## 4 Discussion  
In our experiments, the counts of both _Lactococcus_ spp. and lactobacilli were detected at levels above 7 and 6 log(CFU/mL) in the kefir samples, respectively (Table 1). The obtained results confirm those shown by other researchers [3; 23; 24; 25; 26]. For instance, [PERSON] et al. [27] reported that the addition of inulin did not affect the counts of _Lactococcus_ spp. in the kefir samples. [PERSON] et al. [28] reported that the number of _Lactococcus_ spp. in kefir samples reduced due to the high acidity of kefir. In our study, we did not observe the lactococci population to be dependent on the pH of kefirs, at least at the stage of milk fermentation. As mentioned in the Introduction section, many researchers have shown that, compared to SMP, WPC significantly stimulates the growth of several strains of lactic acid bacteria [4]. However, we did not notice such an effect in the analyzed kefir samples for lactococci and lactobacilli. Usually, fermentation is carried out until the kefir reaches a pH of 4.4-4.6, which takes from 16 to 24 h [2; 26; 29; 30]. [PERSON] et al. [23] described the 24-h fermentation process as the most effective for kefir production in the context of pH changes. In our experiments, we fermented milk for 24 h to analyze how the addition of different milk protein powders could affect the pH changes (Table 1). The typical pH of commercial kefirs ranges from 4.3 to 4.4 [2], which are comparable to the values measured in our research. However, it should be noted that our kefir samples contained added milk protein powders, and as we know, milk proteins have some buffering capacity and thus reduce the observed pH reduction rate [4; 29; 31]. This is probably the reason for the higher pH values (in the range of 4.4-5.2) observed in the samples prepared with milk protein powders compared to the control sample (mean pH 4.5). The acid gel formed during the fermentation of milk with lactic acid bacteria is a three-dimensional protein matrix in which some other components of the milk base, including polysaccharides produced by the microflora of starter cultures, are trapped. Therefore, when analyzing the texture parameters of kefir prepared using lactic acid bacteria starters, it could be expected that the nature and relative proportions of different proteins play a significant role in determining the texture of the final product, which has already been shown by many researchers [32; 33]. However, studies on the textural properties of kefir prepared with milk proteins are not numerous. The protein content in milk dedicated to the production of kefir can be increased by adding WPC or MPC, or possibly retetate, and caseinate. The use of such additives will have different effects on the protein composition of the dairy base, as the ratio of protein to total dry weight and that of whey proteins to caseins will change, which has a significant impact on the texture of the product. The effect of milk base, starter cultures, and process parameters on texture of fermented milk, based on the example of yogurt, were discussed in detail in reference [33]. The exact influence of these proportions on the texture of kefirs is not easy to estimate, given the additional influence of the starter culture. Very few studies have examined the effects of the ratio of whey protein to casein on the texture of fermented milk beverages without changing the proportion of total protein in the dry matter of processing milk [33]. In our research, we added the same amount of milk protein powders to the processing milk, but these powders were characterized by different protein content and different composition of milk proteins, due to which the kefir samples obtained differed in the total protein content and the proportion of proteins from whey to casein as well as the proportion of total whey proteins.