# The Effect of High-Pressure Treatment and Skimming on Caprine Milk Proteins
## 3 Results and Discussion

Table 2: The effect of high-pressure treatment on the content of casein and whey protein in whole and skim caprine milk (g/100 g of whole milk). Electrophoretic bands denoting proteins with a molecular weight of \(\geq\)40 kDa were less intense in skim milk after cream separation (Figure 2, bands 6-10). Proteins with a molecular weight of \(\geq\)40 kDa were not quantified. The presence of proteins with a molecular weight higher than that of \(\beta\)-lactoglobulin and lower than that of \(\kappa\)-casein was observed in HP-treated milk. The results of densitometric analyses of the protein profile are presented in Table 3. The densitometric quantification of electrophoretic bands in untreated caprime milk revealed the highest content of \(\beta\)-CN, whereas the concentrations of the remaining proteins were arranged in the following descending order: \(\alpha\)-CN \(>\beta\)-LG \(>\kappa\)-CN-\(\alpha\)-LA (Figure 2 band 1; Table 3). The composition of caprime milk proteins is determined by breed, lactation stage and diet [29; 30]. In a study by [PERSON] et al. [29], the proportions of casein fractions \(\alpha_{s1}\)-CN, \(\alpha_{s2}\)-CN, \(\beta\)-CN and \(\kappa\)-CN in caprime milk were determined at 1:2:5:2. Ca-sein fractions \(\alpha_{s1}\), \(\alpha_{s2}\), \(\beta\)+\(\gamma\)-CN and \(\kappa\)-CN accounted for 10.4, 14.5, 62 and 13.2% of total casein in caprime milk, respectively. According to [PERSON] et al. [30], caprime milk contains 2.75% protein, 0.433% whey protein, and 0.119% of \(\beta\)-LG, with an average \(\beta\)-LG/\(\alpha\)-LA ratio of 0.59. A quantitative analysis of proteins in HP-treated whole milk also revealed the highest concentration of \(\beta\)-CN, smaller amounts of \(\alpha\)-CN, and the lowest content of \(\kappa\)-CN (Table 3). The results of two-way ANOVA indicate that pressure (p), skimming (s), and the pressure and skimming (p \(\times\) s) interaction did not influence the content of \(\alpha\)-CN, \(\beta\)-CN, \(\kappa\)-CN, \(\alpha\)-LA and \(\beta\)-LG (\(p\geq 0.05\)). Two-way ANOVA revealed that the casein to whey protein ratio was significantly (\(p<0.05\)) influenced by pressure (p). Skimming (s) and the pressure and skimming interaction (p \(\times\) s) had no effect on the casein to whey protein ratio. The casein to whey protein ratio was higher in milk treated at 500 MPa than in untreated milk and milk subjected to 200-400 MPa. A similar relationship was observed in skim milk obtained from whole untreated milk and in skim milk obtained from pressurized whole milk. Skimming (s) and the pressure and skimming interaction (p \(\times\) s) did not exert a significant influence on the casein to whey protein ratio (Table 4). Figure 2: Electrophoresis on polyacrylamide gel (SDS-PAGE) under reducing conditions. Proteins in whole caprime milk: untreated milk (1); HP-treated milk at \(p=200\) MPa (2), \(p=300\) MPa (3), \(p=400\) MPa (4), \(p=500\) MPa (5). Proteins in skim caprime milk: untreated milk (6); HP-treated milk at \(p=200\) MPa (7), \(p=300\) MPa (8), \(p=400\) MPa (9), \(p=500\) MPa (10). The results of electrophoresis performed under reducing conditions revealed that HP-treatment at \(\leq\)400 MPa did not affect the content of casein fractions (\(\alpha\), \(\beta\) and \(\kappa\)), \(\alpha\)-lactalbumin, \(\beta\)-lactoglobulin (Table 3) or the casein to whey protein ratio (Table 4). The protein content of HP-treated milk decreased after skimming (Table 2), but the proportions of the analyzed milk proteins remained unchanged (Table 3). In the present study, the protein profile determined by electrophoretic separation did not support the identification of proteins that interacted with the milk fat globule membrane. Pressure (p), skimming (s) and the pressure and skimming (p \(\times\) s) interaction significantly (\(p<0.05\)) influenced the total color difference \(\Delta\)E between whole and skim milk. The values of \(\Delta\)E increased with a rise in pressure (Figure 3).