## 3 Results and Discussion  
### Sorption Properties of Dried Apple Puree Gels  
The sorption isotherms characterise the relation between water content and the water activity of the product. The selection of the best model describing the sorption isotherms can be a challenge due to the different compositions and complex structure of food materials [46]. Table 3 presents the parameters of analysed models of the sorption isotherms of different dried apple puree snacks. The goodness of fit for models was estimated based on the determination coefficient (\(R^{2}\)), the root mean square error (\(RMS\)), and \(P\) parameter. The lower values of \(RMS\) and \(P\) values and the higher \(R^{2}\), the better goodness of fit [26]. The GAB and Peleg models were described by high values of \(R^{2}\), which ranged from 0.993 to 0.998 for all dried samples (Table 3). The \(RMS\) values varied from 5.99 to 27.59% for GAB model and from 5.79 to 17.64% for Peleg model (Table 3), which can be considered as satisfying. These models were also characterised by \(P\) values lower than 12.24%. [PERSON] and [PERSON] [47] as well as [PERSON] et al. [48] stated that the sorption model can be acceptable when the parameter \(P\) is lower than 10%. It can be noticed that parameter \(P\) of the investigated models was very close to this limit. On the other hand, the Hasley model provided the worst representation of data, with \(P\) values ranging from 10.21 to 39.55%. The GAB equation has been frequently applied to describe the sorption isotherms of many products due to the physical meaning of the model, which facilitates the interpretation of obtained results [38; 49; 50]. However, the constant \(C\) of the GAB model should be in the range of \(5.67\leq C\leq\infty\) because outside of this region the monolayer capacity is estimated with an error larger than 15.5% [51]. Table 3 shows that the values of \(C\) constant for GAB model did not fulfil this requirement in the case of investigated dried gels. For this reason, the Peleg model was selected to describe the sorption isotherms of apple dried snacks, since it presented the highest \(R^{2}\) and the lowest \(RMS\) and \(P\) values. The Peleg model was also the most appropriate for the representation of sorption behaviour of freeze-dried avocado with maltodextrin and inulin [52], freeze-dried vegetable soups [53], as well as apple puree powders [42]. Figure 1 shows the sorption isotherms plots as experimental data and the Peleg fitted model for freeze-dried apple gels with different compositions (Figure 1a) and P40%DE-MC snacks obtained by the application of different drying methods (Figure 1b). The sorption curves obtained for the freeze-dried agar gel without other ingredients (P0%) and the vacuum-dried foamed gel (P40%De-MC) were sigmoidal in shape, which is characteristic for type II isotherms. The application of the procedure described by [PERSON] and [PERSON] [54] based on nonlinear regression analysis of the data and using the plot of \(a_{w}/w\) vs. \(a_{w}\) (plots are not shown) enabled confirming that these isotherms represented type II. Additionally, the values of the \(c\) constant of the BET model obtained for these dried samples were higher than 2, which also indicated the isotherm type II. However, the BET model can be only applied to predict sorption behaviour at the water activity range lower than 0.5 [55]. In the case of isotherms type II, the interactions between water (sorbate) and food material (sorbent) are stronger than between sorbate-sorbate molecules. The freeze-dried agar gels showed a higher hygroscopicity at low water activity, which may affect the softer texture of the material. The material seemed to behave as a sponge. The foamed gel dried by the vacuum-drying method adsorbed a higher amount of moisture than the freeze-dried and the air-dried gels with the same composition at low water activities (\(a_{w}<0.2\)) (Figure 1b).