The extraction of essential oils can easily be carried out. Problems are, on the one hand, the quantitative recovery of the easily volatile components, and on the other hand that the undesired substances may be partly coextracted. For an optimal regulation of the two process parameters, pressure and temperature, the knowledge of the volubility behaviour of the essential oils in dense carbon dioxide is a precondition.
