# Hot Air and Microwave Combined Drying of Potato Monitored by Infrared Thermography
## 3 Results
Figure 6b represents the boundary layer depth for all treatments, where it is possible to observe how the thickness decreases faster depending on the power of the MW treatment. Figure 5: The individual coefficient of heat convection with regard to (**a**) surface temperature in the HAD treatment and (**b**) drying time. In the case of combined drying methods, the microwaves absorbed by the samples represent the quantity of photonic energy at 2.45 GHz that dissipates in electric and thermal energy throughout the microwave penetration depth (\(\delta^{MW}\)). In this frequency range, the main effect of photon radiation is the gamma dispersion, which consists in the orientation and induction of dipolar molecules, water being the main dipolar molecule in potato tissue. The microwave penetration depth can be estimated as follows [6]:  
\[\delta^{MW}=\frac{|\epsilon|}{2\pi f}\frac{\sqrt{\epsilon^{\prime}}}{\epsilon^ {\prime\prime}} \tag{6}\]  
where \(|\epsilon|\) represents the modulus of permittivity at 2.45 GHz, \(\epsilon^{\prime}\) is the dielectric constant, \(\epsilon^{\prime\prime}\) is the loss factor and \(f\) is the frequency expressed in Hz (2.45 GHz). An important characteristic of microwave heating is that when the characteristic dimension of the sample (radius) is less than microwave penetration depth, heating occurs from the center of the sample to the surface [19]. In this experiment, \(\delta^{MW}\) was estimated using Equation (6), obtaining, for raw potato, a value of 1.62 \(\pm\) 0.06 and decreasing throughout the drying process to 0.83 \(\pm\) 0.08 at 440 min, always being greater than the radius measured at each time. The proportion of microwave energy that has been absorbed and transformed into thermal energy can be estimated by the dielectric properties at 2.45 GHz. In order to estimate the amount of microwave energy absorbed as thermal energy, the microwave energy distribution ratio (\(r_{MW}\)) from [6] was used by means of dielectric properties (Equation (7)). Figure 3:0  
Figure 7a shows the relationship between \(r_{MW}\) and sample moisture expressed on a dry basis, where it is possible to observe that the microwave energy distribution ratio remains constant while the moisture is high, and when it reaches a moisture of around 0.7 on a dry basis, it decreases exponentially. This phenomenon may be because the critical moisture of the potato is approximately 0.7, and with that value, the water that remains in the tissue has less mobility, and the ease of inducing water molecules is less than the ease of orienting them. Consequently, the absorption of energy as electrical energy storage increases compared with the dissipation as thermal energy. Figure 3:1 (**a**) Plot of the thermal conductivity of dry air (\(\bullet\)) and vapor (\(\square\)) with regards to the temperature and (**b**) plot of the boundary layer depth with regards to the drying time: HAD treatment (\(\blacksquare\)), HAD + 4 W/g treatment (A) and HAD + 6 W/g treatment (\(\bullet\)). The HAD + MW combined drying equipment measures the total power emitted by the magnetron and the power returned to the emission tube (reflected power). The difference between the two is the microwave power absorbed (\(P_{abs}\)) by the sample. Therefore, with the absorbed power and \(r_{MW}\), the heating by microwaves (q\({}^{MW}\)) can be calculated with the following equation. Figure 3:2  
Figure 7b shows the evolution of microwave heating throughout the treatment. It is possible to observe that the heat remains constant until a process time of 180 min, whereupon the microwave heat decreases. This drying time represents the moment when the sample reaches critical moisture, whereupon the water molecules reduce their mobility and hinder the induction phenomena, while their capacity to store electric energy remains, as seen in Figure 7a.