## 3 Results and Discussion
### Investigation of Drying Kinetics in Different Strategies
Moreover, the results showed that the slope of the graph at a time interval of more than 60 min was lower than that in the graphs obtained in the third strategy. The reason is that, in almost all the treatments, the samples enter the falling phase after the first 60 min and the rate of moisture exit decreases. On the other hand, due to the lower thermal gradient of the product in the hot air-drying method compared to infrared, the evaporation rate decreases and increases the drying time of the samples. The longest drying time was 490 min for the IR250+HA40 method, and the lowest drying time was 90 min for the IR750+HA70 method. In this research, the drying time in the second strategy (HA-IR) (144 min on average) was shorter than that in the other strategies. Thus, in the second strategy, a decrease in drying time was obtained compared to the first, third, and fourth strategies, with an average of 20.48, 30.10, and 42.73%, respectively. Among all the methods, HA70-IR750 had the fastest drying time, with a duration of 80 min. The shortest time in the combined strategies (HA70-IR750) was about 27.2% less than the shortest time in the hot air method (HA70). In a similar study [23], for the IR+HA drying strategy, the drying time varied from 96 to 113 min. In that research, the IR+HA strategy emerged as the best strategy to shorten the drying time. The main reason for this difference in results may be the difference in the levels of infrared radiation intensity used in these two studies because the intensity of the infrared radiation used in that research was higher, and the drying time was also longer in the first stage of that research (90 min), so, in the first stage, a large amount of moisture evaporated from the product, and the drying time was reduced. On the other hand, due to the lower intensity of infrared radiation used in this research, infrared waves cannot alone increase the temperature of the product quickly at the beginning of the drying process, and, due to the little thermal gradient created with the outside air, the moisture exiting rate at the beginning of drying was not very high. However, the reason why the HA-IR combined strategy used in this research was reported as a more favorable option may be because the heat generated in the product intensified when two energy sourceswere used simultaneously. In this case, the infrared waves penetrate the product and raise the internal temperature, and, on the other hand, the hot air changes the temperature of the product's outer parts, therefore the inside temperature of the apple slices increased rapidly at the beginning of the drying process. The reason that the HA-IR strategy was not the best option in that research [25] is probably due to the high air velocity (1.5 m/s) used in that research and the dominance of the reverse effects. Increased air velocity during drying apple slices in a combined dryer with infrared waves sometimes has reverse effects on drying and has also been reported in another study [8]. Another finding of this research is that, in terms of time, it is better to use infrared waves first and then use hot air flow in the two-stage drying strategy because, in the reverse mode, hot air creates a hard layer on the outer parts of the product and makes it difficult for infrared waves to penetrate, and the drying time increases.