## 3 Results and Discussion  
The composition of high-protein milk powders is presented in Table 1. The protein content of high-protein milk powders was as follows: MCC--76.66 \(\pm\) 0.35, SPC--67.75 \(\pm\) 0.33, WPC--60.92 \(\pm\) 0.22, CH--68.12 \(\pm\) 0.23, WPH--68.21 \(\pm\) 0.23, and WPI--91.43 \(\pm\) 0.35. The values of wettability and the insolubility index were determined to evaluate the reconstitution properties of high-protein milk powders. The wettability of all powders was generally >180 s (Table 2). When the powders were wetted, a sticky coating or lumps were formed on the surface, which hindered water absorption by powder particles. Poor wettability could also be attributed to the relatively small size of powder particles and small voids between particles. The low solubility of MCC could be attributed to the cross-linking of casein micelles on the surface of powder particles by non-covalent bonds (e.g., hydrophobic interactions and/or hydrogen bonds). Moreover, the low lactose content of high-protein milk powders contributes to their lower solubility since the interactions between proteins in the presence of lactose result in a different spatial configuration. Micelle agglomeration through non-covalent bonding and high micelle packing were observed due to the low lactose content of the insoluble portion that remained after powder reconstitution. Both phenomena, i.e., the cross-linking of micelles on the powder surface and impeded water transfer into the powder structure are responsible for the poor solubility of MCC [25; 26]. In food powders, solubility is one of the most important attributes which influences other functional properties, such as thickening, gelling, foaming, and emulsifying properties. MCC was characterized by an exceptionally high volume of insoluble particles (>10 cm\({}^{3}\)), whereas SPC was 99.08% soluble. In the tested group of high-protein milk powders, MCC and SPC were characterized by the lowest loose bulk density (0.11-0.16 g/cm\({}^{3}\)) as well as the lowest tapped bulk density (0.14-0.20 g/cm\({}^{3}\)), while the loose and tapped bulk densities of commercial powders achieved higher values. The bulk density of dairy powders is highly dependent on packing density, namely the compressibility of the powder \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Product** & **Fat. \%** & **Protein, \%** & **Water, \%** & **Ash, \%** \\ \hline MCC & 1.46 \(\pm\) 0.08 \({}^{\text{e}}\) & 76.66 \(\pm\) 0.35 \({}^{\text{c}}\) & 5.67 \(\pm\) 0.04 \({}^{\text{d}}\) & 7.11 \(\pm\) 0.14 \({}^{\text{d}}\) \\ \hline SPC & 0.78 \(\pm\) 0.06 \({}^{\text{b}}\) & 67.75 \(\pm\) 0.33 \({}^{\text{b}}\) & 5.11 \(\pm\) 0.03 \({}^{\text{c}}\) & 3.71 \(\pm\) 0.05 \({}^{\text{b}}\) \\ \hline WPC & 1.46 \(\pm\) 0.17 \({}^{\text{e}}\) & 60.92 \(\pm\) 0.22 \({}^{\text{a}}\) & 5.17 \(\pm\) 0.04 \({}^{\text{c}}\) & 4.08 \(\pm\) 0.09 \({}^{\text{c}}\) \\ \hline CH & 1.05 \(\pm\) 0.09 \({}^{\text{d}}\) & 68.12 \(\pm\) 0.23 \({}^{\text{b}}\) & 4.13 \(\pm\) 0.08 \({}^{\text{a}}\) & 12.23 \(\pm\) 0.33 \({}^{\text{e}}\) \\ \hline WPH & 0.99 \(\pm\) 0.09 \({}^{\text{c}}\) & 68.21 \(\pm\) 0.23 \({}^{\text{b}}\) & 4.17 \(\pm\) 0.04 \({}^{\text{a}}\) & 2.98 \(\pm\) 0.17 \({}^{\text{a}}\) \\ \hline WPI & 0.44 \(\pm\) 0.07 \({}^{\text{a}}\) & 91.43 \(\pm\) 0.35 \({}^{\text{d}}\) & 4.89 \(\pm\) 0.05 \({}^{\text{b}}\) & 3.85 \(\pm\) 0.26 \({}^{\text{b}}\) \\ \hline \hline \end{tabular} Results are expressed as means \(\pm\) standard deviation. Mean values with different superscripts in columns differ at \(p<\) 0.05.  
\end{table} 
Table 1: Proximate composition of high-protein milk powders.