## 3 Results and Discussion
### Microstructure Changes after Pretreatments and Drying  
During the technological process of drying, the water content decreases, resulting in the stiffening and shrinkage of the material [68]. Figure 6 shows photos of the morphological structure of dried beetroot tissue, unsubjected and subjected to various types of  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **RR after 3 h [-]** &
\begin{tabular}{c} **Water Content after 72 h over** \\ **the NaCl Solution [g H\({}_{2}\)O/g d.m.]** \\ \end{tabular} \\ \hline Untreated & 4.89 \(\pm\) 0.28 ab * & 0.23 \(\pm\) 0.03 b \\ Blanched & 5.05 \(\pm\) 0.43 abc & 0.21 \(\pm\) 0.03 b \\ Soaked 10’ & 5.11 \(\pm\) 0.10 abc & 0.17 \(\pm\) 0.01 a \\ Soaked 20’ & 5.17 \(\pm\) 0.44 abc & 0.19 \(\pm\) 0.01 b \\ Soaked 30’ & 5.19 \(\pm\) 0.11 abc & 0.18 \(\pm\) 0.01 b \\ US 21 kHz 10’ & 4.68 \(\pm\) 0.17 a & 0.21 \(\pm\) 0.03 b \\ US 21 kHz 20’ & 4.80 \(\pm\) 0.66 ab & 0.19 \(\pm\) 0.01 b \\ US 21 kHz 30’ & 5.31 \(\pm\) 0.21 abc & 0.19 \(\pm\) 0.01 b \\ US 35 kHz 10’ & 5.30 \(\pm\) 0.53 abc & 0.28 \(\pm\) 0.01 c \\ US 35 kHz 20’ & 5.89 \(\pm\) 0.26 c & 0.25 \(\pm\) 0.05 b \\ US 35 kHz 30’ & 5.65 \(\pm\) 0.61 bc & 0.21 \(\pm\) 0.01 b \\ \hline \hline \end{tabular}  
* a–c—homogeneous groups are presented next to mean values (mean \(\pm\) standard deviation) as different letters within the same column, which indicate statistical significance (\(p<0.05\)).  
\end{table}
Table 2: Rehydration ratio (RR) after 3 h of rehydration process and hygroscopic properties expressed as water content after 72 h over NaCl solution [g H\({}_{2}\)O/g d.m.] of untreated dried beetroot and that subjected to pretreatments (blanching, soaking, ultrasound) and infrared–hot air drying.  
pretreatment. Dried beet tissue was characterized by a compact structure, low porosity, and high shrinkage (Table 1). Generally, no differences were observed in the structure of the untreated beet tissue and that treated with soaking before drying, and the effect of ultrasound at 21 and 35 kHz did not differ from the betroot tissue not subjected to pretreatment. Similarly, [PERSON] et al. [69] observed a minor effect of ultrasound treatment for beetroot. They explain that beetroots with lower porosity and smaller cells are less prone to acoustic energy-induced cell wall damage in comparison to samples with larger cells and medium porosity like apples. Also, in the case of eggplant, which is the most porous material among apples, beetroot, and eggplant, ultrasound application did not significantly alter cell size in comparison to the untreated sample. As a consequence, [PERSON] et al. [69] stated that the effects of ultrasound vary across materials with differing initial characteristics, e.g., porosities and cell sizes.  
Due to the cavitation, sponge effect, and accompanying phenomena, the ultrasound treatment of the plant tissue usually results in structural alterations. For example, ultrasound application significantly increased the cross-sectional areas of convectively dried apple tissue cells, particularly with longer sonication times [70]. Prolonged sonication causes further damage to the tissue, resulting in the formation of empty areas [26]. Other researchers have observed a similar effect during the sonication of pineapple [71], melon [72], and berries [73]. In our study, the lack of an effect on the beetroot tissue could also be related to the further drying process, which leads to shrinkage, affecting the final characteristics of the cells in the dried tissue.