A uniform transmembrane pressure (UTMP) crossflow microfiltration (CFMF) system maintains a low but uniform transmembrane pressure (DeltaP(TM)) With high crossflow velocity (CFV), which reduces fouling and cake build-up, and improves the utilization of available filtration area. A CFMF system, with a 0.2 mum nominal pore size ceramic filter, filtration area 0.184 m(2), was operated in both UTMP and non-UTMP modes. The two modes were compared for their effectiveness in maintaining a steady flux during the separation of casein micelles from skim milk up to a concentration factor (CF) 10 at 50 degreesC, Experiments were performed at an average CFV of 7.2 m s(-1) and DeltaP(TM) from 89 to 380 kPa, Up to CF 4 the non-UTMP mode maintained a slightly better flux and process time than the UTMP mode, but reached the minimally acceptable flux (below 0.005 kg m(-2) s(-1)) at CF 6, Depending upon the DeltaP(TM) maintained, the UTMP mode approached the minimal flux at CF 7 or 10 depending upon the combination of DeltaP(TM) and CFV used, Cake resistance (R-cm) was modified to include the effect of an increase in retentate viscosity with concentration, R-cm increased for the non-UTMP mode and decreased for the UTMP mode with a decrease in the ratio of permeation flux/wall shear stress (J(p)/tau (w)) (which occurred as the retentate gets concentrated), This indicated that the cake formed during the non-UTMP mode of operation was more compact and durable (harder to erode) than in the UTMP mode, A central composite rotatable design estimated the optimal operating region at a CFV of 7.1 m s(-1) and DeltaP(TM) of 241 +/- 10 kPa to achieve maximum flux and a high concentration.