## 2 Advancements in Ultrafiltration for Protein and Peptide Separation
### Electroultrafiltration (EUF) and Electrodialysis with UF Membrane (EDUF)
Gradiflow Technology arranges the EUF process with parallel electrodes (cathode and anode), which are overflowed by continuous buffer solution and electrified. In the center of the apparatus, the UF membrane is attached between two restriction membranes to start the electrical gradient in the same direction of voltage going in a perpendicular vector with feed flow. Thus, protein mobility will shift between opposite electrodes. In order to obviate the probability of proteins being denatured by electrolysis and protein deposition on the electrodes, electrodes must not be placed inside the suspension and its permeate entry or exit [90]. Some selected works of protein separation by EUF are summarized in Table 4. EUF accelerated bovine serum albumin (BSA) concentration by 80% with bigger molecular weight cutoff (MWCO) of a polysulfone membrane to concentrate 0.5 g L\({}^{-1}\) BSA solution to 1.0 g L\({}^{-1}\)[73]. Again, with two-sided EUF, not only can high purity and short separation be achieved, but also selectivity of fractionation of binary proteins, i.e., BSA and lysozyme. It was said that the selectivity was enhanced to be above 800, owing to the electrophoretic effects that allowed the filtration velocity to be kept high for a prolonged amount of time [91]. It is interesting to note that the environmental conditions of the solution and the membrane configurations can influence the performance in an agonistic or antagonistic way during EUF. For instance, during the separation of BSA and lysozyme at constant external direct current (DC) electrical field of 1000 V m\({}^{-1}\) and pH 7.4 (between the IEP (isoelectric point) of BSA of 4.7 and lysozyme of 11.0), permeate flux increase between 23.4 and 36.7 L m\({}^{-2}\) h\({}^{-1}\) and retention of lysozyme declined by 53% from 73% [74]. Despite all of those advantages, only proteins that have wide difference of IEP can be separated successfully. However, EUF suffers from productivity limitation only in the level of milligrams per hour. Furthermore, EUF can impose conflicting consequences on performance if operated under high conductivity because of the divided electric field between protein and electrolyte mobility [19]. Therefore, in conventional EUF, continuous current is applied; that consumes high energy requirement due to the heat production and alteration of protein on account of electrolysis and leads to other problems. In order to tackle these challenges, the latest development tailors EUF with intermittent or pulsed electric-field (PEF) UF, where the restoration of permeate flux was found to be better. One study showed that turbulence initiated by the static metal sheet, formation of oxygen bubbles around the membrane skin, and the electrophoretic effect by the pulsed electric field at electric field intensity (E) of 700 V m\({}^{-1}\) provided a 300% increase in permeate flux, parallel to the reduction of membrane abrasion [75]. Higher voltage (until it reaches the critical voltage) and shorter voltage of both pulse interval and pulse duration delivered the highest permeate flux and fully mitigated the fouling, which were useful in membrane cleaning [92]. Here, about 25-40% decrease in resistance can be attained by PEF as opposed to the conventional cross-flow UF with zero electric field [93]. Furthermore, PEF treatment has been investigated to improve antioxidant properties of protein hydrolysates of low molecular weight (10-30 kDa) peptides from egg white antioxidant activity, which increased by 44.23% at 10 kV cm\({}^{-1}\) with a pulse frequency of 2000 Hz [94]. Electrodialysis with UF membrane, abbreviated as EDUF, is another strategy to address these deficiencies by uniting the ion-exchange membrane, UF membrane, and electric field between the cathode and anode. Electrodialysis (ED) utilizes electric potential difference and ion-selective membranes (cation-exchange membrane, CEM and anion-exchange membrane, AEM) for ionic separation [95; 96; 97].