# Current Applications of Ultrasound in Fruit and Vegetables Osmotic Dehydration Processes
## 2 Osmotic Dehydration Process  
Osmotic dehydration (OD) is a non-thermal process for a partial dehydration of plant tissue. It is also called a \"dewatering impregnation soaking process\", since along with the water removal there is a simultaneous impregnation of the tissue with the solutes present in the osmotic solution [33; 34; 35; 36]. This process consists of soaking pieces of fruits and vegetables in hypertonic solution (Figure 1) and the mass transfer is allowed thanks to the differences in the water chemical potential between the product and the solution [20; 37]. During OD, a multidirectional mass exchange occurs through the transfer of water from the material to the osmotic solution, and at the same time the penetration of the osmotic substance into the dehydrated tissue. Low-molecular substances such as vitamins, organic acids, saccharides or mineral salts are rinsed out from the tissue. During such a complex process of mass exchange, the water content decreases and the dry matter increases at the same time, which changes the chemical composition of dehydrated food [33; 36].  
The effectiveness of the dewatering depends on many factors: the concentration, temperature and agitation of the osmotic solution; properties of the osmoactive substance; time and conditions of the process; the type, shape, size and chemical composition of  
Figure 1: The mechanism of the osmotic dehydration process (e.g., kiwifruit tissue).  
raw material subjected to OD; the ratio of the solution to the raw material; etc. The most commonly used osmotic substance is a sucrose solution of high concentration [33; 38; 39], however other sugars as well as sodium chloride, sorbitol or other substances which produce high osmotic pressure and are acceptable for consumer can be used [33; 38]. Recently, new trends are observed, such as replacing sugar solution with fruit syrups and juices enriched with various valuable ingredients in order to produce high value-added, functional products [39; 40; 41].  
OD can be used as process for obtaining minimally processed food [42; 43; 44]; however, it is usually used as a pretreatment before the drying processes [14; 16; 45]. This process also plays an important role in increasing the overall availability of exotic fruits. For example, thanks to the pretreatment by OD followed by drying, it was possible to significantly minimize the loss of papaya during transport. Even 40% of the fruit was unmarketable after traveling from the plantation to the destination countries [46].  
The osmotic dehydration process is long and often requires the acceleration of mass transfer using traditional and innovative methods. OD process allows obtaining semi-dried products with improved stability, thanks to the reduction of their water activity and freezable water content [34; 47]. OD treatment can be applied at room temperature or at temperatures which are slightly higher in order to increase a mass transfer rate, however usually it does not exceed the temperature of 40-45 \({}^{\circ}\)C [34; 39]. Traditionally, agitation and rotation are used for this purpose. However, OD, especially when applied at room temperature, is a time-requiring process, therefore recently the combination of OD with other traditional and innovative techniques has been studied. These combined treatments include the application of ultrasound, pulsed electric field, high hydrostatic pressure, vacuum, irradiation and centrifugal force [47; 48], which are able to increase mass transfer in the treated products. All these techniques were extensively reviewed by [PERSON] et al. [42].