The supercritical fluid commonly utilized is carbon dioxide (CO2). The state diagram of CO2 visualizes the various phases (solid, liquid, gas) depending on pressure and temperature. CO2, at 31,1°C and 73,8 bar, is in its supercritical state, in which there is no distinction between liquid and gaseous phases (as shown in the following picture).
Increasing the temperature and keeping the pressure constant (73,8 bar), CO2 remains in the supercritical state, and so happens when pressure is increased and temperature is constant (31,1°C): this individuates two rays – respectively parallel to the pressure and temperature axes – defining the zone in which CO2 is in the supercritical state; in particular, within this state, the possible combinations of pressure and temperature are shown to variate CO2 solubilizing properties. The reasons for the choice of this supercritical solvent are of economic (CO2 is cheap), environmental (CO2 is not toxic, it does not harm the ozone layer, it does not pollute and it does not contaminate the extracts) and technical (CO2 critical conditions can be reached easily) concern. SC-CO2 assumes the characteristics of a non polar solvent and it is comparable to n-Hexane; it has the characteristic to solubilize compounds which are scarcely soluble in water due to their nature.
If essential oils are wanted, the presence of water in the matrix interferes negatively on the process, because it is extracted together with the oil, hence it is necessary to remove it in a second moment. In order to avoid this problem, vegetable matrices are usually dried before extraction, unless extracts containing also polar substances are wanted. In this case it is necessary to add other solvents (entrainers or co-solvents) directly to the matrix or to the CO2, like ethanol or water, able to extract those compounds. CO2 is chemically inert, so isomerization, oxidation or components hydrolysis are avoided. The advantage of this technique is that at the end of the extraction, the solvent can be removed as a gas, offering the possibility to recover the extracted concentrated compounds. In the industrial processes, CO2 can be recycled minimizing its consume. This technique finds several applications such as oil extraction from seeds, caffeine extraction from coffee, nicotine extraction from tobacco, etc.; it is also very convenient at industrial level. The advantages in using supercritical CO2 are largely of a “health and safety” and environmental nature and relate to increased unease about the presence of organic solvent residues in material for human consumption. It has good solvent characteristics for non-polar and slightly polar solutes.
It has a convenient critical temperature (31ºC). This enables extractions to be carried out at comparatively low temperature (often as low as 40 or 50ºC), decreasing the risk of damage of thermolabile compounds.
Most of the volatile components, which tend to be lost in hydro-distillation, are present in the supercritical extracts. Partly because of this, extracts obtained in this way tend to have flavor and taste, which are well liked by tasty panels. Extraction of natural raw material with supercritical CO2, allows the obtaining of extracts which flavor and taste are perfectly respected and reproducible. The supercritical fluid ability to vaporize non-volatile components (at moderate temperatures) reduces the energy spent, when comparing to distillation. Once the pressure excess in the equipment prevents oxygen entry while extraction occurs, oxidation reactions don’t happen.
In chemistry, we often talk about chemicals and solvents in terms of their polarity. Some chemicals are highly polar (i.e. water) and some chemicals are highly non-polar (i.e. hexane). When describing how a particular solvent will dissolve a chemical – there is a rule of thumb that ‘like dissolves like’. Meaning, a non-polar solvent will dissolve a non-polar chemical. All fats and oils are non-polar, thus using a non-polar solvent is most appropriate.
CO2 like Pentane and Hexane is very unpolar, therefore the best solvent for oils and fats. Water is at the opposit side while Ethanol, Methanol and Acetone in the middle, not very unpolar, not very polar.