## 3 Strategic Development of Ultrafiltration Membrane  
### Membrane Modules and Fiber Diameter  
Different type of membrane module showed changes in efficiency and effectiveness to isolate specific proteins. As performed by the spiral and tubular polymeric modules that produced soy protein isolate, the later was capable of increasing the permeate flux (85 L m\({}^{-2}\) h\({}^{-1}\)) at higher velocity rate (2.5 m s\({}^{-1}\)) and 70 psi for 5% total solids. Meanwhile, the spiral modules only gave 19 L m\({}^{-2}\) h\({}^{-1}\) at 0.5 m s\({}^{-1}\) and 55 psi [174]. Moreover, spiral wound membrane is also limited to achieve higher VRF by high viscosity of retentate while compared to dynamic UF. However, combining this module with tubular ceramic membrane step would allow the VRF enhancement >8 (previously <5-6) [152]. Further consideration to opt the tubular polymeric modules is that they are endowed with lower operating and capital cost to reach the same performance. It should be emphasized that although different module was utilized, similar performance would be obtained if the same parameter \(\upbeta\) is used (detailed in Section 2.1). This result is reported during negatively charged UF of RC 100 kDa in a laboratory-scale flat-sheet and pilot-scale spiral-wound membrane (70\(\times\) larger area) that both were available for retaining more than 98% whey protein [62].  
Interestingly, membrane modules also determine specific amino acid residues retained or passed through the UF process. Of soy protein hydrolysate generated by consecutive pepsin and pancreatin treatment and following fractionation by hollow fiber and spiral-wound membrane with the same 10 kDa MWCO, there are four intriguing phenomena: (1) longer UF time of hollow fiber (3 h) led to higher content of glycine, threonine, valine, methionine, and isoleucine in permeate, representing smaller peptide as opposed to spiral-wound (10 min); (2) primary aromatic amino acids (tyrosine, leucine, and phenylalanine) produced by pancreatin (chymotrypsin) activity shows similar concentration in permeate of both membrane module; (3) amino acids constituting both low and high molecular-weight peptides, such as aspartic acid, serine, histidine, alanine, proline, arginine, and cysteine, have similar concentration in permeate and initial hydrolysate; and (4) highly retained amino acids (glutamic acid and lysine), which belong to high molecular-weight peptides, are more concentrated in the permeate of hollow fiber compared to spiral-wound. Yet, slightly higher antioxidative (ORAC assays) and antiviral activity (PFU assays) was found in the use of hollow fiber configuration [175].  
Not only the type of module, but the diameter of the module influences the overall performance. Here, fiber diameter of a hollow fiber module was investigated to have impact on initial permeate flux and reversible fouling of pea protein concentrates [176]. Smaller lumen internal diameter has lower resistances and therefore must have better permeate flux according to Darcy's Law. It is exemplified that a 1.5 mm fiber module had 1.22 \(\times\) 10\({}^{-5}\) m s\({}^{-1}\) initial flux, while that of 0.5 mm in diameter was 1.40 \(\times\) 10\({}^{-12}\) m s\({}^{-1}\) for the same pressure and viscosity. The lower resistance is demonstrated to be due to minimum cake formation, induced by the predominant drag force that diminishes background of foulants (this was not by means of shear rate).