Majority of solvent extraction process have residual solvents in the final product. The solvents are usually organic solvents and are carcinogenic. SCFE is a safer way for extraction where safe solvent like CO2 is used as solvent under supercritical condition. Fluid beyond their supercritical conditions are known as super critical fluid and they exhibit behavior and physiochemical properties of both liquid and gases. In general, fluids near their critical points have dissolving power comparable to that of liquid and have transport properties of gas. CO2 is most widely used gas for supercritical fluid processing at Tc = 31°C and Pc = 73 atm.). At this condition, CO2 has flowability of a gas but has density comparable to that of liquid and hence has solubilizing power of liquid. After extraction, the solvent can be removed simply by releasing the pressure when CO2 leaves the solute and escapes as gas. Various gases used SCFE process are CO2, ethane, ethylene and N2O.
Preference of CO2 over other gases for SCFE
1. Safer to use for extraction of edible products
2. Ubiquitous in nature, inexpensive and readily available in pure form.
3. It is inert and doesn’t react with solute during extraction
4. Neither inflammable nor toxic
5. Doesn’t corrode the container in its native form or in combination with moisture.
SCFE Process: The material is fed into a cylindrical vessel (extraction section). The vessel is then filled with CO2 or other SCFE solvent and is pressurized using a compressor until desired pressure (usually 75 – 300 atm.) and temperature (35 – 80 °C) is reached. 
The system is allowed to equilibrate for sufficient time with the food solids and the super critical fluid (CO2). Diffusion of SCF into the solvent is slow which may require prolonged contact time for better extraction. The extracted material is then transferred to separation section where the solute is separated from solvent. Once the pressure is released, CO2 loses its super critical nature and hence solubility of solute is drastically reduced. Thus the solute is separated in this separating chamber. The CO2 is either lost off or recycled into the compressor for next extraction. The spent solids are discharged from the extraction chamber subsequently to feed a fresh batch.
About Author
Name : Pratiksha Shrestha
pratiksha.shrestha2001@gmail.com
Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.