## 3 Results and Discussion
### The Kinetics and Energy Intensity of Drying
Once the aforementioned moisture content--frequently designated as the critical moisture content--is surpassed, a falling drying rate period ensues, persisting until the end of the process. By analyzing the temperature curves (Figure 2c), it can be seen that in the microwave-assisted processes (CVMW, CVMWUS) the temperature of the material increased much faster and reached a significantly higher value than for the convection (CV) or ultrasonic-assisted (CVUS) processes. This phenomenon is due to the effective interaction between the microwaves and the moist material being dried, and results in high drying rates and short processing times. The temperature curves also demonstrate the beneficial impact of ultrasound on heat transfer. The application of acoustic waves in the CVUS process resulted in a significantly enhanced heating rate when compared to the convection process (CV). This phenomenon may be attributed to ultrasound-induced effects at the material's surface, which result in a reduction of the boundary layer's thickness and thus enhances heat transfer efficiency and the absorption of ultrasonic energy [26]. An analysis of the shape of the temperature curves (Figure 2c) also reveals the similarity between the CV and CVUS processes, and between the CVMW and CVMWUS processes. In the case of convective (CV) and ultrasound-assisted convective (CVUS) drying, there are no evident characteristic periods, and the curves move asymptotically towards the temperature of the drying agent. For the microwave processes (CVMW and CVMWUS), three stages can be distinguished. During the first, there was intensive heating to a temperature of about 60 \({}^{\circ}\)C. This stage lasted about 1200 s and corresponds to the initial stage on the drying rate curves (Figure 2b). During the next stage, the temperature of the material was approximately constant (about 60 \({}^{\circ}\)C) and lasted up to 3600 s; this coincides with the constant drying rate period (Figure 2b). During the last stage, the temperature increased again to reach a value of about 100 \({}^{\circ}\)C at the end of the process. This period corresponds to a falling drying rate period. Concerning the microwave processes, the intense heating during the initial stage was the result of effective absorption of microwave radiation by the very moist material. In this stage, intense moisture evaporation also began, leading to an increase in the drying rate (Figure 2b). The constant temperature of the material during the constant drying rate period (from 1200 s to 3600 s of the process) was due to the very intensive evaporation of moisture from the sample's surface. Because of this, convectively supplied and microwave-generated heat is used to evaporate moisture and does not heat up the material being dried. This phenomenon occurs only when there is enough moisture in the material, so that it can be transported efficiently to the surface/evaporation front, and that transport within the material does not limit the process rate. Given the constant value of the heat of vaporization, the rate of drying (evaporation) is thus primarily determined by the amount of energy applied per unit time. If drying is conducted under constant process conditions (i.e., constant air temperature and radiation power), it can be assumed that the amount of energy supplied to the surface of the material being dried is also constant. Consequently, the evaporation rate remains constant, resulting in a constant drying rate (Figure 2b) and also in almost constant temperature of material [34]. Once the moisture content of the material has decreased below a critical value, the effectiveness of the moisture transport to the surface/evaporation front drops. The energy applied to the material is then used not only to evaporate the moisture but also to heat the material, resulting in a temperature increase (Figure 2c).