## 3 Results and Discussion  
### Rheological Properties of the Obtained Fermented Whey Beverages  
3.1.1. Apparent and Ultrasonic Viscosity  
The apparent and dynamic viscosity measurements for each tested sample of fermented drink are detailed in Table 2. The first two columns present the results for laboratory-scale beverage productions. The appearance of manufactured beverages was presented in Figure 1.  
Apparent viscosity measurements ranged from 0.21 to 0.93 Pa-s; the lowest value equal to 0.21 Pa-s was recorded for SKKr/R and SSKr/R (both with unpsteurized sour and sweet cow whey with sea buckthorn juice). The lower apparent viscosity of beverages containing sea buckthorn juice compared with that derived from water can be also seen from Table 2. On the contrary, the highest apparent viscosity was found in sample SSK/NP (0.93 Pa-s) and included no fruit juice, since it was sweet goat whey (control sample). These results indicate that the apparent viscosity was affected by the type of whey used and a significant modification abolishing fruit juice. Figure 1: The appearance of the final product with labels. Own photograph. In terms of dynamic viscosity, the lowest value was reported for the SKK/R (2.50 mPas-g/cm\({}^{3}\)) blend containing sour goat whey and sea buckthorn juice; highest one was shown in the control sample SKK/NP (46.33 mPas-g/cm\({}^{3}\)) manufactured with unpasteurized sour goat whey solely without any fruit juice (control sample). On average, all fermented whey beverages without juice showed a higher or not significantly different dynamic viscosity value compared with the same type of beverage with juice and a similar behavior could be distinguished for apparent viscosity. In almost all instances, the cow whey beverages had a lower apparent viscosity compared to other beverages. The variations in the viscosity and texture of the fermented whey beverages are significantly influenced by the composition of whey proteins and the production of exopolysaccharideacharides (EPS) during fermentation. Whey protein hydrolysis by lactic acid bacteria and yeast strains leads to the release of bioactive peptides, enhancing the beverage's rheological properties and nutritional value. Additionally, certain strains produce EPS, improving viscosity, stability, and mouthfeel [30]. Fermentation has also been shown to enhance the bioactive properties of whey proteins. For instance, the process can increase angiotensin-I-converting enzyme (ACE) inhibitory activity, which is associated with antihypertensive effects, highlighting the functional benefits of fermentation [31]. Moreover, research on _Bacillus subtilis_ fermentation has demonstrated the simultaneous production of polysaccharides and bioactive surfactin using rehydrated whey powder, further illustrating the multifunctionality of whey as a fermentation substrate [32]. On the other hand, the dynamic viscosity measurements showed a more pronounced change indicating that the addition of fruit juices, especially sea buckthorn and rosehip that are very rich in bioactive compounds (flavoring extracts), have a great impact in both increasing as well lowering viscosity. The addition of sea buckthorn juice generally resulted in higher dynamic viscosity values compared to rosehip juice. For instance, SKKr/DR (8.50 mPas-g/cm\({}^{3}\)) had a significantly lower dynamic viscosity than