## 3 Results and Discussion
### Acidity of Yogurts during 21 Days of Storage  
Acidity is an important parameter determining product quality [1]. The initial active acidity (pH) of the yogurts made from the control milk (without the addition of herbs) and the herbal milk yogurts were similar, i.e., 4.59 and 4.60, respectively (Figure 2). During storage, the pH value of the yogurts gradually decreased and reached the lowest value, i.e., 4.41 and 4.43, respectively, on storage day 21. Significant differences (\(p\leq 0.05\)) were noted only between the initial (day 0) and final (day 21) storage time. An exception was the experimental yogurt, as its pH value on storage day 7 slightly increased in comparison with the value recorded on day 0. The same changes were noted in the case of potential acidity (Figure 3). In the process of sugar fermentation, lactic acid bacterial strains used to manufacture yogurts produce from 0.6 to 1.0% lactic acid, which is responsible for the specific sensory traits and durability of the product. The titrimetric acidity of the experimental yogurts was within the normal range (at least 0.6% lactic acid content) [31]--Figure 3. The lowest content of lactic acid was recorded on storage day 0, i.e., 0.85% in the control yogurts and 0.83% in the experimental samples. During storage, the acidity of the CY samples gradually increased and reached the highest value on day 21 (0.95%). The potential acidity of the experimental yogurt (EY) slightly decreased by 0.04% after 7 days of storage and then increased significantly (\(p\leq 0.05\)) to 0.98% on day 21. The increase in the lactic acid content and the decrease in pH in stored yogurts are caused by the fermentation activity of microorganisms present in yogurt inocula. In refrigeration conditions, bacteria still decompose lactose, but the process is much slower than at the optimum temperature for thermophilic bacteria [1; 29; 45]. Various authors [46; 47] have reported an increase in active acidity (reduction of pH) with a simultaneous increase in lactic acid content in yogurts during 28-day storage. [PERSON] et al. [48] recorded a decrease in pH to 3.58 and an increase in lactic acid content to 1.24% during 21-day storage of yogurts. [PERSON] et al. [49] found that the acidity of yogurts was influenced by both the storage time and the addition of tea. As expected, the initial pH value steadily decreased from 4.65 to 4.36 (\(p<0.05\)) during 28-day storage. The pH values in the tea-supplemented yogurts were significantly (\(p<0.05\)) lower (by 0.09-0.15 units) than the mean value determined in the control yogurts. A similar trend in pH was found after the addition of _Moringa oleifera_ extracts to milk [50]. The effect of the storage time on pH was also significant (\(p<0.001\)), as significant differences were observed between \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Method** & \multicolumn{2}{c}{**Milk**} & \multicolumn{2}{c}{**Yogurts**} \\ \hline  & CM & EM & CY & EY \\ ABTS & 3.02 \({}^{\text{AX}}\)\(\pm\) 0.26 & 4.03 \({}^{\text{BX}}\)\(\pm\) 0.23 & 4.98 \({}^{\text{AY}}\)\(\pm\) 0.28 & 6.87 \({}^{\text{BY}}\)\(\pm\) 0.86 \\ DPPH & 1.14 \({}^{\text{AX}}\)\(\pm\) 0.15 & 1.25 \({}^{\text{BX}}\)\(\pm\) 0.18 & 2.52 \({}^{\text{AY}}\)\(\pm\) 0.16 & 3.26 \({}^{\text{BY}}\)\(\pm\) 0.28 \\ FRAP & 8.97 \({}^{\text{AX}}\)\(\pm\) 1.13 & 13.1 \({}^{\text{BX}}\)\(\pm\) 2.05 & 17.13 \({}^{\text{AY}}\)\(\pm\) 0.65 & 22.58 \({}^{\text{BY}}\)\(\pm\) 0.98 \\ \hline \hline \end{tabular}  
* ABTS—2,2’-azino-bis(3–ethylbenzothiazelle-6-sulfonic acid) assay; DPPH—2,2-diphenyl-1 picrylhydrazyl assay; FRAP—Ferrie Reducing Antioxidant Power assay. CM—control milk; EM—experimental milk; CY—control yogurts; EY—experimental yogurts; \({}^{\text{Ab}}\)—significant differences at \(p\leq 0.05\); \({}^{\text{A,b}}\)—significant differences at \(p\leq 0.01\). \({}^{\text{XY}}\)—significant differences at \(p\leq 0.01\). \({}^{\text{XY}}\)—significant differences at \(p\leq 0.01\).  
\end{table} 
Table 2: Antioxidant activity of milk and yogurts in mg Trolox/100 mL.