###### Abstract  
This study aimed to examine the impact of the combination of acoustic energy at the nominal powers of 100, 200, 300, and 400 W with moderate heat processing at 40, 50, and 60 \({}^{\circ}\)C on the extraction of phytochemical compounds from _Feniculum vulgar_. Thermosonication processing, based on high-intensity ultrasound combined with an external heat source, can potentialize the extraction of soluble solids from plant material. However, the excessive temperature increase generated by the two energy sources during thermosonication treatment may degrade the thermolabile bioactive compounds. Regardless of the temperature condition, fennel extracts obtained at 400 W presented lower total phenolic content (TPC) than those obtained at 300 W. The cavitation heat and mechanical stress provided at 400 W may have degraded the phenolic compounds. Thereby, the best extraction condition was 300 W and 60 \({}^{\circ}\)C. The fennel extract presented the highest content of TPC (3670 \(\pm\) 67 \(\mu\)g GAE/g) and antioxidant activity determined by DPPH and ABTS methods (1195 \(\pm\) 16 \(\mu\)g TE/g and 2543.12 \(\pm\) 0.00 \(\mu\)g TE/g, respectively) using this treatment. Thermosonication can be an innovative technique for extracting phytochemicals because it provides good results in shorter processing times, with 73% and 88% less energy consumption than Percolation and Soxhlet techniques, respectively. Feniculum vulgarue; high-intensity ultrasound; thermal history; percolation; Soxhlet +
Footnote †: journal: Medical  
temperatures generally above room temperature [6]. Thereby, these conditions can favor the thermal degradation of thermolabile compounds. Thus, emerging technologies have been evaluated to extract these compounds to overcome the drawbacks of traditional techniques. Innovative techniques include extractions that use supercritical CO2 or pressurized liquids as the solvent, microwave, ultrasound, etc. These techniques enable the use of a smaller solvent volume, promoting higher yield, increasing process reproducibility, and generating fewer toxic residues, besides shorter extraction times [3]. Among the emerging technologies, ultrasound stands out for providing highly efficient processes. The application of low-frequency (16 to 100 kHz) and high-power ultrasound (>1 W/cm2) waves in liquid media generates the acoustic cavitation phenomenon. Sonication promotes the sonoporation effect in plant materials. It increases the temperature in the extraction system, favoring the release of phytochemical compounds linked to plant cells and their diffusion into the solvent [7]. The rupture of cellular organelles and the temperature rise can increase the extraction yields [8]. Likewise, acoustic energy with moderate heat processing can be an innovative technique for recovering phytochemicals. This combined treatment is known as thermosonication [9; 10]. Thermosonication is a relatively new technique that has been used for kinetic, microbiological, and enzymatic stabilization of foods and beverages [11; 12]. Thermosonication treatments are based on the combined impact of the acoustic and thermal energies to inactivate pathogenic and spoilage microorganisms and endogenous enzymes. Acoustic energy is provided by an ultrasound system and the thermal energy from an external heat source. Thus, the application of combined acoustic and thermal energies increases the temperature of the sample, favoring the diffusion rate of bioactive compounds into the solvent. The temperature rise may also be unfavorable since some phytochemicals are thermolabile. Thereby, an accurate thermosonication process design is required to know the potential impacts of the combined action of acoustic and thermal energies on the target phytochemical compounds. Thermosonication treatments applied for the extraction of phytochemicals remain scarce. Some studies technically performed thermosonication but did not call it as such; maybe the setup was not the exact setup of the technique [13].