# The Effect of High-Pressure Treatment and Skimming on Caprine Milk Proteins
## 3 Results and Discussion  
The content of nitrogen compounds in caprine milk (Table 1) was similar to that reported in the literature [17; 18; 24]. Pressure (p) induced significant changes in total nitrogen content (_p_ < 0.05), and the content of non-casein nitrogen (_p_ < 0.05), casein (_p_ < 0.05) and serum proteins (_p_ < 0.05) in the analyzed milk samples, but it had no significant effect on the content of non-protein nitrogen. Skimming (s) exerted a significant decrease (_p_ < 0.05) in the content of all nitrogen compounds. The pressure and skimming (p \(\times\) s) interaction had a significant effect on total nitrogen content (_p_ < 0.05), and the content of non-casein nitrogen (_p_ < 0.05) (Table 1), casein (_p_ < 0.05), and serum proteins (_p_ = 0.005) (Table 2). Similar to pressure (p), the pressure and skimming (p \(\times\) s) interaction had no significant influence on the content of non-protein nitrogen (Table 1). In whole milk, HP-treatment induced a significant decrease (_p_ < 0.05) in the content of non-casein nitrogen, an increase (_p_ < 0.05) in casein levels and a decrease (_p_ < 0.05) in whey protein content, but it did not affect (_p_ \(\geq\) 0.05) the content of non-protein nitrogen relative to the control sample. A significant decrease (_p_ < 0.05) in the concentrations of non-casein nitrogen and whey proteins soluble at pH 4.6 was observed with a rise in pressure. In [PERSON]'s LSD test, the average values of casein content in whole milk subjected to HP-treatment at 200-500 MPa formed a homogeneous group (_p_ \(\geq\) 0.05) (Tables 1 and 2). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Milk** & **TN** & **NCN** & **NPN** \\ \hline \multirow{6}{*}{Whole milk} & 0.1 MPa & & 0.112 \(\pm\) 0.001 a & 0.034 \(\pm\) 0.001 a \\  & 200 MPa & & 0.096 \(\pm\) 0.001 c & 0.034 \(\pm\) 0.001 a \\  & 300 MPa & 0.515 \(\pm\) 0.005 a & 0.091 \(\pm\) 0.003 d & 0.034 \(\pm\) 0.001 a \\  & 400 MPa & & 0.089 \(\pm\) 0.004 d,e & 0.034 \(\pm\) 0.002 a \\  & 500 MPa & & 0.083 \(\pm\) 0.003 f & 0.034 \(\pm\) 0.003 a \\ \hline \multirow{6}{*}{Skim milk} & 0.1 MPa & 0.476 \(\pm\) 0.007 b & 0.102 \(\pm\) 0.002 b & 0.033 \(\pm\) 0.003 b \\  & 200 MPa & 0.426 \(\pm\) 0.023 d & 0.087 \(\pm\) 0.001 e & 0.029 \(\pm\) 0.001 b \\  & 300 MPa & 0.409 \(\pm\) 0.001 e & 0.084 \(\pm\) 0.002 f & 0.030 \(\pm\) 0.001 b \\  & 400 MPa & 0.439 \(\pm\) 0.011 c,d & 0.081 \(\pm\) 0.001 f & 0.030 \(\pm\) 0.001 b \\  & 500 MPa & 0.447 \(\pm\) 0.007 c & 0.078 \(\pm\) 0.001 b & 0.030 \(\pm\) 0.001 b \\ \hline \multirow{6}{*}{Significance (_p_ value)} & P & 0.000 & 0.000 & NS \\  & s & 0.000 & 0.000 & 0.000 \\ \cline{1-1}  & p \(\times\) s & 0.000 & 0.000 & NS \\ \hline \hline \end{tabular} Abbreviations: p, pressure; s, skimming; p \(\times\) s, pressure and skimming interaction; TN, total nitrogen; NCN, non-casein nitrogen; NPN, non-protein nitrogen. 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; in column. NPN, mean values with different superscripts differ significantly between whole and skim milk at \(p\) < 0.05; NS—not significant.  
\end{table} 
Table 1: The effect of high-pressure treatment on the content of nitrogen compounds in whole and skim caprine milk (g/100 g of whole milk). The present study revealed that HP-treatment affects the content of nitrogen compounds in caprine milk (Table 2). The observed decrease in whey protein levels suggests that whey proteins are denatured and enter into interactions with casein. HP-induced denaturation of whey proteins leads to loss of their solubility at pH 4.6 after previous interactions with casein in caprine milk [11; 20] and in bovine milk [6]. HP-treatment denatures up to 80% of \(\beta\)-lactoglobulin in bovine milk [13; 25; 26] and caprine milk [11; 20]. According [PERSON] et al.