## 3 Results and Discussion
### Analysis of Fermented Milk, Milk-Soya, and Soya Yoghurt Products  
3.2.1. pH Value  
Table 2 shows the variation in pH value in all yoghurt products. Due to the long fermentation times of the beverages, their initial pH was low, but this resulted in the very good post-fermentation stability of the yoghurt products during 21 days of refrigerated storage. The pH values of almost all samples were significantly unchanged (\(p>0.05\)) after 21 days, but there were significant differences between beverage types. Lower pH values were found in yoghurt products derived from milk-soy beverages than from cow's milk or soy beverages. This may indicate some synergistic effect of the compositions of milk-soy beverages on the acidifying activity of the bacteria used to ferment these yoghurt products. The acidity of the yoghurt products was dependent on the microbial composition of the starter culture used in the experiment; beverages fermented with a yoghurt culture or a mixture of yoghurt and propionic bacteria were the most acidified, compared to beverages fermented with propionic bacteria only. It is worth noting the pH value of the soya beverage fermented by yoghurt bacteria, which increased during the first seven days of storage at low temperature and then began to decrease to a level statistically comparable to that observed immediately after fermentation. Such changes in the pH of yoghurt products were the result of the post-fermentation changes (resulting in different buffering capacities of samples due to changes in the protein fraction and acid content) which did not completely stop at the end of fermentation and continued during the first week of cold storage of the samples.  
It should be mentioned that many authors have noted that the composition of fermentable beverages is important for the final pH value and its changes during cold storage, which are determined by the buffer capacity of the beverage, the different non-protein nitrogen and vitamin contents, and the availability of fermentable carbohydrates which are necessary for the growth of microorganisms [25; 41; 42; 43]. Additionally, the type of starter culture influences the pH of yoghurt samples [26; 44; 45]. Results consistent with this study were obtained by [PERSON] et al. [27], who studied _Propionibacterium shermanii_ subsp. _freudenreichii_ in dairy beverage production. The cited researchers reported significantly more effective pH reduction when _Propionibacterium_ bacteria were used together with lactic acid bacteria than when they were used alone. They also showed that propionic acid bacteria were significantly more effective in lowering the pH of the tested yoghurts when combined with _Lactobacillus acidophilus_, _Bifidobacterium animalis_ subsp. _lactis_, _Lacticaseibacillus rhamnosus_, and _Lacticaseibacillus casei_.  
Many authors have observed changes in the pH of yoghurt products during cold storage; this is explained by the metabolic activity of the starter culture bacteria, which continue to break down the available carbohydrates, albeit at a much slower rate than at their optimum temperature [25; 46; 47; 48; 49; 50; 51; 52]. In the case of the yoghurt products studied in this research, such changes in pH were not observed, which was the result of the initial low pH value (caused by the long fermentation time) and the high degree of attenuation of the available sugars (as described below).