The effect of membrane molecular weight cut off (MWCO) at three levels (10, 20 & 50 kD) on dynamic behavior of permeate flux (JP), hydraulic resistances (total hydraulic resistance, RT; reversible fouling resistance, Rrf; irreversible fouling resistance, Rif and membrane hydraulic resistance, Rm) and milk solutes rejection (protein, RP; fat, RF; lactose, RL; minerals, RM and total solids, RTS) have been studied for ultrafiltration of milk. Experiments were carried out using the pilot plant UF membrane system equipped to a spiral wound module and a polysulfone amide membrane. A three-stage strategy based on a resistance-in-series model (boundary layer-adsorption) was used to determine the different hydraulic resistances. The results showed that the JP decreases greatly with increasing process time, but the JP values for 20 kD obtained considerably higher than 10 kD & 50 kD during whole process. RT increased during operation at all levels of MWCO, but the hydraulic resistances values for 50 kD were significantly greater than 10 & 20 kD. The milk solutes rejection results also showed that the RP and RF were almost constant with process time at the corresponding MWCO, whereas the RL, RM and RTS significantly increased.

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