*3.2. Protein Composition*

Further analysis of proteins by RP-HPLC revealed the results shown in Table 3. It is obvious that the application of crossflow MF, even with a membrane of 1.4 μm pore size, resulted in the fractionation of casein micelles, since protein permeation depends on the interaction between membrane pore and protein size. Both ovine and bovine milk permeates had significantly (*p* < 0.05) higher κ-casein (κ-CN) contents and lower β-casein (β-CN) contents than their counterpart's retentates. Similar, but not statistically significant, results have been reported by Tziloula et al. [28], who showed that micelles of bovine milk with a diameter greater than 550 nm were retained during MF (1.4μm). In ovine permeate, except for β-CN, the αs1-CN and αs2-CN contents were significantly (*p* < 0.05) lower. The loss of casein fractions and especially those of κ-CN in retentate may be due to the damage of the casein micelles surface, which is caused by the shear forces in the membrane circuit pump [11]. In the case of ovine milk, the shear forces were lower because of the lower flux compared to bovine milk and, thus, more κ-CN remained in permeate. On the other hand, α-lactalbumin (α-La) and β-Lg were concentrated in the permeates because of their smaller volume. The particle diameter of bovine α-La and β-Lg was less than 10 nm [29].

**Table 3.** Effect of microfiltration on casein fractions (% of total protein) and the main whey proteins (% of total protein) of partially defatted bovine (B) or ovine (O) milk, bovine permeate (BP), bovine retentate (BR) (mean ± SD, n = 3), ovine permeate (OP) and ovine retentate (OR) (mean ± SD, n = 5).


For the same milk, means with different superscript in the same row differ significantly (*p* < 0.05).
