## 3 Results and Discussion
### Analysis of Fermented Milk, Milk-Soya, and Soya Yoghurt Products
#### 3.2.2 Texture

Table 2: pH values of milk, milk–soya, and soya yoghurt products obtained from beverages fermented using different starter cultures and stored at 5 °C for 21 days (mean values and standard deviations). The firmness of yoghurt products is one of the most important quality criteria. The production of yoghurt and yoghurt-like products that are free from texture defects continues to be a problem in the dairy industry [53; 54]. The texture (i.e., both the hardness and adhesiveness) of yoghurt products depends on the composition of the raw material, the type of bacterial culture used, the fermentation method, and the texturing additives, among other things [47; 48; 52; 55; 56; 57; 58; 59; 60; 61; 62; 63]. In contrast, [PERSON] et al. [64] determined that the rheological properties of fermented milk products depend on the acidity (the higher the acidity, the greater the hardness). Of course, there are also available studies suggesting that the type of milk used and the culture have no effect on the hardness [58; 65; 66] of the analyzed yoghurts [67; 68]. Figure 2: (**a–f**) Changes in the hardness (**a**,**c**,**e**) and adhesiveness (**b**,**d**,**f**) of milk, milk-soya, and soya yoghurt products obtained from beverages fermented using different starter cultures and stored at 5 \({}^{\circ}\)C for 21 days (mean values and standard deviations). \({}^{\text{a,b,c,d,e,f,g,h,i}}\)—The same letters on each chart mean there are no statistically significant differences between the compared numerical values (\(p<0.05\)). The composition of the proteins present in fermented beverages is extremely important [25]. This is supported by this study, which observed differences in the firmness and adhesiveness of milk, milk-soya, and soya yoghurt products. For example, [PERSON] et al. [69] showed that increasing the content of soya \(\beta-\)congplycinin increases the stiffness of the gel, although it significantly reduces the water retention capacity of the gel. The textural characteristics of cow's and soya milk gels can be effectively correlated with their microstructure [70]. However, [PERSON] et al. [25] suggested the formation of some bonds between soya and cow's milk proteins in samples with the addition of soya beverage, which, in their study, appeared to be more compact, with a regular distribution of the protein network and an increase in gel strength during cold storage, compared to milk samples. During fermentation, the proteolysis of both dairy and soya proteins occurs, leading to a homogeneous and dense microstructure. The differences in the hardness and adhesiveness of milk, milk-soya, and soya yoghurt products are also caused by the difference in the isoelectric point between cow's milk and soya beverage; the different protein fractions of soya milk have an electrical point ranging from 5.07 to 5.88, while cow's milk has an electrical point of 4.6 [71; 72]. Since the yoghurt samples in this study were characterized by very low pH values (described above), the above information may explain the recorded hardness and adhesiveness values of the analyzed samples. The acidifying properties of the propionic bacteria, which are weaker than those of the yoghurt starter culture, did not improve the textural characteristics of these yoghurt samples. Furthermore, these results are in line with observations made by [PERSON] et al. [73], who, using a mixture of skim milk and soya beverage in yoghurt production, found that increasing the proportion of soya beverage led to a decrease in the gel strength of the product.