## 3 Results and Discussion
### Isotherms of Water vapour sorption of Fruit Fibres and Pomace  
Based on the results obtained, water vapour adsorption isotherms were determined. The isotherms of the studied fibres had a shape according to the classification of [PERSON] et al. [29] characteristic of type III isotherms. Based on the determined curves, it was found that apple fibre had a significantly better hygroscopic ability compared to the other tested materials (Figure 4). In an environment with a water activity of 0.93, the equilibrium water content in apple fibre was 0.21 g water/g d.m. Under the same conditions for chokeberry fibre, this value was equal to 0.18 g water/g d.m., while for fibre from currant was equal to 0.14 g water/g d.m. The tested materials in the range of water activity 0-0.438 absorbed insignificant amounts of water, while a sudden increase in equilibrium water content begins in an environment with  
Figure 3: Kinetic curves of the dependence of the amount of absorbed water on the time of the tested fruit pomace. water activity above 0.648. Figure 5 shows the course of isotherms of fibre from chokeberry, currant and apple. The GAB model was used to describe water vapour sorption isotherms, which is used to predict optimal storage conditions and storage stability of foods, which are characterised by low and medium water content. From the GAB model, the monolayer capacities and the constants \(C\) and \(k\) were calculated, which are related to the energy of interactions between the first water molecule and the further absorbed ones [30]. The capacity of a monolayer determines availability of polar sites for water vapour, and is determined by the presence of components such as starch or protein and the physical state of these components [31]. The highest water content in the monolayer, which was equal to 0.0563 g water/g d.m., was characterised by apple fibre. The smaller capacity of the monolayer was demonstrated by the other studied fibres. In fibre from chokeberries, 0.0477 g water/g d.m. was found, while in fibre from currant, there was only 0.0430 g water/g d.m. An evaluation of the suitability of the equations to describe the obtained sorption isotherms showed a good fit of the model to the experimental data. The coefficient of determination R\({}^{2}\) was in the range (0.990-0.989), while the mean square deviation did not exceed 10% [32]. Table 1 shows the nutritional values of the tested fibres. The most hygroscopic material, apple fibre, is characterised by a higher carbohydrate content than the other fibres. In contrast, the fibre from currant, which absorbed the least water, was the poorest in sugars. The fibres in question differ in protein content, which may also affect the sorption properties. Currant fibre contains almost twice this component compared to apple fibre. Based on the results, the isotherms of vapour sorption of pomace from apple, currant and chokeberry. Based on the determined isotherms of water vapour sorption of the studied pomace, it was found that the apple pomace absorbed the highest amount of water. The differences between the tested materials were not as significant as in fibres. The equilibrium water content of the apple pomace reached a value of 0.23 g water/g d.m. in an environment with the highest water activity of 0.93. Chokeberry pomace reached a value equal to 0.22 g water/g d.m. Currant pomace showed the worst hygroscopic abilities, with an equilibrium water content of 0.20 g water/g d.m. The tested pomace absorbed a small amount of water in the water activity range of 0-0.438. A sharp increase in water adsorption of water was observed in environments with water activity above 0.648. The dependence of the equilibrium water content on water activity is shown in Figure 5.  
Figure 4: Water vapour sorption isotherms of the studied fibres. The water vapour sorption isotherms of the pomace studied were described by the GAB model. It was found that, as in the case of fibres, the course of the isotherms has a shape similar to Type III isotherms.