## 3 Different Extraction Methods of Polyphenols
### Microwave-Assisted Extraction (MAE)  
MAE is an efficient advanced extraction technique integrating the conventional solvent extraction and microwave approach to recover polyphenols from food wastes. Table 2 represents the recent research in this area. This technique has plenty of advantages due to its rapid extraction rate, short extraction time, low solvent volume, and excellent product quality at a reasonable cost [66; 69]. However, this method has some drawbacks, including high technical complexity, less control on the energy input, reduction of heat-sensitive bioactive compounds, high preliminary cost, and inadequate extraction yield [70]. Microwave technology is nonionizing electromagnetic radiation containing a frequency of 300 MHz to 300 GHz and a wavelength of 1-1000 mm [71; 72]. Microwaves transfer energy from electromagnetic waves into thermal energy without contacting the food. The electromagnetic waves are generated from an emitter exposed to the food matrix [73]. The fundamental principles of microwave technique during the extraction process lie in the movement of energy by an electric field via two corresponding mechanisms [74]. Indeed, MAE is based on the interaction of microwave energy with the polar molecules (e.g., water) in the solvent surrounding the sample to generate heat. The increase in the temperature is due to the ionic conduction and dipole rotation, which results in increased cell wall destruction and efficient extraction of polyphenols [26; 75]. Microwaves selectively warm polar molecules with low molecular weight and high dielectric constant, while nonpolar molecules remain static in the microwave electric field [74]. This selective heating may induce the formation of high-temperature microzones named hotspots compared to the temperature of the reaction medium. Eventually, the temperature of the matrix increases quickly, resulting in increasing the chemical reaction rates [76]. The most prominent parameters in MAE are extraction temperature, solvent composition, extraction time, microwave power, and solid/solvent ratio [69]. The extraction yield of MAE could be improved by raising microwave power and time to a certain extent. However, when these two factors are higher than a threshold, the possibility of a significant loss in heat-sensitive bioactive compounds increases [79]. The stability of the extractive compounds is a deciding factor in selecting the optimum temperature [80]. Overall, the quantity of total recovered polyphenols increases with rising temperature because at higher temperatures, a decrease in solvent viscosity and an increase in intermolecular interaction and intracellular pressure occur, which give rise to a higher molecular motion improving the solubility of polyphenols in the solvent [65]. At low temperatures, the long exposures to microwave radiations diminish the extraction yield due to the loss of the chemical structure  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Food By-Product**} & \multirow{2}{*}{**Solvent**} & **Solid/Solvent** & **Ratio (s/s)** & **Power (W)** & **Temperature (\(\circ\)C)** & **Time (s)** & **TPC \({}^{1}\) in Final** & \multirow{2}{*}{**Reference**} \\ \cline{1-1} \cline{5-7} \cline{8-8} \cline{7-8} \cline{10-10} \cline{10-10} \cline{6-7} \cline{10-10} \cl  
of bioactive compounds [81]. The extraction time starts from only a few seconds to several minutes to prevent oxidative stress and thermal degradation. When a longer exposure time is needed, the thermal degradation of the matrix could be prohibited through the extraction cycle. This can be controlled by providing renewed solvent to the repetitive extraction cycle to guarantee the culmination of extraction [82]. The proper solvents may increase the extraction yield. Many solvents could be utilized in MAE, including water, methanol, acetone, ethanol, and their mixture [69]. They should be chosen according to the food matrix under extraction.