## 3 Different Extraction Methods of Polyphenols
### Ultrasound-Assisted Extraction (UIAE)
There are plenty of merits stemming from the usage of UAE, including low temperature, optimized energy and mass transfer, selective extraction, effective mixing, small equipment size, high extraction rate, fast start-up, and improved production [53,54]. However, some studies have outlined that UAE might have some drawbacks, such as the reflection of the produced off-flavors in the food [55]. During the last few years, UAE has been one of the most utilized advanced techniques for the recovery of polyphenols from food by-products [56]. Table 1 highlights the recent research on the usage of UAE in the recovery of polyphenols from food by-products. Figure 4: Classification of ultrasound waves according to their usage in food science. As declared by previous studies, increasing the extraction time could positively impact the extraction yield of polyphenols [65]. However, it is proven that the UAE with a higher amount of power and shorter times is more efficient because the long duration of sonication could cause the production of some free radicals from water, inducing the deterioration of polyphenols by activating radical chain reactions. In other words, the increase in the power in the UAE method could increase the extraction yield of polyphenols and shorten the time required for the extraction. In addition, the reduction of UAE time can lessen energy consumption significantly [62]. Increasing the extraction time higher than a threshold has no meaningful change in the phenolic content. This condition could be justified by [PERSON]'s second law of diffusion, stating that an equilibrium concentration between a solution and a solid matrix occurs after a certain period. Therefore, it is not necessary to increase the time for further extraction of polyphenols [61]. Since UAE is a nonthermal method, some works in the literature have proposed using thermal profiling to control the temperature of the extraction process [62; 67]. However, when ultrasound is used as an extraction method, the heat generated in the medium solvent may be advantageous for enhancing extraction yield because it alters the membrane structure of herbal cells and simplifies the distribution of the extracted molecules into the solvent [62; 68]. Nevertheless, due to the sensitivity of polyphenols to heat, their antioxidant activity may be lost when the temperature is greater than 55 \({}^{\circ}\)C [61]. In UAE, the polarity of the solvent could increase when there is a growth in the cavitation or bubble formation. This can result in an improved extraction yield [63]. As reported by [PERSON] et al. (2019), the solvent has a substantial influence on the recovery yield of polyphenols. When the combination of organic solvents and water is employed as a medium in this process, the yield rises [52]. [PERSON] et al. (2020) reported that the extraction yield of polyphenols with 70% ethanol is higher than 96% ethanol. Indeed, the addition of water to pure ethanol provides better distribution of polyphenols and enhances the yield of this process [59]. [PERSON] et al. (2021) stated that solid/solvent ratio is a prominent factor in the extraction yield as the increase of the solid weight may decrease the recovery yield of polyphenols because of the extraction of other biomolecules (e.g., proteins and polysaccharides), which could dissolve in the solvent and influence the dissolution of polyphenols [61]. [PERSON] et al. (2017) reported that UAE with a low frequency and high power is much more efficient. Higher extraction yield of polyphenols at a lower frequency might be linked to the increased intensity of acoustic cavitation in the solvent because cavitation