## 3 Results and Discussion
### Titractable Acidity and pH  
The changes in tittatable acidity (TA) and pH are listed in Table 2. The addition of FG significantly increased the content of lactic acid and the active acidity (pH) of the samples (\(p<0.05\)). Generally, the WMP sample had the lowest tittatable acidity during the storage period, which may be attributed to the buffering capacity of milk powder [45]. The pH of the W sample did not change until the last week of storage, when an increase in pH was observed (\(p<0.05\)). This is partially consistent with the results of the study conducted by [PERSON] et al. [46] who did not find any significant differences concerning the acidity of whey beverages during 30 days of storage at 4 \({}^{\circ}\)C. A significant increase in pH (\(p<0.05\)) was observed in the first half of the research period and a stabilization in the second half of the cycle for variants with the addition of FG (WFG1 and WFG2). An overall increase in pH was observed for these samples. The highest acidity during storage was observed for the WFG2 sample (\(p<0.05\)). Generally, according to the information in the literature, the cooling storage of WFB was associated with an increase, decrease or stabilization of the acidity of the product, depending on the type of gum used and the additives. [PERSON] et al. [47] and [PERSON] [48] noted that the presence of gums in the beverage medium can increase pH and decrease acidity. Additionally, [PERSON] and [PERSON] [49] observed a gradual change in tittatable acidity during a monthly storage of WFB. In a study on whey-based mango herbal beverage, [PERSON] et al. [50] found that the pH of the beverage decreased during the storage period; the lowest pH value was recorded on the last day of the study and was 4.15. The acidity of the beverages changed during storage and was found to increase from an initial value of 0.24% citric acid to a final value of 0.32% after 30 days. These results of pH and acidity are in line with those reported by other authors [51]. The pH of the WFB containing fruit juices during storage, according to [PERSON] et al. [41], was in the range of 3.89-4.17, while the content of citric acid was 0.42-0.61%. The value of both parameters was stable during storage. The high acidity of WFB compared to values reported in the present study was due to the high acidity of the whey as well as the fruit juices as flavoring agents. In their work, [PERSON] et al. [8] tested the pH of functional fermented whey-based beverages produced with lactic acid bacteria (_L. delbrueckii_ subsp. _bulgariicus_, _S. thermophilus_ and _L. acidophilus_) and reported on the stabilization, decrease or increase in the pH of beverages during storage (28 days) depending on the experimental variant used. The fluctuation of pH values could be connected to the buffering capacity of whey protein and dairy products. It is noted that this effect could be increased by milk products' treatment, such as heating, high-pressure treatment or salt addition [52]. The same effect was observed in the present study because the samples' pH was no lower than 4.26 \(\pm\) 0.02 and no higher than 4.55 \(\pm\) 0.03. On the other hand, flaxseed gum also consists of a protein fraction with a possible buffering effect [53].