## 3 Results and Discussion
### Analysis of Fermented Milk, Milk-Soya, and Soya Yoghurt Products
#### 3.2.3 Water-Holding Capacity (WHC)  
Water-holding capacity is defined as the ability to retain the aqueous phase in the resulting gel and is usually the inverse of the syneresis resulting from the shrinkage of the protein gel network, which is associated with the inability of the gel network to hold the liquid phase [74]. In this study, the WHC value is determined by the type of starter culture and the type of beverage used to make the yoghurt, as well as the storage time of the samples (Figure 3a-c). The addition of a soya beverage to cow's milk statistically significantly reduced the WHC value, depending on the amount added (in general, the greater the addition, the lower the WHC value). The WHC value was influenced by the microbial composition of the starter culture used, which is probably explained by the acidifying properties of the bacteria used. The lowest WHC values were recorded for samples fermented with propionic bacteria culture only (Figure 3c), while a mixture of yoghurt bacteria and propionic bacteria cultures (Figure 3b) yielded WHC values comparable to those found for samples fermented with yoghurt bacteria culture only (Figure 3a). In some samples, the WHC value changed significantly during refrigerated storage; however, the direction of these changes varied according to the type of starter culture and the type of beverage used to make the yoghurt.  
[PERSON] and [PERSON] [72] indicated that syneresis and the water-holding capacity are determined by the shrinkage of the gel caused by the lowering of the pH value. In this respect, the literature data are highly contradictory. [PERSON] and [PERSON] [49] found that the use of soya beverage in yoghurt production leads to an increase in the amount of whey secreted during cold storage (i.e., a decrease in WHC values). In contrast, [PERSON] et al. [25] showed that the secretion of the aqueous phase decreases (and thus, the WHC value increases) during the cold storage of samples; indeed, the smallest amount of secreted aqueous phase (highest WHC value) was revealed in yoghurt samples in which cow's milk was replaced with the highest amount of soya milk. Additionally, [PERSON] et al. [59] showed a tendency for syneresis to decrease with increased storage time of milk yoghurts.  
Figure 3: (**a**–**c**) Changes in water-holding capacity (WHC) of milk, milk-soya, and soya yoghurt products obtained from beverages fermented by different starter cultures and stored at 5 \({}^{\circ}\)C for 21 days (mean values and standard deviations). \({}^{\text{a,b,c,d,e}}\)—The same letters on each chart mean there are no statistically significant differences between the compared numerical values (\(p<0.05\)).  
However, it should be noted that the structure of the protein gel, and therefore, its hardness, can also determine the WHC value. Research has confirmed this. [PERSON] et al. [25] suggested that the syneresis and WHC value may be due to the total solids of the beverage being fermented. Moreover, [PERSON] et al. [75] indicated that the denaturing effect of the milk and soya proteins and the formation of their joint complexes during heat treatment are decisive for the syneresis and WHC value. In these experiments, cow's milk and soya beverage were separately subjected to UHT treatment, which may have determined the WHC values obtained and influenced the observed changes in these values during refrigerated storage. In addition, [PERSON] and [PERSON] [68] showed a statistically significant effect of the type of starter culture used on the syneresis of different variants of milk yoghurt.