Heat sensitive compounds are those whose chemical composition gets altered when they are subjected to heat. Some of the extraction techniques such as solvent extraction and hydrodistillation use heat to separate the required compound (called target molecule) from the raw material.
Supercritical fluid extraction (SCFE) and particularly one that uses carbon dioxide (CO2) as the supercritical fluid (SCF) has a relatively low process temperature that makes it compatible with the extraction of heat sensitive compounds.
CO2 enters its supercritical state when its:
- Temperature exceeds 31.1 deg-Celsius; and
- Pressure exceeds 73.9 bar.
In this state, small changes in pressure induce large changes in its solvent power i.e. the ability to dissolve the required compounds. These required compounds are called target molecules. In the present case, heat sensitive products are the target molecules. Some example of heat sensitive compounds are as follows:
- Essential Oils
- Flavour & Fragrance molecules
Solvent extraction uses a solvent to first dissolve the target molecule. The solvent and target molecule are then separated. However, the solvent does not completely disassociate from the target molecule. Sometimes, the process also uses heat. Take the case of solvent extraction of seed oil using hexane as the solvent. Hexane does not completely separate from seed oil and the process uses heat.
High temperatures are an essential part of the hydrodistillation process that uses the difference in boiling point of different compounds to separate them. The raw material is first dissolved or drenched in water and the solution is boiled. Different target molecules evaporate (with water) at different temperatures and are separately collected. No wonder, the process is not the first choice for producers looking to extract heat sensitive compounds.
Supercritical fluid extraction uses an SCF to isolate the target molecules. This is how the process works:
- At high pressure, the solvent power of the SCF is high. Here it dissolves the target molecule. This part of the process occurs in the extractor.
- At low pressure, the solvent power of the SCF is low and here the target molecule comes out of solution.
Furthermore, at a certain pressure, the SCF dissolves only the target molecule with very little or no part of other compounds from the raw material. On account of such a mechanism, SCFE is able to extract target molecules in their relatively pure form.
Using CO2 as the supercritical fluid further cements all the natural advantages of the SCFE process. The one primary advantage that we have already taken note of is low process temperature which makes it compatible with heat sensitive compounds.
Here is an exhaustive list of reasons why CO2 is usually the first choice of SCFE equipment operators:
- 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.