## 3 Results and Discussion
### Rehydration Properties and Hygroscopic Properties  
The drying process damages the structure of the material, reduces the ability to bind water, and makes it impossible to restore the original volume of the raw material. There are two important properties in the case of rehydration: increasing the weight of the dried material and the loss of soluble components of the dry matter. They depend on the kind of tested material, pre-treatment, and drying conditions [49]. The values of the relative weight gain expressed as rehydration rate (RR) and the loss of soluble dry matter components (SSL) were determined in dried apples. The response surface plots of the RR and SSL of hybrid dried apples are presented in Figure 4. The RR on the reproducibility of the dried apples showed that US pre-treatment and US power during drying had a significant (quadratic) effect on the reproducibility of the dried material (Figure S3a). The highest weight gain was characteristic for the samples obtained after the average US treatment time (20 min) and at the lowest US power during drying (120 W), and a slightly lower RR was noted for samples subjected to 20 min of US pre-treatment and US-assisted drying with 200 W (T_20\(D\)200W). However, the sample treated for 20 min of US pre-treatment and US-assisted drying with 120 W (T_20\(D\)120W) was significantly different from the others and was characterized by the best reconstitution properties. The formation of microchannels appears to be the key structural change driving improved water-binding during rehydration. Ultrasound applied both before and during drying facilitates the drying process and causes the structural changes of the plant material [2]. Better rehydration properties of dried fruit or vegetables after using the US were observed by [PERSON] et al. [19] for apples, [PERSON] et al. [50] and [PERSON] et al. [51] for carrots, and [PERSON] et al. [52] for beetroot. [PERSON] et al. [50] explain the higher weight gain during the rehydration of US-pre-treated dried materials by the formation of US microchannels, which causes higher porosity and facilitates the penetration of water into the cell. Increasing the time of US pre-treatment from 20 min to 30 min resulted in a decrease in weight gain. [PERSON] et al. [19] also found that increasing the duration of pre-treatment from 5 to 15 min resulted in a decrease in the ability to rehydrate the dried apples. The same conclusions were reached by [PERSON] et al. [50], who indicated that a shorter US treatment of 30 min allowed a higher water-binding capacity of dried carrots to be obtained, compared to the initial process carried out for 60 min. In our study, the result of the best rehydration rate of dried materials obtained after US treatment (with an average time of treatment of 20 min) and US-assisted drying (with the lowest US power applied during drying) can be explained by structural modifications caused by the combination of sonication before and during drying. During the rehydration process, a loss of soluble components of the dry substance, such as vitamins, sugars, or acids, is also observed. The outflow of these substances into the surrounding solution is determined by the chemical composition and internal structure of the material [44]. The loss of soluble components ranged from 0.54 \(\pm\) 0.02 to 0.64 \(\pm\) 0.03 (Figure 4b). The response surface of the loss of soluble components (SSL) of the dry substance (Figure S3b) shows that US treatment time had a significant, quadratic effect on the obtained values. The lowest SSL was obtained for samples after 20 min of US pre-treatment. [PERSON] et al. [45] found that the extension of sonication time from 10 to 30 min resulted in a higher loss of soluble dry substance components in the apples obtained by convection drying at 70 \({}^{\circ}\)C. This was explained by the higher structural damage of the raw material that occurred during the longer pre-treatment. The water content after 24 h was in the range of 13.25 g/100 g d.m. to 16.71 g/100 g d.m.