# Hot Air and Microwave Combined Drying of Potato Monitored by Infrared Thermography
## 3 Results  
The drying process at temperatures below the spontaneous evaporation temperature implies that evaporation occurs through thermodynamic inertia in the air/sample interface, that is, the gradient of water chemical potential between the air surrounding the sample and its surface. Then, water evaporation occurs from the sample surface, and thus, the water transport inside the sample occurs in the liquid state. The terms of the water chemical potential that drive the water transport are those associated with the difference between the surface water activity of the sample and the relative humidity of the drying air and also the difference between the drying air temperature and the sample surface temperature. These differences are produced in a thin air layer, called a boundary layer, with a thickness dependent on the phenomena of water transfer and heat transmission. Figure 2 shows a scheme of heat and mass exchange throughout the boundary layer. In this figure, it is possible to observe two planes in which some mechanisms converge; the first is the interface between the air and the surface of the sample, where heat convection is used to evaporate water (induced by the water chemical potential) and to heat the surface, and the second is the microwave heating plane, which is defined by the microwave penetration depth. Figure 2: Scheme of heat and water mass transport mechanisms in a combined hot air drying (HAD) and MW drying process, where q represents heat, J\({}_{\text{W}}\) represents water flux, \(\varphi^{\text{e}}\) is the drying relative humidity, T\({}^{\text{e}}\) is the drying temperature, T\({}^{\text{s}}\) is the surface of the sample temperature, a\({}_{\text{w}}\)\({}^{\text{s}}\) is the water activity of the sample in the surface, the superscript MW means microwave, CV means convective and cd is conduction. It is necessary to define the variables that characterize each of these planes in order to quantify the phenomena that occur in the combined process of HAD and MW. As explained in the Materials and Methods section, the surface temperature was obtained by FTIR, with a reference material with known emissivity to remove the effect of the reflected energy from the sample surroundings using the equation of [1]. With this expression, it is possible to obtain the emissivity of the sample and, therefore, its surface temperature. Figure 3a shows the emissivity with respect to the sample moisture (expressed on a dry basis), since in the far IR spectrum, the water molecule proportion is one of the main factors that affect the emissivity [6]. It is possible to observe how the emissivity decreases as the moisture decreases. In Figure 3b, it is possible to observe the relationship between the emissivity and the energy emitted by the sample without the effect of the reflected energy from the surroundings. This energy was estimated with two reference materials, one situated close to the sample and the other situated toward the door of the dryer (the door is a ventilation grill with a mesh size of 2 mm, where the air can cross without limitation, as can photons in the infrared range, but photons in the microwave range cannot pass). This energy is needed to obtain, with the emissivity, the real surface temperature. Considering that the K-thermocouples could only be used in the HAD experiment, the emissivity obtained in this experiment was used to correct the surface temperature measured by FTIR in combined HAD-MW experiments, using the moisture of the sample to obtain the value. Figure 4 shows the evolution of surface temperature in each treatment. This figure shows how the surface temperature increases, in all treatments, throughout the drying time. In the case of the HAD treatment, the surface temperature increases and reaches equilibrium with the drying temperature; however, the combined HAD-MW treatments increase the surface temperature throughout the drying time to above that in the HAD treatment; that is, the temperature increases beyond the drying temperature.