## 3 Results and Discussion
Each of the added substances was subjected to spray drying to reduce water content, to prevent the development of undesired microbiota, and to slow down the enzymatic processes. The lowest water content was noted for casein-iron chelate, and the determined value differed significantly from values determined for casein and casein hydrolysate (6.03% vs. 10.17%; \(p<0.05\)). In all analyzed preparations, the DM content was above 90%. A significantly higher DM content was found in casein-iron chelate compared to other variants (90.0% vs. 94.07%; 90.17% vs. 94.1%, \(p<0.05\)). Casein hydrolysate was characterized by lower acidity compared to the acidity determined for c casein and casein-iron chelate (4.63 \({}^{\circ}\)SH vs. 19.55 \({}^{\circ}\)SH; 4.63 \({}^{\circ}\)SH vs. 14.17 \({}^{\circ}\)SH; \(p<0.05\)). In the preparation, lower protein content was also noted as compared to casein and its iron chelate (78.53% vs. 86.97%; 78.53% vs. 84.17%; \(p<0.05\)). Differences were also found in fat content, the highest levels were found in casein hydrolysate and casein-iron chelate, compared to casein (0.46% vs. 1.25%; 0.46% vs. 1.23%; \(p<0.05\)). The iron chelation process resulted in an over 200-fold increase in iron content in the chelate, compared to casein and casein hydrolysate (2.37 mg/kg vs. 683.54 mg/kg; 3.34 mg/kg vs. 683.54 mg/kg; \(p<0.05\)). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Acidity [\({}^{\circ}\)SH]** & **Protein [\%]** & **Fat [\%]** & **DM [\%]** & **Water [\%]** & **Fe [mg/kg]** \\ \hline \multicolumn{8}{c}{\(X\pm\) SD} \\ \hline Isoelectric casein (IC) & 19.55 \(\pm\) 0.09 \({}^{\circ}\) & 86,97 \(\pm\) 1.10 \({}^{\circ}\) & 0.46 \(\pm\) 0.01 \({}^{\circ}\) & 90.0 \(\pm\) 1.00 \({}^{\circ}\) & 10.17 \(\pm\) 0.76 \({}^{\circ}\) & 3.34 \(\pm\) 0.001 \({}^{\circ}\) \\ \hline Casein hydrolysate (CH) & 4.63 \(\pm\) 0.06 \({}^{\circ}\) & 78.53 \(\pm\) 0.25 \({}^{\circ}\) & 1.25 \(\pm\) 0.01 \({}^{\circ}\) & 90.17 \(\pm\) 0.29 \({}^{\circ}\) & 10.17 \(\pm\) 0.29 \({}^{\circ}\) & 2.37 \(\pm\) 0.001 \({}^{\circ}\) \\ \hline Protein–iron chelate (PCh) & 14.17 \(\pm\) 0.001 \({}^{\circ}\) & 84.17 \(\pm\) 0.25 \({}^{\circ}\) & 1.23 \(\pm\) 0.06 \({}^{\circ}\) & 94.07 \(\pm\) 0.12 \({}^{\circ}\) & 6.03 \(\pm\) 0.06 \({}^{\circ}\) & 683.54 \(\pm\) 0.41 \({}^{\circ}\) \\ \hline \multicolumn{8}{c}{\(X\pm\) SD–average \(\pm\) standard deviation; DM–dry matter; Fe–iron content; mean values with different letters (\({}^{\mathrm{a,b,c}}\), etc.) within a single-color component are statistically different (\(p<0.05\)).} \\ \end{tabular}
\end{table} 
Table 1: Acidity and composition of isoelectric casein (IC), isoelectric casein hydrolysate (CH), and protein–iron chelate (PCh) obtained by hydrolysis with serine protease from _Yarrovia lipolytica_ yeast. Figure 3: Chelating activity [\(\upmu\)g Fe\({}^{2+}\)/mg protein] of casein hydrolysates degraded by serine protease from _Yarrovia lipolytica_ yeast during 3 h of hydrolysis [blue], control sample: non-hydrolyzed isoelectric casein [orange]. After the fortified yoghurt production their basic chemical composition was analyzed (Table 2). The DM was determined at a similar level, ranging from 10.13% to 10.92%. Protein content was significantly higher in yoghurts produced with addition of casein hydrolysate and casein-iron chelate as compared to the control variant where skim milk powder was applied (4.61% vs. 6.68%; 4.61% vs. 5.65%; \(p\) < 0.05). It is noteworthy that the qualitative and quantitative standards for fermented milk and yoghurt require that the protein concentration should keep in a range between 2.7% and 10%. The observed differences in the protein content between the variants might be the result of the use of different components for DM elevation, in which their proportion is variable. The fat content did not differ between the variants and was close to the composition of commercially available, natural, defatted yoghurt. In the analyzed variants of the fortified yoghurts the lactose content was also determined.