## 3 Results and Discussion  
### Drying Kinetics and Drying Times  
The course of the infrared-convective drying (IR-CD) of apples with variable process parameters is presented in Figure 1. The obtained curves were characterized by a similar course; at the initial stage, a period of constant drying rate (linear decrease in material moisture) could be observed, followed by a period of decreasing drying rate (curvilinear decrease in material moisture). This is a phenomenon typical for drying this type of matrix, where the free surface water initially evaporates, followed by the water in deeper layers of the material, which is limited by its cellular structure (a barrier to the diffusion process) [23; 24]. As can be seen, the shorter the distance between the infrared lamps and the apple slices, the faster the water removal. A shorter distance from the heat source means higher surface heating and, consequently, more intense water evaporation [9; 25; 26; 27].  
Figure 1: Drying curves depending on the various process parameters (distance between the infrared lamps and the apple slices during infrared–convective drying, IR-CD: 10, 20, and 30 cm; specific energy input during pulsed electric fields pretreatment, PEF: 0, 1, 3.5, and 6 kJ/kg).  
In order to thoroughly compare the differences in the efficiency of the water removal from apples depending on the IR-CD and PEF pretreatment parameters, Figure 2 shows the time required to dry the material to MR = 0.02. Drying time was significantly dependent on both the distance between the infrared lamps and the apple slices (\(\eta^{2}\) = 0.862) and the specific energy input (\(\eta^{2}\) = 0.864). According to the drying curves (Figure 1), the shorter the distance between the infrared lamps and the apple slices, the faster the water removal. Increasing the distance from 10 to 20 cm and from 20 to 30 cm led to an extension of the drying time by an average of 7.4 and 15.2%, respectively. As mentioned above, the closer the infrared lamps were placed to the drying tray, the more intensely they heated the surface of the material being dried, which, in turn, increased the evaporation rate [9; 26; 27]. Moreover, the drying time of apples was reduced by applying pretreatment, i.e., pulsed electric fields (PEF). Supplying specific energy to apples at the levels of 1, 3.5, and 6 kJ/kg resulted in reduction of the drying time of that fruit by approximately 20, 11, and 19%, respectively (in comparison to untreated samples dried at the same IR-CD conditions). Based on those observations, two conclusions can be drawn. First, electroporation of the apple tissue cell membrane reduced the resistance to water diffusion, making it occur faster and more intensely in PEF-pretreated samples [28; 29]. Second, the degree of cell disintegration (CDI) increasing with the amount of supplied energy during PEF treatment had no direct impact on the drying time results. The values of this index were approximately 0.1, 0.2, and 0.4 for the samples with an energy input of 1, 3.5, and 6 kJ/kg, respectively. At the same time, the drying times of the minimally (1 kJ/kg) and maximally (6 kJ/kg) PEF-processed apples were statistically identical, despite significant differences in CDI. This indicates that it is worth considering milder processing of apples with PEF treatment in order to obtain satisfactory results with lower energy consumption [19; 30].