## 3 Results and Discussion
### Water Activity, Synresis, Viscosity and Particle Sizes  
The water activity, synresis, viscosity and particle sizes of the samples are summarized in Table 3. Water activity (\(\mathrm{a_{w}}\)), which is a thermodynamic measure of the chemical potential of water, affects the durability and quality of products, influencing the potential of microorganisms to develop [54]. Fermented whey (W) had the highest \(\mathrm{a_{w}}\) during storage, whereas the lowest values were observed for the WFG2 sample. A significant decrease in the \(\mathrm{a_{w}}\) of all samples during storage was found (\(p<0.05\)), and the greatest decrease was observed for the W sample. The use of FG decreased the \(\mathrm{a_{w}}\) of FWB, but the amount of FG did not significantly differentiate the WFG1 and WFG2 samples (\(p>0.05\)). The lowest synresis (serum release from the gel matrix) during refrigerated storage was observed for the sample containing 1% addition of FG (WFG2) and the highest for fermented whey (W) (\(p<0.05\)). The addition of 0.5% FG limited the syneresis of the samples to a level similar to the addition of milk powder (MP). [PERSON] and [PERSON] [55; 56] reported that FWB are characterized by a significant whey leakage, amounting to one-third of the sample volume, which results in a low assessment of the appearance of this type of product. According to [PERSON] et al. [57], the main problem in the production of FWB is precisely their colloidal instability caused by protein aggregation, mainly \(\beta\)-lactoglobulin. Changes in colloidal stability negatively affect the viscosity and appearance of the product. The results of studies by other authors also confirm the positive effect of FG on the release of whey from fermented beverages. [PERSON] et al. [58] found that the addition of hydrocolloids positively influences the sensory characteristics of FWB. [PERSON] et al. [59] found an increase in the apparent viscosity of the beverages, and thus the lowest release of whey, and recommended the use of gum in the industrial production of FWB. This is consistent with the finding that all adsorbent and non-adsorbent hydrocolloids can stabilize beverages and prevent phase separation by increasing the consistency of the continuous phase and chemically interacting with proteins [59]. The reduction in syneresis observed in our studies after adding FG to the samples is consistent with the results of [PERSON] et al. [44] who reported reduced syneresis of mixed yogurt supplemented with FG. The decreased hydrodynamic properties, as well as syneresis, can be linked to the tendency of FG biopolymers to associate due to the intermolecular associations via hydrogen bonding, leading to pseudogel behavior and the formation of complexes with proteins such as conlinin [60]. In dairy beverages, viscosity is an important characteristic for consumer acceptance, and it depends on the solids content, the type and concentration of the additives, and the fermentation conditions, such as time, temperature and the LAB strains used. The addition of some natural ingredients allows the design of the viscosity of the products [4]. AW has a low total solids content when compared to milk [2; 13; 21]. The addition of FG could fix this problem and may improve the texture parameters because biopolymer complexes formed by protein-polysaccharide interaction can serve as food texture modifiers in food products [61]. [PERSON] and [PERSON] [62] as well as [PERSON] et al. [63] found that under acidic conditions, the FG gel strength decreases as the pH value decreases, and under basic conditions, the gel strength decreases as the pH value increases. In our study, the addition of FG, despite the acidic environment, clearly improved the viscosity of the tested samples. As can be seen in Table 3, the viscosity of fermented whey significantly increased when MP was added (\(p<0.05\)). Moreover, a significant increase in viscosity (8-fold and over 14-fold for samples WFG1 and WFG2, respectively) was noted (\(p<0.05\)).