Supercritical carbon dioxide isn’t just an efficient solvent for apolar compounds. In fact, if it’s combined with fluid modifiers like water, it becomes a very efficient solvent for medium polar and polar substances (caffeine for coffee decaffeination). The extraction efficiency is better than conventional technologies based on chemical solvents, due to the following features offered by supercritical CO2:
- Increase of mass’ transport for the zero superficial tension effect, with correlated advantages in the extractive efficiency and in the duration of the extraction (minutes vs. hours)
- With a minimum modulation of temperature and pressure in the process, the solvent properties are modified in an important way
- Totally miscible like gasses
- CO2 is a low-viscosity, eco-friendly and green solvent, not flammable, inert and non-toxic
- High diffusivity (so it can increase the extractive kinetics)
- Co-solvents (water, ethanol) can further modify the solvent proprieties
- The relatively low extraction temperatures help conserve volatile compounds
- This process is not influenced by oxidation: the extraction vessel, full of supercritical CO2, is an inert space for the oxidation process also at high temperature (50-70 °C)
- Antibacterial and so the final extracts are high quality products, for a microbiology point of view
- Cheap: it’s common in the atmosphere (0.04% and rising) and it’s concentrated in a pure solution (99,9% of CO2, gas state,20-25 bar), simply available in safe cylinders or bottles
- During the process the CO2 is continually recycled and this reduces the primary cost of extraction. At the same time, the small quantity of solvent lost during the process, and so released in the atmosphere, doesn’t increase global CO2 emissions, because the same CO2 was concentrated from the atmosphere in cylinders
- If a polar co-solvent (like water and ethanol) was used for the extraction of polar or medium polar chemical compounds, the extracted substances are easily isolated with the evaporation of the co-solvent
