## 3 Results and Discussion
### The Influence of US or PEF Pretreatment on the Water Content and Microbiological Stability  
Water content is a parameter determining the microbiological stability of the product. The water content in fresh strawberries was on average 90.3 \(\pm\) 0.7% (Figure 3). In all cases, as the dehydration process continued, the water content decreased. This was connected to the osmotic substance permeating the plant tissue. Due to the complex mass transfer occurring because of the pressure gradient at the interface between the raw material and the solution, the water content within the raw material diminished, while the dry substance mass increased [55]. Raw material treated with both ultrasound and PEF displayed a reduced water content, attributed to the enhanced penetration of sucrose from the osmotic solution. Strawberries subjected to a PEF had a lower water content than those subjected to ultrasound. Additionally, the highest changes in water content were recorded in the material exposed to a PEF of 2.5 kJ/kg, which was probably related to greater changes in the structure of the material. Similar results were obtained by [PERSON] et al. [56] when dehydrating kiwifruit treated with a PEF. The water content was approximately 3% lower than in fruits not treated with a PEF.  
Figure 2: Extract content in strawberries subjected to US and PEF treatments and osmotic dehydration in 50% sucrose solution at 30 \({}^{\circ}\)C; SA: osmotic dehydrated strawberries in 50% sucrose solution; SA_PEF1 and SA_PEF2: pulsed electric field-treated strawberries with the energy of 1 or 2.5 kJ/kg and subjected to osmotic dehydration; SA_US1 and SA_US2: ultrasound-treated strawberries for 30 and 90 s and subjected to osmotic dehydration. The same letters above the columns present homogenous groups at \(\alpha\) = 0.05.  
Osmotic dehydration, as well as the PEF and US pretreatments, may affect the microbiological quality of the raw material. This is due to the lower water content in the dehydrated product and, consequently, the lower water activity. In fresh strawberries, the total number of microorganisms was observed at the level of 3.73 \(\pm\) 0.08 CFU/g of product, and osmotic dehydration applied alone did not reduce the number of microorganisms (Figure 4). The use of higher PEF energy (2.5 kJ/kg) and ultrasound pretreatment with both of the parameters used (30, 90 s) resulted in a reduction in the number of microorganisms by approximately 1-1.5 log cycles. Determination of the number of fungi showed a reduction in their level after osmotic dehydration by 1 log cycle, while PEF energy by 2 log cycles. Pretreatment with ultrasound resulted in a reduction in the number of fungi below the detection threshold. This means that the use of proper parameters of the non-thermal pretreatment (both PEF and US) applied before osmotic dehydration positively effects the microbial stability of the final product. After 7 days of refrigerated storage of dehydrated strawberries, an increase in the total number of microorganisms by approximately 3 log cycles was observed in the case of non-dehydrated fruits, while the TVC in the case of dehydrated fruits subjected to pretreatment was a little bit higher or at the same level as immediately after the dehydration process. The same trends were observed in the number of fungi; however, after 7 days, their presence in the US-treated samples was observed at the level of 2 log cycles. [PERSON] et al. [57] showed that the use of ultrasonic treatment on fresh strawberries allows for the reduction in the number of bacteria by approximately 2 log cycles and the number of fungi by 1 log cycle. In turn, in the study by [PERSON] et al. [23] it was shown that storing the US-processed strawberries at refrigerated temperature made it possible to maintain the microbiological stability of the tested fruits.