**6. Conclusions**

From the thermo-acoustic analysis for 0.5 cm-plate apple and tomato samples inside an ultrasound-assisted convection dehydrator assisted by ultrasound waves, and considering the physical dimensioning of the dehydration chamber, the physical parameters of the apple and tomato, and the wave and heat equations, we can determine the spectral behavior of ultrasound waves inside food and the temperature changes on the apple and tomato samples. Based on this analysis, we identify the optimum operating frequencies from the average pressure applied in the dehydration system for apple and tomato samples using ultrasound waves at 34 kHz and 70 kHz, respectively. Using these operating frequencies, we performed a spatial analysis when the ultrasound-assisted convection dehydrator was implemented with one and three piezoelectric transducers radiating a test acoustic field at 2 Pa. However, the spectral analysis showed that using ultrasound waves at 34 kHz for apples, the acoustic field was more uniformly distributed on apple samples located at five levels inside the dehydration chamber using three piezoelectric transducers. The same phenomenon occurred when the tomato was radiated at 70 kHz with three piezoelectric transducers. In addition, considering the distribution of the acoustic field in the two case studies, the temporal analysis to obtain the temperature changes inside the apple and tomato samples at the test frequencies was performed. The results obtained showed that when the five racks were used to place the apple and tomato samples and they were radiated using one piezoelectric transducer, only the temperature of food sample closest to the transducer was increased, while the rest of the food samples remained without significant temperature changes. Note that for the case of a single piezoelectric transducer, the pressure levels ranged between 80 kPa for apples and 22 kPa for the tomato to reach a temperature close to 70°C, which avoids structural and nutritional damage to the foods. When three piezoelectric transducers were used, uniform temperature changes were observed, and a similar temperature increase was observed in food samples of the five racks. Under these conditions, the apple and tomato samples reached temperatures between 67 and 70°C. When apple and tomato samples were radiated with three piezoelectric transducers using pressure between 52 kPa and 5 kPa respectively, less energy was required than when the food samples were radiated with one transducer. Thus, from the FEM analysis, it was possible to determine the optimal operating conditions at which an ultrasound-assisted convection dehydrator for apple and tomato samples can operate most efficiently based on its spectral and thermodynamic behavior.
