Subcritical extraction liquids (SEL)
Subcritical state is achieved when the pressure is higher than the critical pressure (Pc) but the temperature is lower than the critical temperature (Tc), or conversely. The most common fluids for this method are water and CO2. low viscosity, density close to the that of the liquids and diffusivity between that of the gas and liquid are good properties of the liquids obtained in this method [65, 66]. Other positive aspects of using this method are simplicity, low cost and favorable impact on the environment. The most important advantages of this technique over traditional extraction techniques are: shorter extraction time, higher extract quality, lower extraction costs, an environmentally friendly method and low solvent consumption [57]. The subcritical state of CO2 occurs when the temperature is between 31 and 55 °C and the pressure is between 5.5 MPa and 4.7 MPa. In this form, CO2 behaves as a non-polar solvent [67]. The subcritical state of CO2 prevents degradation observed due to temperature during other common extraction methods.
Subcritical water extraction (SWE), also known as ”pressurized hot water”, has dynamic conditions (pressure high enough to maintain water under liquid state and temperature in the range of 100 to 374 °C) [4]. In general, extracts obtained from subcritical carbon dioxide have better quality and functional in comparison to SWE [68]. According to studies, based on a comparison between supercritical CO2 and SWE [69], It was concluded that although the SWE process is cheaper and softer than supercritical CO2but it is still expensive to operate due to the need for special equipment [70]. The following schematic (figure 10) compares supercritical CO2 with subcritical water [57, 70]. For SWE, temperature of 130 °C and 20 min extraction time were found optimal for obtaining high content of bioactive compounds. The results of this method were evaluated with other novel methods such as microwave-assisted extraction and showed lower efficiency [71].