# A Model for Probiotic Fermented Food Production
## 3 Results and Discussion  
### Growth and Acidification Ability of L. rhamn Sus Strains in Milk  
The growth and acidification capacity of the three probiotic strains were tested under two different temperatures, 37 \({}^{\circ}\)C and 52 \({}^{\circ}\)C, to demonstrate the efficacy of the impedometric test in screening multiple bacteria strains both for their ability to grow in a specific ingredient and at different technological temperatures. For example, dairy milk products like yogurt and kefir have an optimal temperature of up to 37 \({}^{\circ}\)C [70]. The same probiotic strain can also survive at higher temperatures of 52 \({}^{\circ}\)C for a limited amount of time, leading to the production of some types of raw milk cheeses. Traditional aged raw milk hard cheeses such as Swiss Emmentaler PDO, Parmigiano Reggiano PDO, Cheddar, Grana Padano PDO, and Emmentaler PDO typically have a scaling condition, a sub-pasteurization heat treatment applied for the manufacture, which can vary between 52 and 55 \({}^{\circ}\)C \(\times\) 30-40 min [71; 72; 73].  
The impedometric analysis shown in Figure 1 revealed significant variation among the strains in their ability to grow in milk ([PERSON] multiple comparisons of means, \(p\) < 0.005). The probiotic strains HN001 and LR06 showed a higher acidifying capacity associated with an increase in conductivity (M-value), meaning that they were able to grow, metabolize and acidify milk. At the same time, they showed different adaptation behaviors to the temperatures and milk environments. The HN001 strain reached maximum exponential growth, with an M-value of 30 at 30 h, both at the sub-optimal temperature and at the stress temperature, with no significant differences between the two conditions (_p_ = 0.9). On the other hand, the LR06 strain showed an extended _Lag_ phase, reaching the highest M-value of 30 at 50 h under the stress conditions of 52 \({}^{\circ}\)C and after 70 h at the sub-optimal temperature of 37 \({}^{\circ}\)C (_p_ = 0.39). Despite the LR006 strain showing a longer _Lag_ phase, it was able to reach the same level of acidification as strain HN001 under the stress condition, with no significant differences among the two strains at the end of the incubation (_p_ = 0.9). In contrast, the strain LR04 showed a very limited capacity to grow in milk at both temperatures, reaching an M-value max of 10 at the end of incubation.  
The impedometric results were also confirmed via pH evaluation, in which the HN001 and LR06 strains showed the highest acidity level with a pH value of 3.7 \(\pm\) 0.2 and 3.5 \(\pm\) 0.1, respectively (Table 1); strain LR04 showed a limited reduction in pH values compared to the control sample under stress conditions with a pH value of 6.7 \(\pm\) 0.2 versus 7.2 \(\pm\) 0.1. At 37 \({}^{\circ}\)C, the pH of LR04 was higher than the control, reaching 7.3 \(\pm\) 0.5. The ability to lower the pH is an important food safety quality in fermented products since it is linked to the production of organic acids with important antimicrobial features that inhibit spoilage and pathogen bacteria growth [24; 74]. Based on the impedometric findings together with the pH values, the two strains, LR06 and HN001, were chosen for further trial studies to test their capacity to grow and acidify in the cheese environment.