Pressurized fluid extraction thermal lability

Supercritical fluid extraction Supercritical fluid extraction (SFE) uses compressed gas as the extraction medium and circumvents some of the problems associated with the use of classical separation techniques involving organic solvents. This technique combines features of distillation (i.e., separation because of differences in component volatiles) and liquid extraction (i.e., separation of components that exhibit little difference in their relative volatilities or that are thermally labile). A number of gases, when compressed isothermally at a temperature greater than their critical temperature and to pressures greater than their critical pressure, exhibit an enhanced solvating power (136), which has been known since the nineteenth century (137, 138), but its actual applications did not come to practice until the late twentieth century. 

Microwave assisted extraction (MAE), supercritical fluid extraction (SEE) and accelerated solvent extraction (ASE) have made the extraction of thermally labile antioxidants possible. With ground Aims or thin Alms, MAE has been shown to have a high extraction efficiency with short extraction times and low solvent consumption, giving cost-effective methods. SEE has also been shown to give high extraction yields. There are drawbacks with the high pressure techniques the optimization of the extraction parameters can be difficult and a greater investment in laboratory equipment is needed. 

As noted above, even for supercritical fluids at hquid-Hke densities, there is no heat of vaporisation to move the fluid to gas-hke densities -- just lower the pressm-e at a given temperature. With typical hquids, heat must be supplied to evaporate them. That means that when a chemist wants to remove the solvent from a solution of liquid solvent-plus-extract, heat is usually appHed and the temperature of the solution is elevated to the boiling point of the liquid solvent. If the raffinate (what is left after extraction) is the desired product, usually there is some liquid solvent left in it as well, so heat is often used again to elevate the temperature. This can he a problem when working with thermally labile components and matrices. With a supercritical flvud, when pressure is lowered sufficiently, the supercritical solvent is effectively removed from both the extract and the raffinate. Usually heat is only input to offset the cooling due to the expansion process so very mild temperatures can be used during supercritical solvent removal. 

---