## 3 Results and Discussion  
### Viability of Microbial Load and pH  
The survival of microorganisms during processing and storage in fermented milk is highly important [40]. As can be seen in Figure 1, the type of diet, storage time and lactation stage did not significantly affect the number of viable microorganisms of all milk fermented by YO-MIX(tm) 300 starter culture. The sum of the number of viable cells of _Streptococcus thermophilus_ and _Lactobacillus delbrueckii_ spp. _bulgaricus_ was \(>\)10\({}^{7}\) CFU/mL (colony forming units CFU) and the ratio 2:1 (_S. thermophilus: L. bulgaricus_) was maintained during refrigerated storage, being the established requirement for yogurt fermented milk by CODEX ALIMENTARIUS [25; 40; 41; 42]. The symbiosis between these microorganisms has been extensively studied, the protocooperation. [PERSON] et al. (2020) [26] explained the sum of the acid production in a mixed culture is greater than in a single culture. _Streptococcus thermophilus_ produce several components (formic acid, pyruvate acid, carbon dioxide, long chain fatty acids, among others) which stimulate the growth of _L. delbrueckii_ spp. _bulgaricus_, which is highly proteolytic. This causes a pH reduction due to the transformation of lactose into lactic acid, and then, the denaturation of milk proteins and the release of aminoacids that are essential for the growth of _Streptococcus_. According to [PERSON] et al. (2016) [13] the fermentation time decreases when symbiotic cultures are used. Previously, [PERSON] et al. (2019) [25] found differences in the bacterial count of _S. thermophilus_ and _L. delbrueckii_ spp. _bulgaricus_ in fermented goat milk, increasing the viability of _S. thermophilus_ even under low temperature conditions and after 28 days of storage, while the viability of _L. delbrueckii_ spp. _bulgaricus_ decreased. In our study, a general reduction of viable cells was detected with the advance of the lactation stage; counts on FMs at PE and EL productions (three diets) were higher than those on FM from LL stage. As to FMs by MA400 starter culture, a general decrease in the number of viable cells was observed due to the storage time from T2 to T30 of refrigeration (_p_ < 0.05) for the three studied diets and at the four lactation stages (PE; EL; ML and LL). Considering lactation stage and diet, although some significant differences were detected they were considered irrelevant as they were minor in quantitative terms. No scientific studies on fermented millks by MA400 culture were found in the scientific literature as it is especially used for the manufacture of cheese, and also butter and buttermilk. The decrease in viability during storage cannot be directly attributed to any of the microorganisms that are part of this starter culture, but it could be potentially linked to _Lactococcus lactis. L. lactis_ has been used together with other microorganisms in cheese making and it has been observed after six days of storage at 6 \({}^{\circ}\)C, that the decrease in the number of viable cells was due to _L. Lactis_ reduction (from 2.3 \(\times\) 10\({}^{9}\) CFU/g to 1.7 \(\times\) 10\({}^{9}\) CFU/g) [43]. This decrease was attributed to the fact that while LAB grows, lactate is increasing which causes a reduction in pH, inhibiting the growth of _L. lactis_[44]  
One of the key parameters in FMs production is pH decrease in milk; it is the direct consequence of the activity of the inoculated microorganisms. Fast acidification ensures food safety and pH is used as an indicator of the end point of fermentation. No significant differences were found among diets and lactation stage (Figure 2). FMs by YO-MIX(tm) 300 (range 4.6 and 4.18) had higher pH values than FMs by MA400 (range 4.36 and 4.02). A slight decrease of pH was noticed between T2 and T30 for both cultures. These results agree to what was previously found by other authors, were no significant variations of pH during storage for FMs was noticed.