# Current Applications of Ultrasound in Fruit and Vegetables Osmotic Dehydration Processes
## 3 Sonication
### Structure Changes of Material Subjected to Sonication and Osmotic Process  
The US treatment in the plant tissue results in the structural changes of the tissue, due to several different effects undergoing through the sonication describe in previousparagraph. The plant material treated with US is characterized by multiple breakdowns of cells causing changes in tissue structure, as a consequence of the cavitation effect, the sponge effect and the accompanying effects [49]. Furthermore, the sonication causes changes in the structure of the material, dependent on the applied sonication parameters. Longer US pretreatment usually results in higher damage of the plant tissue. The examples of changes in the structure are shown in Figure 2 for apple and potato tissue treated with US. Fresh samples and those after 30 min of US treatment (conducted in ultrasonic bath at 21 kHz for pear and 35 kHz for potato) were freeze-dried to preserve the structure of the material [28; 67]. The photos (examined with a scanning electron microscope--Hitachi, model TM-3000, Tokyo, Japan) show alteration in a structure after the sonication. Sonicated apple tissue is characterized by distortions, and the coalescence of cells into larger clusters can also be observed (Figure 2a). In the case of potato tissue, the US caused breakdowns of the tissue and the exposition of the starch grains (Figure 2b). This effect was also observed by many researchers in apple [27; 68; 69], carrot [28], pineapple [31], melon [70], berries [71], basil [63], etc. For example, in the case of US-treated carrot [28], microchannels were formed, especially with shorter sonication time, i.e., 10 min. The image analysis revealed that the cross-section area of those samples and interspaces were smaller than in the material which was treated for a longer time of ultrasonic waves. The changes in structure of the carrot tissue were generated mainly by the direct US effect, i.e., the sponge effect, due to reduced cavitation with the used packaging. The US generated the formation of microchannels, especially at the beginning of the treatment, and then over time the sonication led to distortion of the structure, forming larger spaces in the tissue of apple [27] and carrot [28]. In addition, other researchers showed that the formation of microchannels occurred during sonication, and this phenomenon was observed in the structure of kiwifruit [32], apple [27; 69], potato [72] and carrot tissue [28] after ultrasound's application. It is well known that during the OD process the fruits lose water and gain the osmotic substance [70]. With longer immersion time in osmotic solution, greater changes in the texture occur and this impacts the structure. After 1 h of OD process, a gradual disconnection and breakdown of the tissue, a loss of shape of cellular walls and loss of turgor pressure are noticed [11]. As aforementioned, the use of sonication causes the formation of microchannels and the alteration of the tissue structure. Furthermore, the increased bidirectional exchange during OD causes greater alteration to the microstructure when compared with the dehydrated material, but not pretreated with US [24; 32]. After the application of US, the maximum cross-section area of cells increased in kiwifruits, especially at longer sonication time (30 min) (Figure 3a). The analysis allowed expressing in numbers the changes in size of cells and intracellular spaces, leading to better understanding of the alterations in microstructure of the plant tissue [28]. [PERSON] et al. [24] observed greater changes in microstructure in arils tissue when combination of sonication and OD was applied. The cell wall between two cells of arils tissue, after 30 min of OD, was partially broken and some cells were merged together, while for combined OD and US treatments the cells were characterized by larger cell interspace and microchannels. However, in the case of pumpkin subjected to 10 min sonication and 12 h of OD, the changes were small and comparable with US treatment applied for 20 and 30 min before OD.