# The Effect of Pre-Treatment on the Rehydration of Dried Apple Cube
## 3 Results and Discussion
This may be because high microwave power causes permanent cell rupture, tissue displacement and loss of structural integrity, resulting in significant shrinkage, dense structures and severely contracted capillaries. The reduced hydrophilic properties demonstrate lower water absorption capacity and prevent water accumulation, leaving pores unfilled [59; 60; 61]. In addition, the degree of rehydration of the obtained dried material was determined depending on the method of drying the material and the preceding preparatory treatments of the raw material before the drying process. Lower values of the degree of rehydration were obtained for samples dried without air circulation (Figure 7b). Due to the type of pre-treatment preceding the drying process, no significant changes were observed only in the case of samples not subjected to any pre-treatment (see Figure 7a,b). The lowest degree of rehydration was achieved for samples subjected to osmotic drying (Figure 7b). Blanching or microwave heating provided the same degree of material hydration. Slightly different results were obtained by [PERSON] et al. [62], who studied the effect of pretreating apples for drying (high-intensity electric pulses and osmotic dehydration in a sucrose solution) on their rehydration capacity. The authors concluded that the rehydration capacity of apples increased with the duration of osmotic dehydration, while blanched and pre-frozen samples exhibited higher rehydration capacity and were also firmer. The electrical  
Figure 5: Stabilization of the rehydration process; (**a**) air-circulated drying, (**b**) non-air-circulated drying. conductivity of the immersion medium increased with rehydration time but decreased with osmotic dehydration. Figure 6: Stabilization of the rehydration process; (**a**) samples without pre-treatment, (**b**) blanched samples, (**c**) microwave heated samples, (**d**) osmotically dried samples. On the basis of the conducted tests, the kinetics of rehydration were plotted (Figures 8-10). The research showed that the content of absolute humidity during low-temperature rehydration is most effective for dried apples, which were subjected to osmotic drying in a 10% starch solution in the previous treatment, Figure 8a,b. In addition, all samples that were dried in the circulating air dryer contained a higher absolute humidity content than those that were dried without the flow of the drying agent, regardless of the pretreatment. Interesting conclusions regarding the impact of the drying method on the rehydration degree of apple cubes can be found in the work of [PERSON] [63]. The author dried apple cubes using two methods: convection drying with an air stream at a velocity of 4 m/s and a temperature of 55 \({}^{\circ}\)C, and a combined method involving preliminary convection drying followed by microwave-vacuum drying (with preliminary drying times of 40, 100 and 160 min and microwave power levels of 240, 480 and 720 W). After drying, the apple cubes were rehydrated in distilled water at 21 \({}^{\circ}\)C. [PERSON] reported that the samples dried using the combined method exhibited better rehydration properties, as evidenced by their absorption capacity, dry matter retention and rehydration ability compared to those dried solely by convection. Based on the obtained results, the rate of rehydration was determined at the temperature of 20.0 \({}^{\circ}\)C \(\pm\) 1.0 \({}^{\circ}\)C. The rate of rehydration decreased during the process [64], and after about two hours it asymptotically approached zero (Figure 9). In the first 10 min of the irrigation process, a higher rate of water absorption was observed in samples dried without forced air flow than in the case of samples dried in an air-flow dryer (Figure 9a,b). Figure 7: Degree of rehydration R(t) of dried apple cubes; (**a**) air-circulated drying, (**b**) non-air-circulated drying. The high rate of water uptake during the initial stage of rehydration can be explained by the rapid filling of capillaries and cavities located near the surface [65].