## 5 Conclusions  
The development of UF for protein concentration and selective separation encounters more often than not fouling and concentration polarization phenomena. The resurgence of concentration polarization is initialized during the first minute of UF and has reversible feature, in contrast with fouling that is induced over the process duration. These two challenges influence the performance in an individual manner, depending on protein being employed, operating conditions, and membrane configurations. Overall, they generate the trade-off between productivity and selectivity of the process. In order to tackle these challenges, some advancements are developed in protein ultrafiltration. Some reported studies show the advantages of using charged membrane during protein separation, such as higher separation rate, flexible design, and simpler system than pressurized UF. However, the separation is only effective for proteins that have far different IEP, low electrolyte conductivity, and productivity (only in the level of milligrams per hour). Combining UF with ED is useful for separating protein with similar molecular weight but with different charge. Amidst all of the advantages in operating EDUF, water splitting, fouling, and vulnerable membrane integrity still obstruct the scale-up strategies. EDUF could ensure minor degradation of UFM, but the protein fouling on ion-exchange membrane cannot be ignored. Fouling is the major problem of membrane operation, including in ultrafiltration of protein. Numerous studies have been devoted to find a method for controlling fouling formation, such as ultrasonic-assisted UF, dynamic UF, and HPTFF. These methods can suppress fouling phenomena by breaking up cake layer on the membrane surface and decreasing concentration polarization. Another way for controlling fouling formation is by tuning filtration conditions, such as concentration, pH, ionic strength, transmembrane, and temperature. Optimum adjustment of operating condition can result in both high separation performance and fouling reduction. Membrane modification is also an interesting way for performance improvement of UF. Surface modification aims to get the double advantages of structural support and hydrophilicity. Interestingly, low membrane permeability could be tailored for more to improve permeates flux owing to fouling reduction at the same time with solving the selectivity needs. Until now, not so many of membrane modification have been tested by complex protein hydrolysate mixtures, and thus open up the chances of further research. Yet, the effect of electrostatic interactions on the observed protein sieving could be more elaborated. Surface modification may be directed to introduce positive or negative charges on the membrane surface providing electrostatic repulsion. The repulsion mechanism may help to reduce the deposition of foulant on the membrane surface. As the UF in commercial process is joined together with other separation units, each performance, including energy requirements, purity, yields, bioactivities, and economical flux, needs to be evaluated as a whole. In dairy industries, for example, the deficiency of UF-NF arrangement was investigated during treatment of dairy effluent. Advantageously, diafiltration can be incorporated to achieve the desired flux. Other techniques such as integration of UF and MF, cascaded UF, UF membrane stacking, and jet-cookingwith enzyme-assisted UF was revealed to increase the protein content while delivering reasonable flux. Writing--original draft preparation, E.R., R.R. and K.K. ; writing--review and editing, R.B., and I.G.W. ; conceptualization, I.G.W. ; funding acquisition, I.G.W. All authors have read and agreed to the published version of the manuscript. This research is funded by Indonesian Ministry of Research and Technology /National Agency for Research and Innovation, and Indonesian Ministry of Education and Culture under WCU Program managed by Institut Teknologi Bandung. Not applicable. No new data were created or analyzed in this study.