One of the limitations of supercritical carbon dioxide (sCO2) that comes to the fore during the supercritical fluid extraction of phytochemicals is its incapacity to extract polar phytochemicals. Being naturally non-polar, sCO2 is:
- More suited for the extraction of non-polar phytochemicals as well as those that are volatile and of low molecular weight.
- Less suited for the extraction of polar phytochemicals, particularly those located inside the cell wall.
It is for this reason that operators of supercritical fluid extraction equipment use a co-solvent along with sCO2 – to improve its capacity to extract polar phytochemicals. Co-solvents used are:
Addition of cosolvents is an excellent way to further expand the utility of sCO2. This is because carbon dioxide offers a whole range of benefits as a supercritical fluid (SCF). Supercritical carbon dioxide:
- Has a critical temperature of 31.10C, which is around the ambient temperature improving its compatibility with temperature-sensitive compounds;
- Has a more manageable critical pressure of 73.9 bar;
- Is non-flammable and non-toxic;
- Has a customizable density to upgrade its solvent power;
- Is available in ample quantities and in pure form; and
- Has a comparatively low cost.
Phytochemicals are found in plants and are a part of the plant’s defence mechanism. Research has proved their utility to keep at bay a number of human disorders such as cancer, inflammation, mutation, and oxidation. As many as 4,000 phytochemicals are identified and 150 of these have been investigated in detail.
There are two ways of adding the co-solvent to the SCF:
- Actuating both the co-solvent pump and the sCO2 pump at the same time while maintaining the restrictor valve in the open position in order to have the correct mixing ratio. This method requires the use of a co-solvent pump.
- Operationalizing the sCO2 pump after addition of the exact quantity of co-solvent in the sample that is already placed in the extractor. This does not require a co-solvent pump.
We have to remember that the supercritical fluid (SCF) pump is different from the co-solvent pump – the former pumps the SCF (sCO2 in this case) and the latter pumps the co-solvent. Different types of pump serve these two different purposes:
- Operators prefer high performance liquid chromatography (HPLC) pumps for co-solvent addition.
- Producers and particularly those in the dietary supplement, pharma, food, and cosmetic industries prefer plunger pumps of high capacity as SCF pumps.
To improve the yield of phytochemical extracts, CO2 SCFE is often deployed in combination with:
- Ultrasound Extraction
- Microwave Radiation
- Pre-treatment with Enzymes
- Organic Co-solvents