## 3 Different Extraction Methods of Polyphenols
### Microwave-Assisted Extraction (MAE)
This choice depends on the solubility of the compound in the solvent, the penetration power of solvent into the matrix, as well as its dielectric constant. Organic solvents, such as ethanol, acetone, and methanol, could be effectively employed in the extraction process. Since the presence of water could enhance the penetration power of solvent into the food matrix and increase heating efficiencies, water-based solvents are the most preferred ones for the extraction of bioactive compounds [83; 84]. In this respect, [PERSON] et al. (2019) reported that pure ethanol gave the lowest yield of polyphenols from lime peel waste, while 50-60% ethanol concentration (diluted with water) resulted in higher yields of polyphenols. Since polyphenols are polar/hydrophilic molecules, the addition of water to the organic solvents increases their polarity index, which results in higher recovery of polyphenols in the extraction process. In addition, water could increase the dielectric constant of the solvent and absorption of microwave energy, leading to a higher temperature inside the sample, which leads to the disruption of cells and easier release of polyphenols [66]. In MAE, ethanol can increase the degree of sample cell membrane breakage and improve phenolic compounds solubility. However, as ethanol concentration increases, the polarity of the solvent changes, which may lead to increased impurities being extracted, therefore reducing the number of polyphenols extracted. Moreover, increased diffusion resistance due to the coagulation of proteins at high ethanol concentrations may inhibit the dissolution of polyphenols and affect the extraction rate [26]. It should be kept in mind that the selection of solvent for MAE is not consistent with the traditional extraction process. For instance, diethyl ether is not proper for MAE as a solvent, while it is employed in conventional methods [85]. Moreover, a modifier could be used to improve the performance of the solvent. For instance, acetone can be added as a modifier to methanol in the extraction of curcumin from Curcuma longa using MAE [24]. Furthermore, room-temperature ionic liquids are becoming one of the most attractive solvent modifiers in the extraction process due to their outstanding solvent properties. They have many advantages, including good low vapor pressure, thermal stability, wide liquid range, miscibility with organic solvents and water, and perfect solubility of different bioactive compounds [22; 37]. Thus, they are preferable for unstable compounds since high solvent power can improve the extraction efficiency, and it lessens the overexposure to microwave heating [86]. In MAE, the sample should be in dried and powder form for an optimum extraction yield. If the particle size is too small, it causes a problem in the extraction and may require an extra washing stage [87]. Recently, microwave-assisted drying and extraction technique has emerged as a novel notion where microwave drying is combined with a condenser. The vapors (containing polyphenols) vaporized from the sample (to be dried) pass through the condenser and condense to the liquid extract. Using this approach, the bioactive compounds are extracted without the use of external solvents. The technique is suitable for the sustainable extraction of bioactive compounds from different fruits, vegetables, and herbal foods during drying [88]. Figure 5 shows a simplified schematic of extraction of polyphenols using MAE technique with the help of a condenser. As reported by [PERSON] et al. (2021), MAE has a better performance compared to the UAE method in the extraction of polyphenols from Brazilian olive leaves. This could be attributed to the greater cell wall disruption under microwave processing, resulting in faster release of the cell compounds into the solvent. Moreover, the temperature applied in MAE could enhance the permeation and the solubilization processes to wash the intracellular compounds out of the matrix.