Extraction of Flavourings

The most important procedures for producing natural flavourings and volatile aromatic oils are steam distillation and selective C02-extraction, both of which will be compared more in detail. 

Steam distillation is based on an aceotropic or carrier-gas distillation of two immiscible liquids. Due to the unfavourable ratio of vapour pressures and thus of mole fractions in the distillate, large amounts of water must be evaporated for the separation of small amounts of essential oils. This is connected to long distillation times at around 100°C and a considerable thermal stress leading to the formation of artefacts, oxidation and isomerisation to a certain extent. Moreover the water itself can be a reactant and hydrolyse terpene esters that make up the core of a flavour terpene alcohols remain partially dissolved in the water and thus are lost from the essential oil. All this can modify the essential oil composition and change the original typical flavour impression. 

The positive aspects of steam distillation are the simple method, the universal application and the inexpensive equipment. Also, steam distillation is suitable for fresh plant material and production in place or even in the field is possible. 

C02-extraction, in comparison, is a more complex technology using more expensive equipment than steam distillation. The method has a limited suitability for fresh materials and is not recommended for floral fragrances which should be produced from fresh flowers immediately in place. The C02-oils are slightly more coloured than steam distillates and they can contain some wax traces. 

The first example is clove bud oil which is composed mainly of caryophylene, eugenol and eugenylacetate. For oil production clove buds were cold milled and one part of the powder was C02-extracted, the other part steam distilled. The yield was 14.8% in both cases. The oils were analysed by GC-FID and the composition of the 

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Mukhopadhyay, Natural Extracts Using Supercritical Carbon Dioxide, CRC Press, Boca Raton, Fla., 2000 Moyler, Extraction of flavours and fragrances with compressed CO2, in Extraction of Natural Products Using Near-Critical Solvents, King and Bott (eds.), Blackie Academic Professional, London, 1993. 

A different mode of operation, characterised as bubble flow, is possible if the continuous phase is the liquid to be extracted. The liquid level is kept constant at the top of the column and the supercritical gas which is introduced at the bottom is bubbled through the liquid for extraction. This method is mainly used if small amounts of dense gas are sufficient for complete extraction, i.e. if the solvent ratio of dense gas/ liquid feed is around 1 kg/kg. Examples are the extraction of flavours from wine and fruit juice where only very small amounts of extract can be expected. 

As shown above flavour substances may be mobilized by destruction of cells and framing structures of plants and by extraction of flavour materials dissolved in the plant juices. In addition, flavour precursors may also be liberated from covalent bonds [18a], The majority of these bonds are glycosidic links of flavour chemicals to molecular or particulate plant components. The enzymes used for cleavage of these bonds (p-glycosidases) tend to form equilibria of bound and free forms. 

Many of the near-critical extraction processes in use at the present time (e.g. decaffeination of coffee (chapter 5), hop extraction (chapter 4) and the extraction of flavour essences (chapter 6) involve leaching materials from a solid matrix. It is not surprising therefore that many of the mass transfer rate studies reported in the literature are for the extraction of components from a bed of solids. 

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