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].