# Recovery of Polyphenols from Agri-Food By-Products: The Olive Oil and Winery Industries Cases
## 5 From Agri-Food Wastes to Polyphenols: Processing Techniques
### Extraction Processes  
Several techniques have been used to recover polyphenols from olive oil and wine by-products. Conventional extraction with solvents, also called solid-liquid extraction (SLE) or maceration, is the most applied technique from an industrial point of view, to extract bioactive compounds from matrices of vegetable origin [7,81]. Many solvents have been studied for polyphenols extraction, but the preferred systems for food, pharmaceutical, or cosmetic applications are those based on water and ethanol [82]. Figure 6: Normalized results of the spectrophotometric assays and HPLC-UV technique from the olive mill (O1 and O2) and winery residues (W1 and W2). New non-conventional techniques appeared due to the growing need for the use of more efficient recovery procedures. These new methods can reduce extraction time, process temperature, and solvent consumption, thus contributing to a higher extraction efficiency and lower consumption of energy. Some of the most relevant technologies are: ultrasound-assisted extraction (UAE), microwave-assisted extraction (MAE), supercritical fluid extraction (SFE), pressurized liquid extraction (PLE), enzyme assisted extraction (EAE), ohmic heating (OH), and pulsed electrical fields (PEF) [1; 82; 83; 84; 85; 86]. Ultrasound-assisted extraction (UAE). UAE consists of the use of high-frequency waves (\(\geq\)2 MHz) that produce negative pressure, change the physicochemical properties of the medium, and allow the formation of cavitation bubbles. This last phenomenon is responsible for the rupture of the cell membranes in the samples, promoting contact between the solvent and the matrix. The use of this technique is increasing because of cavitation, which improves the transfer of heat and mass through the rupture of cell walls. UAE can be done using an ultrasonic bath, or more efficiently with an ultrasonic probe [7]. Microwave-assisted extraction (MAE). MAE is an extraction method that is based on the direct impact of microwave radiation on polar molecules [7]. The microwave energy is used to heat the polar solvents in contact with solid samples and to extract compounds of interest from the sample to the solvent, increasing the internal pressure inside the cell. This facilitates the rupture of the cell wall and the release of active compounds to the solvent [1]. Supercritical fluid extraction (SFE). The extraction with supercritical fluids is a technology that replaces the use of organic solvents with carbon dioxide in a supercritical state. The increase of the pressure and the temperature of the liquid/gas, above the critical point, generate fluids in the supercritical state. CO\({}_{2}\) is the most widely used supercritical fluid due to its availability, non-toxicity, and flammability, low cost compared to liquid organic solvents, and easiness to handle critical temperature and pressure (31.1 \({}^{\circ}\)C and 7.38 MPa). When compared to liquid organic solvents, supercritical CO\({}_{2}\) has a relatively low viscosity, high molecular diffusivity, and low surface tension of the system. All these properties lead to the improvement of mass transfer. Due to the non-polar character of CO\({}_{2}\), it is especially suited for the extraction of non-polar compounds. The addition of a co-solvent, such as ethanol, methanol, acetone, increases the polarity of CO\({}_{2}\) and allows the extraction of polar compounds [1; 7]. Pressurized liquid extraction (PLE). PLE is based on the use of the extraction solvent at high temperatures and pressure. Working at high pressure allows maintaining the solvent in a liquid state at temperatures above its boiling point. This technique can be used in three modes: dynamic, static, and dynamic-static. In dynamic mode, the solvent is constantly driven through the cell during extraction; in the static mode, the solvent is only pumped  
Figure 7: Recovery of polyphenols from agri-food wastes. between the extraction cycles; while the dynamic-static mode represents a combination of both [13]. Enzyme assisted extraction (EAE).