## 3 Results and Discussion
### Water Adsorption Isotherms of Kefir Powder Samples  
The ability of a powder, such as kefir powder, to adsorb moisture from the environment (hygroscopicity) has an impact on a product's stability and quality during storage, since, for most foods, increased hygroscopicity accelerates non-enzymatic browning and oxidation [32]. For many food powders, moisture sorption results in stickiness and caking, making them difficult to handle and process. The adsorption isotherms of the samples shown in Figure 4 follow the type III isotherm [33], which is in accordance with our previously published results for kefir powder produced with spray drying [34] and the results for the kefir powder samples prepared with freeze drying [35]. This type of isotherms occurs where the adsorbate-adsorbate interaction is large compared to the adsorbate-sorbent interaction [33], indicating that binding energy is so high that water activity is suppressed, while water is absorbed [35]. This means that when all the binding sites are filled, the increase in moisture content causes a significant increase in water activity. This type of behavior is typical for foods rich in soluble components such as kefir, which contains lactose, salts, short-chain fatty acids, bioactive peptides, and exopolsaccharides that are characterized by their soluble nature [35]. According to [PERSON] et al. [36], who studied the water vapor sorption of milk powder, at low relative humidity environments, water is preferentially bound by casein. As the relative humidity increases, lactose binds water more strongly and acquires sufficient moisture to undergo a phase transition, while at relative humidity above 50%, the salts bind sufficient water to set the conditions for protein destabilization. The storage conditions for these types of foods, including kefir powder, must be carefully selected (low relative humidity at ambient temperature, 18-20 \({}^{\circ}\)C) so as to protect them from moisture intake that could potentially result in poor product stability and quality.  
As can be seen in Figure 4, at low relative humidity environments, all samples exhibit increased hygroscopicity values compared to the CONTROL. As the relative humidity increased, the hygroscopicity of sample 5.2-0 became similar to that of the CONTROL, and at 95% relative humidity, sample 5.2-7 showed the highest hygroscopicity, followed by the CONTROL and 4.4-7. Sample 5.2-0 showed the lowest hygroscopicity of all. To the best of our knowledge, no data exist in the literature concerning the adsorption isotherms of spray dried kefir samples. The only available data are for freeze dried kefir [35]. According to the results of the aforementioned research, freeze dried kefir showed increased hygroscopicity at all relative humidity environments studied (at 25 \({}^{\circ}\)C) when compared to the samples of the present study. The increased hygroscopicity of the powder samples prepared with kefir grains when compared to the CONTROL might be attributed to the hydrophilic nature of kefir. As discussed in the following subsection (3.4), the variety of microorganisms present in kefir grains alongside their unique symbiotic relationship might be responsible for an increase in the production of the exopolsaccharide.