### Innovative and Sustainable Extraction Methods
#### 2.2.8. Supercritical CO\({}_{2}\)  
Carbon dioxide (CO\({}_{2}\)) is widely employed as chemical feedstocks for a variety of applications because it is nontoxic, nonflammable, abundant, economical, and intrinsically renewable. Among the applications of carbon dioxide in science, green extraction technologies like supercritical fluid extraction (SCFE) can conform to all the current, as well as likely future, regulations pertaining to safety, health, and the environment [89]. Advantages of using CO\({}_{2}\) as a solvent include chemical inertness, nontoxicity, nonflammability, nonexplosiveness, and availability with high purity at a reasonable cost; moreover, thanks to its physical properties, any further solvent removal procedure is not needed, making it interesting for industrial application [90; 91]. Furthermore, relatively low critical properties (Tc = 304.1 K, Pc = 73.8 bars) make CO\({}_{2}\) an ideal solvent when it comes to the extraction of thermally labile components such as terpen and carotenoids (and represent a good aspect for further industrial application involving lower plant costs), allowing excellent handling properties and recovery of solvent [92; 93]. With these remarks, supercritical fluid extraction based on carbon dioxide (SFE-CO\({}_{2}\)) has been evaluated as a solvent for the recovering of interesting and added-value compounds, especially from wastes of the agri-food industry. In this application, SFE-CO\({}_{2}\) turns out to be a very advantageous technique considering the particular supercritical state properties: in fact, it is characterized by both gas and liquid properties, i.e., gas-like viscosity and diffusivity and liquid-like density and solvating properties, which allow the fluid to diffuse easily through the matrix, giving faster extraction yield [93]. Additionally, the density of supercritical CO\({}_{2}\) (sccO\({}_{2}\)) can be modified by changing its pressure and/or temperature, modifying the solvent strength of the fluid [92; 94]. A general scheme of an apparatus for SFE-CO\({}_{2}\) is shown in Figure 3.  
Figure 2: Scheme of an apparatus for instant controlled pressure drop extraction. The main compartments of the instrumentation are the DIC chamber where the sample is placed and extracted, the pressure-drop valve that releases the pressurized steam into the vacuum chamber, and a trap where the extract is recovered. Focusing on scCO\({}_{2}\) as extraction solvent, thanks to its sustainability and tunable-solvation properties, it has been studied over the years to valorize byproducts from the agri-food industry, enhancing the circular economy standpoint [95]. Natural bioactive compounds from agri-food waste constitute a wide variety of molecules with different structures and functionalities to produce nutraceuticals, functional foods, and cosmetics. Even if scCO\({}_{2}\) is an apolar solvent, it is shown to be an effective solvent for a very large class of compounds such as polyphenols, lycopene, anthocyanins, lipids, sugars, alkaloids, proteins, dietary fibers, and flavors, because the extraction selectivity can be tuned by the use of cosolvent (ethanol, water), forming tailorable solvent mixtures with different polarities [96]. In a recent work carried out by our group, scCO\({}_{2}\) turned out to be the best solvent for the selective recovery of fatty acids (FAs) from discarded waste pomace [97]. In this study, six different processed wastes were considered: blackberry, raspberry, blackcurrant, wild strawberry, pomegranate, and blueberry. To investigate scCO\({}_{2}\) efficiency as a green solvent for the recovery of such compounds (especially FAs), it was compared with traditional Soxhlet liquid extractions using hexane as solvent. The extraction conditions for SFE-CO\({}_{2}\) (P, T, CO\({}_{2}\) flow, time) were optimized based on the recovered yield, showing that 300 bar and 70 \({}^{\circ}\)C with a flow of 5 mL/min for 5 h were the best conditions to recover the highest amount of target compounds from all the matrices.