The effects of pretreatment with gelatin and bentonite, pressure (Delta P) and molecular weight cut-off (MWCO) on flux performance of apple juice during ultrafiltration were studied. Filtration data (volume versus time) were satisfactorily fitted to De La Garza and Boulton's exponential model to find the exponential fouling coefficient (k) and the membrane resistance (R-m). Increasing the amounts of gelatin and bentonite used for pretreatment of apple juice decreased k values during ultrafiltration which was a clear evidence of flux improvement by means of delaying the membrane fouling. The molecules in apple juice which are responsible for membrane fouling could be successfully retained by the aggregate formed by gelatin and bentonite. Both k and R-m values increased as Delta P increased during ultrafiltration of apple juice. A decrease in R-m was observed as MWCO of membrane increased. However, the membrane having MWCO of 100 kDa had the highest k value which indicated the increased tendency of this membrane to foul. (c) 2006 Elsevier Ltd. All rights reserved.