## 3 Results and Discussion
### pH Values  
It was determined that the pH value of the raw milk used in the study averaged a value of 7.30 \(\pm\) 0.07. The results obtained on day 1 of the cold storage experiment illustrate the effect of fermentation on the pH value. The samples without fermentation (C samples) exhibited higher pH values compared to the fermented samples (B samples). Furthermore, samples containing the BB-12 monoculture (B and C samples) maintained a higher pH throughout the cold storage period (21 days) compared to the A, K, and OK samples, where either a mixture of different cultures (K and OK samples) or a single LA-5 monoculture (A samples) was used. Both _L. acidophilus_ LA-5 and _B. animalis_ subsp. _lactis_ BB-12 exhibit varying sensitivities to changes in pH. The LA-5 strain demonstrates greater tolerance to low pH values, as evidenced by its relatively high survival rate in OK samples despite significant acidification. In contrast, BB-12 is more sensitive to low pH, as confirmed by its high mortality in OK samples characterized by extremely low pH. Thus, changes in environmental pH can significantly affect the survival of these strains and, consequently, the probiotic properties of fermented mare's milk beverages. Throughout the experiment, we observed that the pH values varied depending on the type of fermented beverage (Table 3), with the highest values (4.23-5.92) recorded in the B samples containing the _Bifidobacterium animalis_ subsp. _lactis_ monoculture, while the lowest values were recorded in the OK samples (3.17-3.25). Statistically significant pH changes were noted during cold storage in all fermented beverage tests. In the case of the A samples, these changes were observed as early as day 7 of storage, whereas significant pH changes in the B and K samples were detected from day 14 onward (\(p<0.05\)). Notable differences in pH between the A samples, fermented with _Lactobacillus acidophilus_, and the B samples, fermented with _Bifidobacterium animalis_ subsp. _lactis_, were also observed. These differences may indicate varying levels of fermentation activity of the monocultures based on the populations of both species (Tables 2 and 4). The K and OK samples had significantly lower pH values compared to the other samples; however, the pH changes in the OK samples were not statistically significant throughout the duration of the experiment. In contrast, the statistical analysis confirmed that the decrease in pH in the C samples was statistically significant. Based on the results, it can be concluded that the BB-12 monoculture preserved its metabolic activity in the B and C samples during the 21-day storage period at 5 \({}^{\circ}\)C, as indicated by the gradual decline in pH. Moreover, the bacterial population in the B and C samples (both containing the BB-12 monoculture) decreased slightly over the 21-day storage period, which may be attributed to slow acidification of the environment, thus promoting the survival of the BB-12 monoculture in this matrix. Interestingly, the pH value of the K samples is close to the pH value of the A samples. It can be assumed that the K samples are characterized by \"metabolic behavior\", like samples containing only the LA-5 monoculture (A samples). We may conclude that the pH value in the co-culture (K samples) is influenced by mutual interactions between the LA-5 and BB-12 cultures, with the predominance of metabolic influence of the LA-5 culture and its metabolic products. In addition, the decrease in BB-12 bacteria in the K samples may be associated with the production of hydrogen peroxide by the LA-5 bacteria present in this sample. On the other hand, the OK samples are characterized by the lowest pH values compared to all other samples, which may result from the presence of specific bacterial cultures. We may speculate that the bacteria present in the original kumis have metabolic activity characterized by the production of metabolic products, leading to a strong acidification of the environment.