# Hot Air and Microwave Combined Drying of Potato Monitored by Infrared Thermography
## 1 Introduction  
During hot air drying (HAD) at temperatures below the spontaneous evaporation temperature, the main force that drives water removal from the product is the gradient of water chemical potential generated between the air, with a very low relative humidity, and the product, with high water activity, causing a water flux from the product to the air [1]. Liquid water is transported from inside the sample to the surface, where it evaporates [2,3]. This state change requires the input of energy, which is generally obtained from the internal energy of the air in the sample surroundings, decreasing its temperature. However, it can also be obtained from the internal energy of the surface of the product, thus cooling it. The diversity of water molecule interactions that occur inside the potato tissue [4], as well as the different ways that water molecules have to reach the surface, make the internal water transport fast at the beginning of drying, as the water molecules with high mobility are mobilized through the fastest route (apoplastic pathways). In this drying period, surface evaporation is the limiting factor of the overall transport. After this period, more retained molecules (critical moisture) are mobilized through slower routes, causing slower internal transport. In this period, the internal transport becomes the limiting factor [5].  
Another possibility is to combine the thermodynamic inertia of HAD with another source of heat, such as microwave radiation [6]. The interaction of photons with biological systems at low frequencies of the electromagnetic spectrum (Hz to GHz) causes orientation and induction phenomena. Gamma (\(\gamma\)) dispersion occurs at microwave frequencies (between MHz and GHz), and it is produced by the orientation and induction of dipolar molecules [7], water being the most important dipolar molecule in biological systems. Gamma dispersion produces the transformation of photonic energy into electrical energy by molecular spin orientation and into thermal energy by molecular collisions and frictions. Many food products have an important aqueous phase; therefore, the application of microwaves in heating processes is a possible source of energy. Microwave radiation has a high penetration depth, producing an internal heating front [8; 9]. Therefore, combined HAD-MW drying could reduce the drying time [9; 10]. This technique has been applied in different foods: in orange peel [11], in nectarine slices [8] and in shiitake mushrooms [12].  
In order to evaluate the effect of HAD and MW, it is necessary to analyze thermal conditions on the surface of the product. Infrared thermography (FTIR) is a technique that measures the surface temperature of materials in a non-destructive way [13; 14; 15]. This technique has been used in the food industry to evaluate pork quality [16], to detect differences between raw and cooked chicken meat [17; 18], to evaluate the freezing process of potato [4], to control disinfection with the steam of carrots [19], to detect the position and depth of bruises in apples [20] and to evaluate injuries of cold-stored guava [14]. Thermal imaging has also been demonstrated to be a useful tool in monitoring the surface temperature in drying processes in the food industry in combined dryings of kiwifruits [21], HAD combined with microwaves to dry mushrooms [22], HAD of corn kernels [23] and vacuum drying of banana slices [24].  
The aim of this paper is to determine the effect of MW in the combined HAD-MW drying of raw potato at a drying air temperature below the spontaneous evaporation temperature, with the potato having a smaller characteristic dimension than the MW penetration depth, in order to obtain the real driving forces and the mechanisms involved in the water transport, with the purpose of optimizing the MW power used.