# Hot Air and Microwave Combined Drying of Potato Monitored by Infrared Thermography
## 3 Results
If an energy balance is applied to the total sample (Equation (9)), the heat contribution by convection (q\({}^{cr}\)) and by microwaves (q\({}^{MW}\)) will be distributed in heating by conduction from the surface (\(Q^{U}-q^{cd}\)), the internal energy variation associated with microwave heating and surface water evaporation. This balance is shown in the following equation:  
Figure 3:3  
where \(U\) is the internal energy of the sample and E\({}^{v}\) is the evaporation energy. With Equation (9), it is possible to estimate the internal energy variation associated with microwave heating. Figure 8a shows the internal energy variation associated with microwave heating, obtained from Equation (9), with regard to the sample moisture on a dry basis. In order to better understand the heating process of microwave radiation, Figure 8c shows a diagram of the photon input at 2.45 GHz in the potato sphere, with a radius smaller than the microwave penetration depth, producing an accumulation of heat dissipation at the center of the sample. This phenomenon generates heat, which flows by conduction from the center to the surface. However, a small part of the dissipation of the electromagnetic energy of the photons is lost along the way, which is more evident in water molecules in the gas state because of their high mobility, flowing through the boundary layer, increasing the air surface temperature or producing more evaporation. Figure 3:4 Microwave sample effect: (**a**) The relationship between the microwave energy distribution ratio and the sample moisture expressed on a dry basis and (**b**) microwave heating throughout the drying time for HAD + 4 W/g (a) and HAD + 6 W/g (b) treatments. Considering that the evolution of the individual coefficient of heat convection is similar in all treatments, it is possible to calculate the surface heating by microwaves (\(E_{MW}\)) by comparing, at each time, the surface temperature of each combined treatment with the HAD treatment, according to the following equation:  
Figure 3:5  
where \(\rho_{AIR}\) is the air density (kg/m\({}^{3}\)) obtained by the thermodynamic properties or air [6], \(X\) is the absolute moisture of air (kg\({}_{\mathrm{w}}\)/kg\({}_{\mathrm{dryair}}\)) and \(C_{P}{}^{\mathrm{v}}\) is the specific heat of vapor at the surface temperature. Figure 8b shows the surface heating by microwave evolution for combined drying treatments, where it is possible to observe that internal heating (Figure 8a) is 10,000 times higher than the surface heating by microwaves (Figure 8b). Consequently, in a combined drying process of hot air and microwaves at temperatures lower than the spontaneous evaporation temperature and in samples with a characteristic dimension less than the microwave penetration depth, it is possible to conclude that the convection heating is mostly transformed into surface water evaporation, and microwave heating is mostly transformed into an increase in the internal energy of the food, that is, an increase in the internal temperature. Therefore, the measurement of the surface temperature by FTIR in the HAD process combined with microwave heating allows the HAD process and surface water evaporation to be monitored, but not the microwave heating process, because MW heats with high penetration. Thus, the FTIR technology allows the segregation of the effect of HAD and microwave heating in combined drying processes, making it possible to determine water removal. In the future, IR cameras with a higher frequency spectrum will be able to make direct measurements of the surface water content.