# The Effect of High-Pressure Treatment and Skimming on Caprine Milk Proteins
## 3 Results and Discussion
[27], the degree of whey protein denaturation in caprine milk treated at 500 MPa reached 54.1% after one day of storage at temperature of 4 \({}^{\circ}\)C.  
In skim milk obtained from the control sample and from HP-treated samples subjected to different pressure, nitrogen compounds were transferred from whole milk to the cream layer after centrifugation. The above was confirmed by [PERSON]'s LSD test which revealed that the average content of the analyzed nitrogen compounds tended to decrease (\(p<0.05\)) in untreated and HP-treated skim milk relative to the corresponding samples of whole milk. The concentration of nitrogen compounds was higher (\(p<0.05\)) in untreated skim milk than in HP-treated skim milk. Subject to the applied process parameters, HP-treatment decreased (\(p<0.05\)) the content of whey proteins in skim milk with a rise in pressure and decreased total nitrogen content and the content of casein in skim milk which reached minimum values at 300 MPa (\(p<0.05\)) (Tables 1 and 2). Liquid-phase whey proteins consist mainly of \(\beta\)-lactoglobulin, \(\alpha\)-lactalbumin and, to a smaller extent, serum albumin, immunoglobulin, lactoferrin, transferrin, ferritin, proteose peptone, calmodulin, prolactin and folate-binding protein. Non-protein nitrogen compounds include ammonia, urea, creatinine, creatine, uric acid and amino acids [18]. The decrease in the protein content of milk after centrifugation can probably be attributed to the interactions between milk proteins and emulsion phase components. In a study by [PERSON] et al. [16], \(\beta\)-lactoglobulin, \(\alpha\)-lactalbumin and \(\kappa\)-casein were associated with milk fat globule membranes in cow's milk under exposure to \(>\)100 MPa, \(\geq\)700 MPa and \(\geq\)500 MPa, respectively. The effect of HP-treatment on whole milk proteins and protein distribution after the separation of the cream layer was determined by electrophoresis. The protein profile of caprine milk samples is presented in Figure 2. Casein fractions, including \(\alpha\)-casein (\(\alpha\)-CN), \(\beta\)-casein (\(\beta\)-CN) and \(\kappa\)-casein (\(\kappa\)-CN), and whey proteins, including \(\alpha\)-lactalbumin (\(\alpha\)-LA) and \(\beta\)-lactoglobulin (\(\beta\)-LG), were identified and quantified in whole milk and skim milk. The results of electrophoretic separation were used to identify proteins with a molecular weight of \(\geq\)40 kDa, including milk fat globule membrane proteins [28]. \begin{table}
\begin{tabular}{c c c c} \hline
**Milk** & & **CN** & **WP** \\ \hline \multirow{4}{*}{Whole milk} & 0.1 MPa & 2.571 \(\pm\) 0.038 \({}^{\text{b}}\) & 0.502 \(\pm\) 0.009 \({}^{\text{a}}\) \\  & 200 MPa & 2.675 \(\pm\) 0.034 \({}^{\text{a}}\) & 0.397 \(\pm\) 0.008 \({}^{\text{c}}\) \\  & 300 MPa & 2.706 \(\pm\) 0.019 \({}^{\text{a}}\) & 0.366 \(\pm\) 0.015 \({}^{\text{d}}\) \\  & 400 MPa & 2.720 \(\pm\) 0.014 \({}^{\text{a}}\) & 0.352 \(\pm\) 0.017 \({}^{\text{d,e}}\) \\  & 500 MPa & 2.758 \(\pm\) 0.019 \({}^{\text{a}}\) & 0.314 \(\pm\) 0.017 \({}^{\text{t,b}}\) \\ \hline \multirow{4}{*}{Skim milk} & 0.1 MPa & 2.388 \(\pm\) 0.048 \({}^{\text{c}}\) & 0.442 \(\pm\) 0.015 \({}^{\text{b}}\) \\  & 200 MPa & 2.162 \(\pm\) 0.137 \({}^{\text{e}}\) & 0.369 \(\pm\) 0.008 \({}^{\text{d}}\) \\ \cline{1-1}  & 300 MPa & 2.074 \(\pm\) 0.015 \({}^{\text{f}}\) & 0.344 \(\pm\) 0.012 \({}^{\text{e}}\) \\ \cline{1-1}  & 400 MPa & 2.286 \(\pm\) 0.077 \({}^{\text{d}}\) & 0.323 \(\pm\) 0.008 \({}^{\text{f}}\) \\ \cline{1-1}  & 500 MPa & 2.356 \(\pm\) 0.052 \({}^{\text{c}}\) & 0.304 \(\pm\) 0.008 \({}^{\text{g}}\) \\ \hline \multirow{4}{*}{Significance (\(p\) value)} & P & 0.000 & 0.000 \\  & S & 0.000 & 0.000 \\ \cline{1-1}  & p \(\times\) s & 0.000 & 0.005 \\ \hline \end{tabular} Abbreviations: p, pressure; s, skimming; p \(\times\) s, pressure and skimming interaction; CN, casein; WP, whey protein. Mean values \(\pm\) SD; n = 4. Values with different superscripts in columns differ significantly at \(p<0.05\), depending on the pressure and skimming interaction.  
\end{table}