# Current Applications of Ultrasound in Fruit and Vegetables Osmotic Dehydration Processes
## 3 Sonication
### Ultrasound Mechanism  
The damage to the material's structure and activation of various chemical reactions as a result of US propagation is connected with its mechanism of action. Among the phenomena suggested in the literature, the following are predominantly reported [18; 49; 50]:  
* \"sponge effect\";
* cavitation;
* absorption of acoustic energy; and
* effects accompanying cavitation (in particular at the solid/liquid border), such as microstreaming, microjetting or standing wave pattern. Thermal, mechanical and physicochemical effects of sonication result from the absorption of the ultrasonic wave. The temperature increase is dependent on the US frequency, power or the kind of treatment (immersive, contact and airborne) [49; 58]. Along with the propagation of high intensity ultrasound in a solid matter (such as plant tissue), due to its mechanical impact, a series of compression and decompression of the structure of the medium occurs, called \"sponge effect\", because the material then resembles a compressed and released sponge [49; 50]. It is assumed that the major changes in the material are due to cavitation and the effects associated with cavitation. Cavitation takes place in liquids or solids (which contain moisture), after the application of high intensity ultrasound. The phenomenon consisting in formation of gas microbubbles (micro-bubbles of water vapor), which is caused by local drops in liquid pressure [49; 59]. The cavitation causes thermal, physical (collapse pressure, turbulences and shear stresses) and chemical (free radical formation) effects [61]. Furthermore, the mechanical energy and pressure of sound waves cause a million bubbles to cyclically oscillate around their stable size in a very short time. With regard to plant tissue, disturbances intensify the flow of mass and heat. In the available literature, this phenomenon is called microstreaming. The bubbles that reached unstable size suddenly collapse [18; 49; 50]. The energy released during the collapse of one bubble is extremely small, but the simultaneous collapse of a million bubbles has a cumulative huge effect. This results in point (located in a small area) generation of very high pressures (up to 50-100 MPa) and temperature (up to 5500 \({}^{\circ}\)C) [18; 50; 62]. Considering cavitation regarding plant tissue immersed in solution (or inside the cells), it should be highlighted that bubbles near the solid surface collapse asymmetrically, releasing an irregular fluid beam (fountain of microbubbles) towards the solid surface, which microjet the tissue with a speed of 200 m/s. This in turn leads to erosion of the plant tissue. This effect is called microjetting [18; 49]. The \"sponge effect\" and cavitation (especially asymmetric collapse) cause permanent damage to cell walls and cell membranes causing creation of microchannels in the material [27; 32; 50; 63], which are discussed in the next section. The aforementioned, together with turbulences of solution reducing the laminar sublayer, shock waves and shear stresses in the fluid medium, facilitate the exchange of water with low-molecular substances and osmotic substance [49; 59; 64]. The ultrasound-assisted OD (UOD) is thus especially important in the case of heat-sensitive raw materials, as usually OD is accelerated by the increase of temperature of osmotic solution. However, the effectiveness of the impact of US depends primarily on the use of an appropriate frequency of US and the time of its application [64]. Moreover, the type of liquid medium in which the material is subjected to immersive US treatment is important. [PERSON] and [PERSON] [65] found that treating bananas with US in water allows more water to be removed from the tissue during drying than when an osmotic solution was used. It is believed that one of the reasons for the observed behavior may be the sugar saturation of the microchannels formed by the action of US.