## 3 Results and Discussion
During fermentation, milk undergoes a series of biochemical changes conducted by starter cultures which transform milk into intermediary products that finally achieve the desired consistency and texture. Rheology analysis plays an important role to understand the gelation process, micro-structure, and flow behavior of the obtained product [42]. Different authors [36] analyzed the effect of shear rate on the apparent viscosity of yoghurt and pointed out the inversely proportional correlation between those parameters. In the analyzed yoghurts, the rheological properties were evaluated by the viscosity analysis (Table 4). In our study, no statistically significant (\(p<0.05\)) changes in viscosity  
Figure 4: Changes in acidity [pH] during the storage of yoghurts fortified with the addition of 2% of skim milk powder [blue], casein hydrolysate [orange], casein–iron chelate [grey] (mean values with different letters (a,b,c, etc.) within a single-color component are statistically different (\(p<0.05\))). Figure 5: Changes in tittatable acidity [\({}^{\circ}\)SH] during the storage of yoghurts fortified with the addition of 2% of skim milk powder [blue], casein hydrolysate [orange], and casein–iron chelate [grey] (mean values with different letters (a,b,c, etc.) within a single-color component are statistically different (\(p<0.05\))). were found between the yoghurt variants supplemented with casein hydrolysate and its iron chelate. However, directly after production, their viscosity was about 3 timers lower in comparison to the yoghurt prepared with skim milk powder (0.0659 1/s vs. 0.1889 1/s; 0.0670 1/s vs. 0.1889 1/s; \(p\) < 0.05). In each of the analyzed variants, the apparent viscosity decreased about 3 times during the analyzed storage time, keeping the trend of 3 times lower viscosity in yoghurts with casein hydrolysate and iron chelate in comparison to the control produced with skim milk powder. Additionally, [PERSON] et al. [42] showed that the apparent viscosity of enriched yogurt samples during their 28 days storage at 4 \({}^{\circ}\)C was lower in comparison to the control and decreased with time. The authors suggested that the addition of different additives to yoghurt may interrupt the gel structure of the enriched sample. The same results were reported by [PERSON] et al. [43] and [PERSON] et al. [44], who analyzed viscosity changes in fortified yoghurts. Different observations were made by [PERSON] et al. [45], who reported that apparent viscosity increased with storage time due to the rearrangement of protein and protein-protein interactions. In food production, an important element is the assessment of the product sensory parameters. The presence of iron ions may negatively influence the taste causing the metallic taste and greenish color, and therefore negatively impact consumer acceptance. [PERSON] et al. [36], who analyzed the effect of the addition of various iron salts on the sensory evaluation of yoghurt during 7 days of their storage at 4 \({}^{\circ}\)C \(\pm\) 1 \({}^{\circ}\)C, showed that the product samples evaluated directly after production exhibit a typical color, taste and texture. Authors observed that the storage time had a negative effect on the taste of the final product. Texture and overall acceptability in products assessed directly after production received a higher score compared to those stored for 3 days or 7 days. Different results were obtained by [PERSON] and [PERSON] [46], who concluded that the assessment by consumer panels did not show a significant difference in the appearance of the iron-fortified yoghurt. Our results were similar to those presented by [PERSON] and [PERSON] [46]. The analysis of yoghurt color, consistency, and aroma did not show significant differences between the analyzed samples (Table 5). Independently of the applied additive, yoghurts exhibit the characteristic yoghurt aroma and taste, with no aftertaste or off-flavors.