# High-Pressure Technologies for the Recovery of Bioactive Molecules from Agro-Industrial Waste
## 2 Presurized Liquid Extraction Processes  
In recent years, as a consequence of the deepening of research on biologically active natural substances, many extraction methods have been developed in order to obtain active compounds directly from natural sources. Phenolic compounds, carotenoids, flavonoids, lipids, proteins, polysaccharides, and essential oils are examples of active compounds obtainable from natural sources. Among the emerging extraction technologies that can enhance the performance of conventional extraction techniques, pressure liquid extraction (PLE) is considered to  
Figure 1: Biofinerinery concept applied to food waste. be environmentally friendly and efficient, and generates a small amount of waste while reducing cost and time [8,9]. The methodology is based on using solvents below their critical point in order to preserve the liquid phase during extraction. Firstly, PLE was introduced as Accelerated Solvent Extraction (ASE) technology by Dionex Corporation [8] in 1995 at the Pittcon Conference. It is also known as pressurized solvent extraction (PSE), enhanced solvent extraction (ESE) or pressurized fluid extraction (PFE) [10, 11, 12, 13]. In the beginning, this technology was used for trace analysis of organic contaminants [8,14,15]. Currently, this technique is mainly applied to the extraction of environmental pollutants in soil, sediments, sewage sludge, and fly ash. However, in recent years, due to the advantages of PLE that include significantly shortened extraction time and low solvent volume required, easy operation, and good performance, this technology is being applied to different fields, including biology, pharmaceuticals, and the food industry [15, 16, 17, 18]. As a result, the number of papers related to pressure-assisted processes in this field have increased significantly (Figure 2). This growing interest is mainly due to some impressive features of PLE; indeed, it is a process that can be automated, is characterized by reduced extraction time and solvent consumption, and its settings are particularly suitable for compounds that are sensitive to oxygen and light. PLE requires minimal sample pre-treatment, especially for non-fat samples such as homogenization and/or drying [7]. Regarding the mechanism of extraction, PLE combines elevated pressure and temperature with liquid solvents, without exceeding the critical points of the solvent or solvent mixtures, which remains in a liquid state. In this way, it is possible to increase the mass transfer rate from the matrix compounds to the extraction solvent to achieve a fast and efficient solid-liquid extraction. PLE often demonstrates better recovery when compared to other extraction techniques such as microwave-assisted extraction (MAE) or ultrasound-assisted extraction (UAE) [19,20]. Other significant advantages of PLE include the elimination of post-extraction steps, such as filtration and centrifugation, the high level of automation, and the possibility of coupling with different techniques, such as ultrasound [8,21,22]. Regarding the operation mode, there are three possible different modalities for applying the PLE process: static [23, 24, 25], dynamic [26], and static-dynamic. In the static mode, a fixed volume of extracting solvent is used; in the dynamic mode, the extracting solvent flows continuously through the sample; in the static-dynamic layout, a combination of the two modes is proposed. Figure 3a shows that static pressurized liquid extraction generally consists of the following components: (1) a pump to feed the solvent through the system; (2) an extraction vessel. The extractor consists of an extraction cell (EC) in which the solid matrix is contained, surrounded by a heating system. The layout is completed by  
Figure 2: Number of research articles on the main active substances extracted by PLE from 2010 to 2021 (ScienceDirect).