# Effect of PEF Treatment on Chosen Properties of Raw and Hot Air- and Freeze-Dried Poultry Meat
## 3 Results and Discussion
### The Influence of PEF Pretreatment on the Structure of the Hot Air- and Freeze-Dried Poultry Meat
#### 3.3.1 Dry Matter Content and Water Activity  
The dry matter content of the dried meat samples untreated and treated with the PEF ranged from 90.2 to 97.2% (Table 3). For the hot air (HA)-dried meat, only the sample  
Figure 3: Scanning electron microscope photographs of intact and PEF-treated (15 and 30 kJ/kg) poultry meat dried with hot air (HA) and freeze-drying (FD) methods (100\(\times\) magnification). pretreated with the PEF at an energy intake of 30 kJ/kg achieved a significantly lower (\(p<0.05\)) dry matter content compared to the untreated reference sample. In the case of the freeze-dried (FD) samples, there were no significant differences (\(p>0.05\)) in this parameter between the PEF pretreated samples and the untreated ones. As is shown in Table 3, the FD samples were characterized by about 5% higher dry matter content compared to the hot air-dried samples, irrespective of the energy input setting. Although the PEF increased the permeability of the cell membrane [56], accelerating the mass and heat transfer during further processing [57], there is the risk of quantitative losses of soluble substances. In addition, an increase in the PEF energy input may result in the cross-linking of cell membrane fragments, creating structures with high water-holding capacity [58]. As [PERSON] et al. [58] showed in the example of beef semitendinosus muscles, a PEF induces the rupture of myofibrils along the \(Z\) lines, which reorganizes the myofibrillar structure and makes the muscles more porous. The simultaneous loss of water and heat exposure during conventional drying may enhance this ability, as myofibrillar proteins are denatured and a rigid surface layer is formed [15]. In turn, freeze-drying can increase the intercellular and extracellular space in the muscles through ice crystal formation and improve the removal of water during the sublimation process. Therefore, the final product has a low water content and great susceptibility to rehydration [14]. As [PERSON] et al. [9] summarized, the freeze-drying method allows the preservation of the dried meat's original features to the maximum possible degree. Dried products usually have water activity (a\({}_{w}\)) below 0.7 [8], but the critical a\({}_{w}\) at which microorganisms can grow is 0.60 [59]. Nevertheless, the greatest stability of dried food can be achieved for aw in the range of 0.07-0.35 [60]. The water activity of the dried poultry meat varied from 0.080 to 0.486 (Table 3). Statistical analysis showed that PEF pretreatment at 30 kJ/kg increased the water activity of the HA-dried meat compared to other samples preserved with the same drying method. The PEF application did not have a meaningful influence (\(p>0.05\)) on the water activity of the FD samples (Table 3). The opposite effect was obtained by [PERSON] et al. [61] in the case of freeze-dried apples. Although the water activity and water content are not directly proportional, it was observed that lower water activity was associated with a higher dry matter content in all the meat samples. Therefore, the FD meat exhibited about a 4-fold lower water activity than the HA-dried poultry meat (Table 3). Greater differences in the water activity of dried meat could appear as a function of its storage time, because at a\({}_{w}\leq 0.1\) the activity of lipases increases [9]. Accordingly, this parameter could be higher for untreated meat compared to PEF pretreated samples, in which presumably lipases were partially or completely inactivated (depending on the electric field strength) [62]. Taking into account the above-mentioned considerations, the results suggest that the drying method, rather than the use