## 3 Results and Discussion
### Impact of Thermosonication on the Total Phenolic Content  
Figure 3 presents the influence of the nominal power and the extraction temperature on the fennel extracts' total phenolic content (TPC). The increase in nominal power until 300 W and rise in temperature increased the TPC (\(p\)-value < 0.001). The TPC varied from 636 \(\pm\) 4 to 3670 \(\pm\) 67 ug GAE/g. The fennel extract produced at 60 \({}^{\circ}\)C and 300 W presented the highest phenolic content.  
Acoustic cavitation is the phenomenon responsible for the extraction of interest compounds by ultrasound [14]. The mechanical and thermal effects provided by the acoustic cavitation can break the cell walls of the plant material, increasing the surface area of  
Figure 3: Thermosonication effects on the total phenolic content (TPC) of fennel extracts.  
Figure 2: Thermosonication effects on the visual appearance of the fennel extracts.  
contact between the solid and liquid phases. The increase in contact area favors greater penetration of the solvent into plant cells, facilitating the diffusion and mass transfer of phytochemical compounds from the raw material to the solvent. The nominal power increase from 100 W to 300 W enhanced the extraction of the phenolic compounds from fennel. [PERSON] et al. [23] observed the same behavior increasing the ultrasound intensity from 1.1 W/cm\({}^{2}\) to 13 W/cm\({}^{2}\). This ultrasound intensity increase resulted in a high yield of anthocyanins recovered from the jabuticaba peel. The intensification of the acoustic cavitation provided by the increase in the ultrasound intensity favored the mass transfer rates. A similar study reported better efficiency in extracting phenolic compounds from orange peel, increasing the ultrasound nominal power from 50 to 150 W [24].  
Otherwise, the fennel extract produced with 400 W and 60 \({}^{\circ}\)C presented a lower TPC than the extracts obtained with lower nominal power at the same working temperature. This result can be associated with the cushioning effect proportionate to high temperatures during the thermosonication process, since the extraction carried out at 60 \({}^{\circ}\)C reached up to 77 \({}^{\circ}\)C throughout the thermosonication performed at 400 W (Figure 1). The temperature rise can increase the vapor pressure and decrease the solvent's viscosity, favoring the formation of larger cavitation bubbles. In this case, the amount of steam inside the bubbles is greater, which dampens the implosion of the bubbles during cavitation and, consequently, reduces the acoustic cavitation performance on the extraction rate. In the thermosonication processes, there is an optimum temperature for maximum cavitation in liquid media at which the strongest effects of ultrasound emerge.  
The global yield obtained at 400 W was higher than those obtained by the other nominal powers studied (Table 1), but the content of total phenolic compounds was lower (Figure 3). Thus, the high temperature achieved during the thermosonication-assisted extraction may also have degraded the extracted phenolic compounds. Moreover, the thermosonication treatment performed at 40 \({}^{\circ}\)C extracted more phenolic compounds than at the other temperatures evaluated. [PERSON] et al. [25] also observed a decrease in phenolic compounds' content of carrot juice after increasing the working temperature of the thermosonication process (110 W, 40 kHz) from 40 to 60 \({}^{\circ}\)C. The authors stated that an increase in temperature might result in the degradation of phenolic compounds.