BACKGROUND: Membrane fouling in downstream processing has become a major obstacle in enzyme production. The use of a membrane with high surface hydrophilicity may be an acceptable way to overcome this problem. In this study, the effects of dip time on chitosan incorporation were investigated to produce a low fouling ultrafiltration membrane for trypsin separation. RESULT: Pristine ultrafiltration membrane with a polymer concentration of 15% was developed via phase inversion. Membrane surface modification was performed using chitosan solution with different dip times. Membranes with a 60-min dip time provided optimum trypsin transmission (about 91.8%). Such membranes have a high permeability coefficient (71 L m-2 h-1) and good porosity (about 89.6%). The hydrophilicity of this modified membrane was also improved by 50% compared with the native membrane, and its flux recovery was about 89.8%. The successful assembly of chitosan onto the membrane's surface was ascertained by ATR-FTIR and X-ray diffractometry (XRD). The morphology of this membrane was significantly different from that of native membrane. CONCLUSION: The experimental results show that membrane dip time exerts a significant influence on the self-assembly of chitosan particles onto the membrane's surface, and this process can be used to produce a potentially low-fouling UF membrane for trypsin separation. Copyright (C) 2012 Society of Chemical Industry