# High-Pressure Technologies for the Recovery of Bioactive Molecules from Agro-Industrial Waste
## 2 Presurized Liquid Extraction Processes
### Effect of Operative Parameters  
In order to improve the extraction efficiency of an active substance, the key parameters that can affect the efficiency of the extraction process must be investigated. For pressurized liquid extraction, the factors that affect the extraction include the type of residue, solvent type, extraction time, pressure, temperature, pH, liquid-to-solid ratio, stirring speed, particle size, and sample moisture [38; 39].  
Pressure-assisted extractions can employ a wide range of extraction temperatures, usually from room temperature to 200 \({}^{\circ}\)C, and pressures up to 200 bar [11]. Temperature is a very important parameter to improve the mass transfer rate of the target compound from the solid matrix to the extraction solvent. Due to the non-compressibility of the liquid (the change in density with pressure is usually insignificant), the effect of pressure does not have a direct effect on the solubility of the analyte in some cases. However, operating at higher pressures can ensure that the solvent remains in a subcritical liquid state, while pressure forces the solvent to penetrate into the matrix pores and facilitates the desorption of the target compound due to a synergistic effect with temperature. The combined effect of high temperature and high pressure provides conditions that are able to significantly increase the solubility and diffusivity of the solute, to perform the rupture of the solute-matrix bond, and to reduce the surface tension and viscosity of the solvent [40; 41; 42; 43].  
Solvent selection is a key step in the design of industrial extraction processes. The extraction solvent must be able to dissolve the target solute and minimize the co-extraction of other matrix components. In addition, compatibility with subsequent processing steps (extract purification, target molecule preconcentration, or analytical techniques) and solvent volatility (if extract concentration is required) must also be considered [9; 44].  
The use of green solvents can have a positive impact on the environmental sustainability of processes; however, extraction yield can be negatively impacted by the selection of solvent less performant compared to the classical solvent. However, when non-conventional processes are performed, operating conditions are able to provide extraction yields and selectivity even higher than conventional processes using conventional solvents. Among the green solvents investigated in the literature, especially for agri-food waste recovery, there are subcritical water and pressurized ethanol aqueous solutions. Since they do not produce residues during the extraction process, they are non-toxic and eco-friendly.  
Indeed, as reported by [PERSON] et al. [45], subcritical water extraction (100 \(\leq\) T \(\leq\) 140 \({}^{\circ}\)C and 8 \(\leq\) P \(\leq\) 15 MPa, for 130 min) provided a total flavonoid and polyphenol 12- and 19-fold higher than conventional solid liquid extraction performed with water at 25 \({}^{\circ}\)C for 19 h, and with a higher antiradical power. In addition, polyphenol extraction from _Teucrium montanum_ L. carried out by subcritical water gave higher concentrations of total phenolics in comparison to aqueous, methanolic, acetone, ethyl acetate, and petroleum ether extracts obtained by conventional solid/liquid extraction [46]. At the same time, the comparison of HPTE with other extraction techniques, using ethanol as solvent, showed the best performance of the former on the recovery of bioactive compounds from _Arthrospira platensis_, while solid-liquid extraction provided the worst performance [47].