Flux decline during dead-end ultrafiltration (UF) of the fermentation broths of Bacillus subtilis ATCC (American Type Culture Collection) 21332 culture was studied, in which polyethersulfone membrane with a molecular weight cut-off of 100 kDa was used. Prior to UF, the broth was treated by centrifugation at 10,000 x g. All experiments were performed at a feed pH of 7, a feed surfactin concentration of 0.56 g L-1, and a stirring speed of 300 rpm but at different applied pressures (Delta P, 86-430 kPa). The resistance-in-series model was used to analyze flux behavior, which involves the resistances of membrane itself and the cake as well as those due to adsorption and solute concentration polarization. It was shown that the resistance due to solute concentration polarization dominated the flux decline under the conditions studied. The resistances due to cake formation and solute adsorption were comparable, and their sum contributed below 3% of the overall resistance. (c) 2008 Elsevier B.V. All rights reserved