## 2 Advancements in Ultrafiltration for Protein and Peptide Separation
### Dynamic UF  
Dynamic UF aims to impede the development of fouling and concentration polarization by leveling up the shear rate [137], which thereafter stimulates turbulences from induced rotational speeds (centrifugal and radial drag forces), notably in mass-transfer-controlled regime [138; 139]. This can be generated by two ways: (1) arrangement of relative motion by assembling the membrane with a rotating disk, impeller, or vibration system as a moving part and (2) developing self-rotating membrane UF. In one review, rotating cylindrical membranes, a rotating disk system, and a vibrating system were extensively discussed with regards to the design, mechanism, and application for general membrane filtration in MF, NF, UF, and reverse osmosis (RO) [33]. Conversely toward conventional filtration, in dynamic UF, the energy requirements and TMP reduction can be minimized since participation of tangential velocity that generates large axial pressure gradients is unnecessary [140]. The major advantages of dynamic filtration are more profound in low MWCO membrane as they will enable operation at high TMP and rise flux until 4000 kPa owing to the extended pressure-controlled regime and lower concentration polarization. Both promising concentration factor (CF) (defined as the ratio between concentration of the concentrate recovered after filtration to initial concentration of solution filtered) and membrane selectivity can be gained with highly viscous solution, and these are implicated in high water recovery [141; 142]. Moreover, using the second generation of shear-enhancing modules like vibratory shear-enhanced processing (VSEP) or multishaft rotating membranes (RDM) and choosing ceramic (not polymeric) membranes, the performance of dynamic UF can be powerfully enhanced [33; 143]. Applications of dynamic UF to reach high VRF are rapidly increasing; one of them is for protein recovery in mung bean wastewater [83]. At TMP 1.2 bar, rotating speed 1403 rpm, and VRF 5.0, they achieved tenfold higher flux at 42 L m\({}^{-2}\) h\({}^{-1}\) as opposed to the cross-flow system. Yet, stabilization (recycling back permeates to feed tank, and gradually increase TMP from 0.4 to 1.0 bar) was found not to positively contribute to fouling minimization and flux enhancement, but in fact, it prolonged the operation time. RDM application can be used to isolate therapeutic small peptides effectively near the IEP, and this has been developed for sesame protein hydrolysate, from which they comprehended that stirrer rotation was limited in use for generating suitable shear rate [144]. Numerous research studies applied rotating disk membrane to fractionate \(\alpha\)-lactalbumin and \(\beta\)-lactoglobulin from whey solution. Since these proteins have similar molecular weight, standard dynamic UF can only see low yield and selectivity. For example, a work with \(\alpha\)-lactalbumin transmission was stagnant between 0.2 and 0.13, the yields were from 28% to 34% [143]. Either rotating disk membranes [140; 141; 143; 144; 145; 146; 147] or vibrating membranes (VSEP) [84; 148; 149; 150; 151] have been intensively used for protein concentration or fractionation in several studies. On the opposite, self-rotating membrane, although it can supplement the complexity of conventional dynamic UF, to the best of our knowledge, is not well-known as an up-to-date application for UF. One experiment of [PERSON] et al. (2015) performed dynamic UF with self-rotating membrane for concentrating skim milk protein by a pilot-scale apparatus, and compared the result with spiral wound membrane (SWM) [152]. They described an intriguing phenomenon that there was a critical flux controlling permeate flux in dynamic UF by self-rotating membrane as medium rotational speed (1000 min\({}^{-1}\)) gave the highest flux (35 L m\({}^{-2}\) h\({}^{-1}\)) due to permeate backpressure or altercation of the smaller particle deposition beyond a critical speed. Flux was also found to increase linearly until TMP 100 kPa.