# High-Pressure Technologies for the Recovery of Bioactive Molecules from Agro-Industrial Waste
## 2 Presurized Liquid Extraction Processes
### PLE of Bioactive Compounds from Waste
#### 2.2.4 Polyphenolic Compounds  
Among the secondary metabolites produced by plants, phenolic compounds are a relatively abundant category. Generally, phenolic compounds are most often classified into flavonoids and non-flavonoids. Flavonoids are the most abundant plant polyphenols consumed by humans. They are polyphenol compounds with two aromatic rings connected by a three-carbon bridge. For example, more than 800 phenolic compounds have been identified in plants. Chemically, phenolic compounds have at least one aromatic ring and one or more hydroxyl groups [122]. Although phenolic compounds are not necessary for human metabolism, if they are present in the diet, they can improve health and may reduce the risk of serious diseases. In fact, phenolic compounds are widely studied because they can reduce the reactive oxygen species that cause oxidative stress and are related to the prevention and treatment of cardiovascular and chronic degenerative diseases. In addition, phenolic compounds are due to their antibacterial, anti-inflammatory, antidiabetic, and anticancer properties [123; 124; 125; 126; 127; 128]. At the same time, the antioxidant capacity of phenolic compounds indicates that they can be used as natural additives for functional foods. For example, the phenolic compounds recovered from olive mill wastewater [129] were evaluated as an agent to extend the shelf life of baked products due to their antibacterial properties. In the same way, [PERSON] et al. [130] extracted phenolic compounds from sugar-exhausted _Prunus avium_ homogenate to be loaded into low-methoxy-pectin film as an antioxidant barrier for food preservation.  
However, the extraction of these compounds is challenging because they may be unstable and their biological activity can be affected by extraction process parameters and external factors such as the presence of oxygen and light. Phenolic compounds have been extracted for decades using traditional methods such as Soxhet extraction and maceration [131; 132; 133].  
Alternative and more effective extraction processes are needed to overcome these shortcomings and produce higher extraction yields while maintaining the integrity of the compound.  
Due to the many advantages of pressure-assisted extraction processes, the research on the extraction of polyphenic compounds by this technology has become increasingly intense in recent years.  
[PERSON] et al. [134] recovered phenolic compounds from betroot (_Beta vulgaris_ L.) residues, leaves, and stems by PLE at a temperature of 40 \({}^{\circ}\)C, pressures of 7.5, 10 and 12.5 MPa, and a flow rate of 3 mL min\({}^{-1}\). [PERSON] et al. [135] demonstrated that PLE was effective in enhancing the extraction rate of flavanols from industrial by-product cocoa shells. [PERSON] et al. [136] optimized extraction conditions for total phenolics and caffeine from waste coffee grounds using the PLE method with water and ethanol.  
Considering the thermosensitivity of several polyphenols, the positive effect of pressure in improving the extraction process is of a great interest, allowing good extraction yields even at low temperatures [137] or enabling the use of a very high temperature but for short times [138].