## 2 Materials and Methods
### Experimental Products Made from Mare's Milk  
Fermented mare's milk was produced using the thermostat method, which, alongside the tank method, is widely practiced in the production of various types of fermented milk products, such as yogurt and kefir. The thermostat method is characterized by the fermentation of milk (following inoculation with an appropriate amount of starter culture), occurring directly in individual packaging, with the curd remaining undisturbed by mixing. In the tank method, fermentation takes place in a fermentation tank, after which the curd is dispensed into sterile individual packages, a process that disrupts the curd structure. The method for producing fermented mare's milk, like the one used in this study, is described in the publications by [PERSON] et al. [26] and [PERSON] et al. [27]. The end of fermentation was determined based on the pH and fermentation curve specified in the culture's specifications. For the preparation of the experimental samples (A, B, C, and K), the freeze-dried probiotictic monocultures, LA-5 and BB-12, were activated in sterilized skimmed milk. At the time of inoculation, the concentration of _Lactobacillus acidophilus_ in the starter culture was 8.1 \(\pm\) 0.4 log CFU/mL, while the concentration of _Bifidobacterium_ sp. was 9.0 \(\pm\) 0.2 log CFU/mL. The bacterial counts were also determined in the original Bio-Kumis (Kumylac\({}^{\circled{B}}\)), which was directly used as the starter culture for the laboratory-prepared kumis (K). Based on the obtained results, it was determined that in the original product, the average count of _Lactobacillus acidophilus_ was 7.1 \(\pm\) 0.3 log CFU/mL, and _Bifidobacterium_ sp. was 6.5 \(\pm\) 0.3 log CFU/mL. The experiment analyzed four fermented products and one non-fermented product. The non-fermented product was included to preliminarily assess the survivability of probiotic microflora when the beverage is not subjected to a fermentation process. The fermented beverages included mare's milk fermented with a _Lactobacillus acidophilus_ LA-5 monoculture, mare's milk fermented with a _Bifidobacterium animalis_ subsp. _lactis_ BB-12 monoculture, an original Bio-Kumis purchased in Germany, and kumis produced using a mixture of original commercial Bio-Kumis (40%) and a _Lactobacillus acidophilus_ LA-5 monoculture (7%). According to the literature [28], the microflora of kumis, in addition to specific yeast strains, consists of bacteria from the genus _Lactobacillus_, including _Lactobacillus delbrueckii_ subsp. _bulgaricus_ and _Lactobacillus acidophilus_. The latter is characterized by limited survivability under certain conditions. To produce the non-fermented beverage, a _Bifidobacterium animalis_ subsp. _lactis_ BB-12 monoculture was used. The following experimental sample variants were obtained:  
A sample--mare's milk fermented with _Lactobacillus acidophilus_ LA-5 monoculture;  
B sample--mare's milk fermented with _Bifidobacterium animalis_ subsp. _lactis_ BB-12 monoculture;  
C sample--mare's milk with _Bifidobacterium animalis_ subsp. _lactis_ BB-12 monoculture, without fermentation,  
OK sample--the original commercial Bio-Kumis, purchased at retail;  
K sample--produced with 40% of original commercial Bio-Kumis relative to the amount of mare's milk used and with 7% of _Lactobacillus acidophilus_ LA-5 monoculture. For the preparation of experimental beverages (A, B, C, and K), lyophilized cultures from [PERSON] were used, which were activated in 0% fat UHT milk at 37 \(\pm\) 1 \({}^{\circ}\)C for 10 h for the LA-5 culture and 14 h for the BB-12 culture before use. The end of fermentation was determined by measuring the pH of the samples and comparing the obtained values with the fermentation curve provided in the culture specification. The resulting starters were added to pasteurized mare's milk at a dose of 7% (_w/w_).