## 3 Different Extraction Methods of Polyphenols
### Ultrasound-Assisted Extraction (UIAE)  
In this process, mechanical vibrations are produced by the passage of ultrasound waves through a liquid medium. The transmission of sound waves induces compression and rarefaction areas in the medium. This leads to the formation of many bubbles exploding at the next level. The collapse of these bubbles causes a phenomenon named acoustic cavitation. In other words, the growth and collapse of bubbles by ultrasonic effect is known as the acoustic cavitation phenomenon. The rarefaction phase is the growth of bubbles due to the reduction in local pressure, while the compression phase is the decrease in the bubble surface area. During acoustic cavitation, both the rarefaction and compression phases occur simultaneously. When the size of bubbles increases to above their critical extent, they implode and cause the cavitation phenomenon [47]. The cavitation phenomenon is divided into two categories, namely, stable cavitation and transient cavitation. When the acoustic pressure is inadequate and the bubbles do not reach their critical size, it is called stable cavitation. Conversely, when the pressure is adequate to generate the implosion, it is called transient cavitation. The growth and collapse of bubbles and release of energy at the molecular level can result in arising a high temperature (up to 5000 K) and pressure (up to 1000 atm) [47]. Indeed, UAE involves physical and chemical forces different from those involved in conventional solvent extraction. The cavitation resulting from the physical forces causes the breakdown of the plant cell membrane and enhances the extraction process of polyphenols [48]. In other words, the cavitation may cause hydration and swelling of herbal food matrices, which produce micro-bubbles and micro-jets rupturing the cell of the food matrix and favoring the penetration of the solvent into them. This could simplify the release of polyphenols, resulting in an increased extraction yield [49]. Ultrasound processes could be employed using various types of tools and apparatus depending on the aim of its usage. This variation could be ranged from basic ultrasonic water baths to more advanced high-power ultrasonic generators [47]. Figure 3 illustrates a simplified schematic of the extraction of polyphenols using UAE. The efficacy of the UAE is impacted by some factors, including extraction time, acoustic intensity, solid/solvent ratio, solvent type, temperature, and the height of the solvent around the sample within the extraction container [48]. Depending on the food matrix, volume, and ultrasound processing states, the temperature of the sonicated medium  
Figure 3: Simplified schematic of ultrasound assisted extraction of polyphenols. increases with the formation of hot areas due to cavitation. As shown in Figure 4, ultrasound waves are divided into two groups according to their usage in the food industry [47]. The processing parameters in UAE are power, amplitude, frequency, intensity, treatment time, volume, and composition of food [50]. The intensity of ultrasound indicates the amount of power added per unit area [47]. This factor is calculated by the relation between the acoustic power and the surface area of the probe (Equation (1)) [51]. The higher the ultrasound intensity is applied to the solvent mixture, the more violently the cavitation bubbles implode [52]. Ultrasonic power can be determined calorimetrically using Equation (2). By fitting the temperature data versus time and extrapolating to the initial time, one can calculate dT/dt. Moreover, acoustic energy density (AED) is measured using the relation between acoustic power and the volume of the treatment medium (cm\({}^{3}\) or mL) (Equation (3)). \[I=\frac{P}{A} \tag{1}\]  
\[P=mC_{p}\left(\frac{dT}{dt}\right)_{t=0} \tag{2}\]  
\[AED=\frac{P}{V} \tag{3}\]  
where \(I\) is the intensity, \(P\) is the power, \(A\) is the area, \(m\) is the mass, \(C_{p}\) is the specific heat capacity, and dT/\(dt\) is the initial rate of change in temperature during sonication [51].