# Current Applications of Ultrasound in Fruit and Vegetables Osmotic Dehydration Processes
## 3 Sonication
### Effect of Sonication and Osmotic Dehydration Process on Microbial Growth  
The use of the sufficient power of US positively affects the microbial aspects. The US can be applied individually or in combination with different sanitizers [81; 82] or with elevated temperature [83]. [PERSON] et al. [84] studied the effect of acoustic energy on the microbial safety of apple cider. Results showed that sonication increased _E. coli_ K12 cell destruction and with the use of higher temperature from 40 to 60 \({}^{\circ}\)C caused bacteria reduction by 0.1-5.3-log cycles. Similarly, [PERSON] et al. [83] mentioned that ultrasound-assisted thermal treatment of _E. coli_ is better than when the sonication is applied separately. On the other hand, [PERSON] et al. [82] stated that the use of sonication as a pretreatment at higher frequency (850 kHz) immediately followed by hypochlorite addition is as good as, or even better than, using a short period with chlorination simultaneous (1-5 min)  
Figure 3: Structural changes of: (**a**) kiwifruits subjected to ultrasound treatment (10 min) and osmotic dehydration (30 min); and (**b**) cranberries treated with ultrasound-assisted osmotic dehydration (30 min). Own unpublished data.  
at 20 kHz to kill microorganism. [PERSON] et al. [83] explained that the reduction of microorganisms under the influence of ultrasound results from the mechanism related to localized heating even to 5500 \({}^{\circ}\)C and high pressure to even 500 MPa and formation of free radicals, which cause the sonolysis of water. Furthermore, the collapsing of the cavitation bubble may cause the destruction of the cell surface. However, [PERSON] et al. [85] stated that US induced cell death is not be related to membrane damage.  
According to the mechanism of the OD process, the reduced amount of freezeable water content and water activity and therefore the inferior availability of water for the degradative reactions and growth of microorganisms can guarantee a higher microbial stability [34; 86; 87; 88] For example, pumpkin subjected to OD process in different osmotic solutions, after 90 days of storage was characterized by low values up to 3-4 log CFU, while the initial microbial counts of total aerobic bacteria, yeast and molds after OD process were at undetectable levels (<10 CFU/g) [89]. Moreover, [PERSON] et al. [39] studied the growth of mesophilic aerobic bacteria and yeast in cranberries subjected to the UOD process with different osmotic solutions and stored for eight weeks at the temperature of 10 \({}^{\circ}\)C. They noticed that the sucrose solution enabled for longer shelf-life in comparison to other two solutions used (trehalose solution or reduced concentration of sucrose with the addition of steviol glycosides), due to the lower water activity achieved in the final products. However, any significant differences in microbial growth were noticed between the samples pretreated with or without US assistance [39].  
[PERSON] et al. [90], instead, observed a positive effect on the microbial load reduction in UOD-pretreated and coated dried apricots compared to the OD pretreated ones. The decrease of 0.29-0.52 log cycles of total mesophilic aerobic was noticed in the UOD-pretreated and coated dried samples compared to the control ones, showing the lowest values (35-45 CFU/g), when UOD at 35 kHz and temperature of 30 and 40 \({}^{\circ}\)C was used in combination with coating (pectin + ascorbic or citric acid). Similar results were observed as concern the total yeast and mold count [90].