Plant materials are usually extracted by organic solvents. Traditional solid-organic solvent extraction methods are time-consuming, require relatively large quantities of solvents, leave toxic solvent residue, and cause degradation of unsaturated compounds due to the heat. Because of these facts, there is an increasing demand for different extraction techniques with shorten extraction time, reduced organic solvent consumption and decreased environmental impact. In the last decades, supercritical carbon dioxide extraction (SC-CO2), powerful separation technique, is gaining interest regarding its commercial application, most particularly thanks to the technical and environmental advantages compared to current classical extraction methods by organic solvents.
Carbon dioxide is an environmentally friendly solvent. It is most widely used compressed fluid, especially for the extraction of natural products, because it is non-toxic, non-explosive, readily available, easily removable from the product and possesses convenient critical properties (Tc=31.1°C, Pc=73.8 bar). It is the solvent of choice for obtaining valuable substances that could be utilized not only in the food but also in the pharmaceutical industry. CO2 is generally recognized as safe (GRAS) solvent so products containing extracts obtained with “food grade” carbon dioxide are safe with respect to human health. SC-CO2 is an alternative process to traditional techniques suitable for the production and isolation of seed oils (Liu et al., 2009), essential oils, volatile aromatic compounds (Filip et al., 2014), tocopherols (Vagi et al., 2007), fatty acids (Brunner, 2013), chlorophylls and carotenoids (Pavlić et al., 2015).
Fig. 1. Main families of compounds found in the extracts obtained by SFE of vegetable
biomass, and examples from each one.
The most direct property prone to be affected by pressure variations is density, which can be used to perceive how close SC-CO2 reaches a liquid-like solvent power. When studying the influence of density on SC-CO2 behavior, pressure is a much preferable variable to tune its values as it offers considerably wider manipulation margins than temperature (de Melo et al., 2014). The efficiency of supercritical extraction can be affected by several variables, alone or in combination, which includes the operation temperature and pressure, extraction time and carbon dioxide flow rate. However, the most dominant effect on extraction process is provided by operating pres sure.
Fig.2. Laboratory-scale high-pressure extraction plant. GC – gas cylinder, CU – compressor unit, C – compressor with diaphragm, E – extractor, S – separator, HE – heat exchanger, UT – ultra thermostat, RV – regulation valve, V – on-off valve, MF – micro-filter, CV – cut-off valve, RD – rapture disc, PI – pressure indicator, TI – temperature indicator, FI – flow indicator.
Extracts obtained using carbon dioxide as extractant are solvent free/without trace of toxic extraction solvents, and thereby highly evaluated. Currently, SC-CO2 at commercial scale is limited to decaffeination, production of soluble hops extracts and sesame seed oil, extraction of certain petroleum products and extraction of certain valuable medicinal plants. This is caused mainly due to very high investment costs of SC-CO2 equipment. According to that fact, the optimization of operational conditions used in SC-CO2 is of great importance to make this technique economically viable.