## 3 Results and Discussion
With the time of storage, the highest influence on the count of LAB was noted for casein-iron chelate (Table 3). The statistically significant differences were observed after 1 and 2 weeks of yoghurt storage for _Lactobacillus_ (1 week 7.08 vs. 7.25 vs. 7.84; \(p<0.05/2\) weeks: 7.2, 7.25 vs. 7.52; \(p<0.05\)) and _Streptococci_ (1 week: 7.25, 7.25 vs. 7.48; \(p<0.05/2\) weeks: 7.23, 7.28 vs. 7.41; \(p<0.05\)). The growth of starter cultures also determines the fermentation activity and the time of milk acidification. Titractable acidity and pH measurement are the main factors in determining the degree of development of the casein network responsible for yogurt structure formation. During the production and storage of fortified yoghurt, the acidification process was analyzed by pH (Figure 4) and by the titratable acidity determination (Figure 5). During production, all variants reached the pH of 4.6 in the given production time. The titratable acidity of all yoghurts samples during storage increased by 2-3 \({}^{\circ}\)SH from 37-38 \({}^{\circ}\)SH to 41 \({}^{\circ}\)SH. Higher acidity values were determined in the variants with the addition of hydrolysate and chelate, both after production and after 7 days of storage. Similar results were obtained by [PERSON] et al. [40], who enriched yoghurt with iron sulphate and iron chloride and analyzed the usefulness of the additives directly after production and after 21 days of storage at 5 \(\pm\) 1 \({}^{\circ}\)C. In their research, there were no significant differences (\(p<0.05\)) between the pH values of the control yoghurt and the yoghurt supplemented with iron sulphate. The same authors showed that the addition of the analyzed preparation before and after the heat treatment had no effect on the pH of the yogurt as well as on the starter culture. During storage, the authors observed a gradual decrease in the pH value of all samples. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{3}{*}{\ \begin{tabular}{c} **Lactic Acid** \\ **Bacteria** \\ \end{tabular}  } & \multirow{3}{*}{ \begin{tabular}{c} **Week of** \\ **Storage** \\ \end{tabular}  } & \multicolumn{3}{c}{**Additive:**} \\ \cline{3-5}  & & & **Skin Milk Powder** & **Casein Hydrolysate** & **Casein–Iron Chelate** \\ \cline{3-5}  & & & **log10 [X \(\pm\) SD]** & \\ \hline \multirow{3}{*}{_Lactobacillus_} & 0 & 7.34 \(\pm\) 0.04 \({}^{\text{a}}\) & 7.38 \(\pm\) 0.02 \({}^{\text{ab}}\) & 7.43 \(\pm\) 0.03 \({}^{\text{b}}\) \\ \cline{2-5}  & 1 & 7.08 \(\pm\) 0.04 \({}^{\text{a}}\) & 7.25 \(\pm\) 0.02 \({}^{\text{b}}\) & 7.48 \(\pm\) 0.04 \({}^{\text{c}}\) \\ \cline{2-5}  & 2 & 7.2 \(\pm\) 0.03 \({}^{\text{a}}\) & 7.25 \(\pm\) 0.05 \({}^{\text{a}}\) & 7.52 \(\pm\) 0.04 \({}^{\text{b}}\) \\ \hline \multirow{3}{*}{_Streptococci_} & 0 & 7.59 \(\pm\) 0.02 \({}^{\text{a}}\) & 7.63 \(\pm\) 0.02 \({}^{\text{a}}\) & 7.58 \(\pm\) 0.02 \({}^{\text{a}}\) \\ \cline{2-5}  & 1 & 7.23 \(\pm\) 0.05 \({}^{\text{a}}\) & 7.25 \(\pm\) 0.02 \({}^{\text{a}}\) & 7.48 \(\pm\) 0.04 \({}^{\text{b}}\) \\ \cline{2-5}  & 2 & 7.23 \(\pm\) 0.03 \({}^{\text{a}}\) & 7.28 \(\pm\) 0.05 \({}^{\text{a}}\) & 7.41 \(\pm\) 0.02 \({}^{Yoghurt, with its special structure, is a good example of shear-diluted non-Newtonian fluid, which shows different viscosity with changes in shear rate. The parameter depends upon the size, shape, and chemical nature of molecules and in yoghurt is affected by the strength and number of bonds between casein micelles, their structure and spatial distribution [41]. It increased with elevated amounts of dissolved solids. Many factors were identified that influence the yoghurt structure and textural properties, such as the source of milk, fat content, starter culture, production process, storage, and applied additives. Their application has a significant impact on the rheological properties, and results from the interaction between additives and yoghurt components, such as protein, fat, carbohydrates, and microorganisms conducting fermentation processes.