*3.1. Physicochemical Composition*

The gross composition, pH value and acidity of both ovine and bovine milk are shown in Table 1. First, as feed milks were not skim, i.e., 0.05% fat, a significant (*p* < 0.05) reduction of fat contents of permeates was observed. This was expected since MF, even through a membrane of 1.4 μm pore diameter, removes fat [4]. Second, protein content was significantly (*p* < 0.05) lower in both ovine and bovine permeate than in the respective feed milks O and B. This was probably due to the significant lower casein nitrogen (Table 2). In contrast, the water-soluble nitrogen components did not exhibit a significant difference between permeates and retentates of both milks. The casein nitrogen to total nitrogen (CN/TN) ratio showed 1.3% and 5.1% retention of casein for bovine and ovine permeate, respectively, despite the similar average diameter of casein micelles, i.e., 193 nm in ovine and 180 nm in bovine milk [26]. Because MF operates at relatively low pressures, it is susceptible to fouling and to formation of a secondary layer on the membrane by gelatinous material when higher pressure and fluxes are applied [3]. In addition, the length of the tubular membrane that was used was long enough (1.02 m) for the formation of a secondary layer, since it has been shown that tubular membranes 1.2 m long usually operate under a deposit layer [27]. The flux in this experiment was constant during the 15 min of MF process. Therefore, the retention of casein micelles could be due to the formation of a secondary membrane on membrane surface. Other researchers have reported non-significant casein retention during MF (1.4 μm) of bovine milk [13,28].

**Table 1.** Effect of microfiltration on pH, acidity, somatic cells counts (SCC) and chemical composition (%) 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).

The total solids content of permeates were similarly affected by the casein retention (Table 1). Casein retention affects also the phosphorus and calcium contents, since part of these inorganic elements are associated with casein micelles. Because of this, the mean phosphorus content of bovine permeate milk was significantly lower than of bovine feed milk and its retentate. The same trend was observed for the mean phosphorus content of ovine permeate milk. Moreover, the mean calcium content of ovine permeate milk was 148.56 ± 7.77 mg/100g, and it was significantly (*p* < 0.05) lower than in feed milk (198.01 ± 15.86 mg/100g) and retentate (191.06 ± 6.94 mg/100g). Ash contents were like phosphorus and calcium contents, i.e., significantly lower in ovine and bovine permeates than in the respective feed milk (Table 1).


**Table 2.** Effect of microfiltration on the nitrogen components (%) 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).

Finally, pH values were not affected, but acidities of both OP and BP were significantly (*p* < 0.05) lower than in feed milks. Milk acidity is related to protein content and, thus, the lower acidity values of permeates could be associated with the lower protein content that both permeates presented.
