The pH value and TA acidity play a significant role in the viability of lactic acid bacteria during fermentation and storage, as well as the quality of fermented dairy products. The main function of a fermented milk starter is to initiate and facilitate the production of lactic acid through the metabolic activity of lactic acid bacteria during the fermentation process [
18]. The pH quality parameters for T1, T2, T3, and T4 were recorded concurrently.
Figure 3A illustrates the pH variations among different treatment groups with varying generations (
p < 0.05). Based on the statistical analysis, the pH values of T1, T2, T3, and T4 were found to be significantly different. The pH values of T1 were found to be significantly lower than those of other treatment groups (
p < 0.05). As per Jovanović M et al. (2021) [
19], lactic strains possess the ability to ferment lactose into lactic acid, which leads to an increase in acidity and a decrease in the pH of yogurt. The production of lactic acid ultimately results in a reduction in pH. During the first generation of yogurt in T1, the pH value was recorded as 4.22, which is lower than the pH values of T2 (4.77), T3 (4.66), and T4 (4.50), respectively (
Table 2). The average pH value for eight generations, from the first to the eighth generation, was 4.34 for T1, 4.47 for T4, 4.48 for T3, and 4.49 for T2. The average pH value of T1 yogurt is lower than that of the other treatment groups, indicating that lactic acid bacteria quickly become the dominant group during the fermentation process of T1 yogurt, resulting in better fermentation quality compared to the other treatment groups of yogurts. Undoubtedly, pH and acidity control are significant parameters in yogurt processing, as they play a significant role in curd coagulation, ripening, and shelf life of the final product [
20]. According to the present study, the pH value of the T1 group decreased as a result of the addition of lactic acid bacteria, consistent with the findings of Akgun et al. (2018) [
21]. In their study, Akgun et al. observed that the pH values of the milk during processing, from the time of inoculation with bacterial cultures to the time of yogurt manufacturing, decreased from 6.70 to 4.34. It has been demonstrated that the pH value typically decreases to a range of 4–5 during the yogurt fermentation process, which is attributed to the production of lactic acid that lowers the pH. At the isoelectric pH (pH 4.6) approach, the casein micelles lose their steric stability, resulting in their flocculation, precipitation, and subsequent formation of a coagulum, as noted by Loveday et al. in 2013 [
22]. The decrease in pH during yogurt fermentation is attributed to the activity of microorganisms that utilize residual carbohydrates and produce lactic acid, small amounts of CO
2, and formic acid, as reported by Vital et al. in 2015 [
23].
The acidity analysis was conducted by calculating the mean values of different treatment groups across different generations simultaneously. The results were analyzed using one-way ANOVA, and the means were compared accordingly.
Figure 3B shows that the average acidity values of T1, T2, T3, and T4 yogurts during the passage process were 0.82%, 0.75%, 0.76%, and 0.75%, respectively. The average acidity value of T1 yogurt was observed to be higher than that of the other yogurt groups (
Table 3). According to Servili et al. (2011), the elevation in acidity levels can be attributed to the growth and activity of lactic acid bacteria [
24]. In the control condition, the commercially available plain yogurt, which did not undergo any special treatment, showed nonsignificant results over time. However, it was also observed that the peach oat flavor yogurt demonstrated significant acidic values (
Table 3). Vinderola et al. (2019) reported that the co-fermentation of
L. bulgaricus and the starter could reduce the postacidification of yogurt and improve the stability of products during storage [
25]. Typically, there exists an inverse relationship between pH and acidity, with lower pH values indicating higher acidity levels. Throughout the yogurt fermentation process, a general trend of decreasing pH values accompanied by increasing acidity levels was observed [
26]. As suggested by Wang et al. (2021), this phenomenon may be attributed to the continuous production of lactic acid and other organic acids resulting from the consumption of lactose by lactic acid bacteria and starter cultures [
27].