## 3 Strategic Development of Ultrafiltration Membrane
### pH in Pretreatment  
Various research studies provided information on the effect of pH on UF performance. Severe fouling can occur in operation at IEP, leading to lower permeate flux [30; 157; 204; 205]. However, it is also expected that at IEP, the transmission of protein become the highest due to the minimum electrostatic repulsion (charged effect) [157]. Interestingly, because of the denser fouling layer developed by employing smaller protein size, the permeability was the lowest at IEP [206]. Intermolecular electrostatic repulsion could be achieved by arranging operating pH higher or lower than majority's IEP, so there will be an added sum of negative- or positive charged molecules that inhibits fouling. However, this condition does encumber the target of permeate flux. While limited peptides exist in the medium, one can easily arrange the process at IEP of targeted compound, demonstrated here by fractioning myoglobin from hemoglobin of yellowfin tuna red muscle. They implemented pH 8.6 which was the IEP of myoglobin and therefore, the charged hemoglobin (IEP 7.3) will have positive charge and minimal mass flow in contrast to myoglobin. Furthermore, in pH 8.6, with higher TMP, the mass flow tended to increase (myoglobin, 12 g m\({}^{-2}\) min\({}^{-1}\) and hemoglobin, 0.8 g m\({}^{-2}\) min\({}^{-1}\)). As the net charge of myoglobin is zero, its molecule expressed the lowest bulk mass transfer diffusion and thereby concentration polarization can contribute in a positive way that increase protein transmission [196]. The same procedure of tuning the pH also takes place while it comes to complex protein solution even though unlike experiment using binary solution model, permeate flux might be lower and its control is harder due to intervention of other factors such as concentration, TMP, etc. Represented during CFPH fractionation, after 20 min at pH 4.0 and 9.0, permeate flux was 37.63 and 39.20 L m\({}^{-2}\) h\({}^{-1}\), respectively, but in their IEP (pH = 5.1), it was only 35.54 L m\({}^{-2}\) h\({}^{-1}\). Additionally, the flux reduction while operation in IEP was nearly 10 L m\({}^{-2}\) h\({}^{-1}\) as opposed to in average 5 L m\({}^{-2}\) h\({}^{-1}\) for pH 4.0 and 9.0 [197]. Plausible reason for this was that in highly acidic or basic environment, high electrostatic repulsion hindered the peptide transmission (40% in pH 4.0, 7.0, and 9.0) which did in fact reach maximum at IEP (48.81%) [197] (Figure 7b). After all, as transmission of desired peptide is far more important that permeate flux of all proteins, operation is usually selected in their IEP point. With respect to plant-based protein, antinutrients like phytic acid and trypsin inhibitors are aimed to be separated from the rest. Removals of phytic acid in soy protein isolate using diafiltration technique showed that pH of 9.0 was not favorable compared to 6.5 [207]. This was because the higher positive charge of phytic acid could stimulate ternary complex of cation-phytic acid, which made removal efficiency to decline. Interestingly, in pH 6.0 of tangential UF of protein concentrates from chickpea with 50 kDa hollow fiber membrane, UF pH 9.0 followed by DF pH 6.0 was capable of reducing the phytic acid to the lowest level that cannot be achieved by isoelectric precipitation at pH 4.5 [208]. Meanwhile trypsin inhibitor content also did not see any changes with all employed variation of single stage UF. So, the reduction of phenolic compounds was better held in the system of UF pH 9.0 and DF pH 9.0. Furthermore, in another finding for producing low-phytate soy protein isolate by EDBM, reducing pH of UF operation from 9.0 to 6.0 will not only accelerate the rate of acidification by two times, but also lower fouling tendencies (total cell resistances) of protein deposition in ED spacers which give 20%-increase on permeate flux [199].