## 3 Results and Discussion
### Effect of Water Activity on the Glass Transition Temperature of Dried Gels  
The water activity and glass transition temperature are crucial parameters in the evaluation of changes occurring in food products. Sugar recrystallisation is the main problem of stored products that contain low-molecular sugars in an amorphous state. Dried fruit products are commonly in an amorphous state. These materials are not stable due to a lack of thermodynamic equilibrium [24]. Values of glass transition temperature \(T_{g}\) obtained for selected dried apple puree gels and freeze-dried agar gel as a function of equilibrium water content are shown in Figure 3a. The experimental data and the Gordon-Taylor fitted model were presented. The glass transition temperature decreased with increasing water content due to the plasticising effect of water. Additionally, the addition of maltodextrin to apple puree gels caused the increase in \(T_{g}\) of the freeze-dried sample. A similar effect of a carrier on the \(T_{g}\) of different dried products was observed for mango powder [60], apple puree powder [42] and date syrup powder [61]. The highest glass transition temperature at the investigated range of water content was observed for freeze-dried agar gel without the addition of apple puree. The application of the air-dried method enabled obtaining apple snacks with slightly higher values of \(T_{g}\) in comparison with other dried apple puree gels. It means that the air-dried product will be more stable during the storage because the phase transitions occur at higher water activity at room conditions than for other dried gels. In addition, based on the sorption behaviour, it can be stated that the air-dried gel was less hygroscopic than other materials. The parameter \(T_{gs}\) of the Gordon-Taylor model (Equation (8)) showed that the drying method of gels with the same composition (P40%DE) did not affect the \(T_{gs}\) values, but the \(k\) parameter obtained for the vacuum-dried sample was significantly higher than for the air-dried gel (Table 4). The \(k\) parameter is related to the plasticising effect of water. Higher value of \(k\) can be linked with a decrease in the glass transition temperature of the sample [60; 62]. Lower susceptibility to water sorption of air-dried gels can be linked to the higher glass transition temperature. The increase in pulp addition caused a slight increase in \(T_{gs}\) (Table 4). [PERSON] et al. [60] stated that the glass transition temperature of anhydrous sample \(T_{gs}\) should be independent of pulp content in freeze-dried mango powder because the solid matrix of pulp is intrinsically crystalline. However, the increase in apple pulp addition in freeze-dried gels caused the increase in low molecular sugars and their presence affected a slight decrease in glass transition temperature. The addition of maltodextrin  
Figure 2: Effect of composition of gels and drying method on the water kinetics uptake of apple puree snacks. and a lower amount of incorporated pulp affected a fabrication of dried gels with higher stability (the possibility of structure collapse and the changes in texture of the product can be limited). The relation between the water activity and glass transition temperature can be described by Roos model (Figure 3b). However, the calculated _RMS_, \(R^{2}\) and \(P\) values showed that the experimental data obtained in this study did not fit well for the Roos equation (Equation (9)). A higher value of critical water content and critical water activity at the glass transition temperature of 25 \({}^{\circ}\)C obtained from the relations of water content-water activity and water content-glass transition temperature may indicate the greater stability of physical properties of products during storage. Dried apple puree gels with maltodextrin were characterised by a significantly higher \(a_{\text{src}}\) than dried gel without carrier addition. A similar tendency has been observed for grapefruit and mango powders [20; 60].