## 3 Results and Discussion
### Investigation of Drying Kinetics in Different Strategies
At the same time, the temperature of the outer parts increases with the hot airflow, and, gradually, the temperature difference between the slices with the surrounding environment increases, which ultimately increases the moisture removal rate of the product [43]. In similar research [8], by using the combined HA-IR strategy, the drying time was reduced by 40.8% and 29.8% compared to hot air and infrared drying, respectively. According to their results, the optimal drying time (225 min) was obtained via the combined strategy of hot air at 60 \({}^{\circ}\)C with an infrared intensity of 0.3 W/cm\({}^{2}\). In the third drying strategy (HA+IR), the highest drying time (300 min) was related to the HA40+IR250 method, and the lowest drying time (120 min) was for the HA70+IR750 method. Similarly, in research [23], the drying time of sweet potato slices in drying with the HA+IR strategy was in the range of 125-180 min. Figure 2c is the plot of the dimensionless moisture ratio related to the third strategy, in which the drying process was in two stages (HA+IR), first starting with hot air and then continuing with infrared waves. As shown in the figure, the slope of the moisture ratio curve changes suddenly at the beginning of the second stage (after 60 min). When a low temperature was used in the first step and  
Figure 2: Dimensionless moisture ratio against time for different strategies: (**a**) first strategy, (**b**) second strategy, (**c**) third strategy, (**d**) fourth strategy. followed by a high level of infrared intensity in the second step (such as HA40+IR750), this change in the curve slope was quite noticeable. Conversely, in the first stage, there was a high level of air temperature, and then, in the second stage, a lower level of infrared radiation intensity was used (such as HA70+IR250). The reason for this is that, in the first case, using the hot air flow with a low temperature in the first 60 min could not remove the free moisture of the apple slices and bring the drying product to the falling phase of drying. Therefore, when infrared waves with high radiation intensity are used in the second stage, the thermal gradient increases at once, and this causes the moisture of the apple slices to remove at a high rate. On the contrary, when hot air flow with a high temperature is used to dry the product in the first stage, free moisture and surface moisture are removed. Finally, a hard layer is formed on the apple slices when the surface moisture evaporates. Therefore, when infrared waves with low radiation intensity are used in the second stage, the infrared waves will not be able to penetrate the apple slices, and the moisture removal rate suddenly drops sharply. In both cases, due to the significant and sudden change in the energy level received by the apple slices, the slope of the dimensionless moisture ratio plot undergoes a noticeable variation at the moment of changing the drying method. In general, it can be observed from the results that, in two-stage drying, it is better not to suddenly reduce the energy level that reaches the apple slice in the second stage. It is better to use higher air temperature or radiation intensity for the second stage of two-stage drying. Figure 2d is related to the fourth drying strategy (IR+HA), which used infrared radiation in the first stage and hot air in the second stage. The plot shows that, when infrared waves with high radiation intensity were used in the first 60 min, the slope of these plots was even higher than that in the plots obtained in the third strategy. The reason is that, in the first 60 min, infrared waves focusing on the drying product cause a high-temperature increase inside the apple slices, and a high thermal gradient is created with the surrounding environment. Finally, this causes the free moisture of the product to evaporate. However, when low radiation intensity was used, due to the delay in increasing the internal temperature, the slope of the graph was less than that in the third strategy.