## 4 Discussion  
Table 1 shows the rise in the dry substances, fats, and proteins during UF. This could be explained by the passage of the water and low molecular weight substances (such as lactose, salts, and some vitamins) through the membrane [45]. [PERSON] et al., in 1991 [46], established that UF caused a slightly more pronounced increase in fats than in proteins. Ultrafiltration and constant-volume diafiltration were used for obtaining the milk protein concentrates of protein contents > 80% on a dry basis [47]. The ultrafiltration of skim milk was used to partially remove water, lactose, peptides, and other solutes. [PERSON] et al., in 2018 [48], proved that the molecular weight cut-off of the membrane affects the retention of the main components--an UF membrane of a molecular weight cut-off of 10 kDa showed better protein retention in comparison to that of a 50 kDa cut-off. [PERSON], in 2012 [49], found a significant increase in the acidity of the reetentates; however, the pH stayed mostly the same after a two-fold level of an UF concentration of goat's milk with a 30 kDa membrane. Higher acidity in reetentates could be explained by the change in the buffering capacity, which depends on the milk composition, including protein content [50]. The values of specific microorganisms showed that the yoghurts obtained were safe for consumption. The availability of _E. coli_, _S. aureus_, _Salmonella_ spp., fungi, and yeasts, is important for the production of safe and quality products because these microorganisms cause gastrointestinal diseases in human bodies [44]. As expected, the dry matter, fats, and proteins in the yoghurts increased with the rise in the concentration level during UF (Table 2). In the past few years, consumer interest in high-protein yoghurt has increased because of its better taste and texture. There is also more scientific data on the health benefits of dairy proteins. Different processing techniques, including membrane processes, affect the composition, rheology, structure, and sensory characteristics of yoghurts [51]. Upon comparing the microbiological properties of the yoghurts with different starters, it could be concluded that the probiotic yoghurts with the highest counts of viable cells  
Figure 5: Viscosity curves for probiotic Bulgarian yoghurts with starters: (**a**) MZ\({}_{2}\)f; (**b**) MZ\({}_{2}\)f + _Lb. acidophilus_ LAB-8; (**c**) MZ\({}_{2}\)f + _B. bifidium_ BB-87. were those with MZ\({}_{2}\)f + _B. bifidum_ BB-87, followed by those with MZ\({}_{2}\)f + _Lb. acidophilus_ LAB-8 and those with MZ\({}_{2}\)f alone (Figure 1). According to [PERSON] et al., in 2007 [52], UF could be applied to increase the protein content, which led to higher concentrations of _Lb. acidophilus_, _B. bifidum_, _S. thermophilus_, and _Lb. bulgarcius_. According to [PERSON] et al., in 2019 [53], the application of a 50 kDa UF membrane to obtain a retentate with a 10.6% protein content resulted in a rise in the concentrations of _Lb. helveticus_ R0052 and _S. thermophilus_. The application of UF increased the peptides and amino acids and, thus, improved the survival of probiotic strains [54]. Ultrafiltration causes a concentration of caseins that leads to a rise in milk's buffering capacity and a reduction in pH. This leads to a better growth of lactic acid bacteria in UF retentates during fermentation compared to goat's milk without UF. The UF increased the protein content of yoghurts and, thus, favoured the growth of lactic acid bacteria. This provoked a rise in the titratable acidity of yoghurts obtained by UF [55; 56]. The highest value of pH was established for MZ\({}_{2}\)f at the beginning of storage--5.05. The lowest value was obtained for _Lb. acidophilus_ LAB-8 at the end of the storage period (Figure 2)--4.08. This was probably due to the lactic and succinic acid produced by _Lb. acidophilus_ LAB-8 [57]. Figure 2 indicates that UF caused a decrease in pH for all starters studied.