*3.4. Microbial Counts*

The populations of microorganisms are presented in Figure 2. It is obvious that MF affected the microbial groups in a different way, since the retention of specific bacteria depends on their cellular volume [31,32] and on their initials counts in the feed milk. In bovine permeate, total mesophilic microorganisms, coliforms, thermophilic lactococci and lactobacilli were reduced from 1.5 to 2.5 Log, while in ovine permeate, the microbial counts reduction was more efficient, e.g., total mesophilic microflora was reduced about 4 Log. Similar reduction of bacterial load has been reported for bovine permeate [13,17,31], while a reduction 2–3 Log has been reported for ovine permeate obtained with flux 200 L·m−2·h−<sup>1</sup> under pressure 0.6 bar at 40 ◦C [17]. It has been shown that by using a membrane of 1.4 <sup>μ</sup>m pore size and fluxes of over 640 L·m−2·h<sup>−</sup>1, 99.7% of the bacteria can be removed from skim bovine milk [33]. Moreover, by cold MF (1.4 μm) at 6 ◦C, a method to inhibit bacteria growth in the system, an average of 3.4 Log reduction in vegetative bacteria can be achieved [9]. In the present study, the MF system and the applied conditions should meet the nominal reduction of bacteria counts 3–4 Log at flux 166 L·m−2·h−1. In the case of ovine milk, the flux was 105 <sup>±</sup> 32 L·m−2·h−<sup>1</sup> and hence, this milk kind in combination with its higher protein content, which probably caused a thicker deposit layer on the membrane, retained more microorganisms than bovine milk.

Regarding sporeforming microorganisms, they were completely removed from ovine permeate, while in bovine permeate, they were reduced about 0.5–2 Log. The total retention of such microorganisms in the case of ovine milk was also attributed to a thicker protein layer formation on the membrane. In contrast, the insufficient retention of them in bovine retentate followed the trend of all other microbial groups and was attributed to the MF applied conditions in combination with the kinds of the present microorganisms. For example, *Lactobacillus casei*, a thermophilic microorganism, has cell width size 0.5–0.8 μm, whereas *B. cereus*, an aerobic sporeforming microorganism, is 1 μm [32]. Griep et al. [7], using cold MF (1.4 μm) for skim bovine milk, showed that *B. licheniformis* spores were reduced 2.17 log, while *Geobacillus sp*. spores were completely removed.

**Figure 2.** *Cont*.

**Figure 2.** Effect of microfiltration on microbial counts (Log cfu.mL<sup>−</sup>1) 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).
