The flux behavior during the industrial cross-flow ultrafiltration of apple juice in a batch process was modeled using a combination of the fouling and concentration polarization models. It was observed that the major flux reduction was at the beginning and at the end of operation due to fouling and increasing solute concentration in the feed tank, respectively. The fouling phenomenon was analyzed by classical and empirical models and it was shown that the empirical one has the best correlation within less than 0.3% error for each experiment. The most significant advantage of this model is its ability to predict a steady flux, while other models predict zero flux at infinite time, which is contrary to the observation. With continued permeation, the solute concentration in the feed becomes important in which the back diffusion of solute from the membrane surface to the bulk or concentration polarization becomes controlling. Consequently about 4:53 h after starting the operation, corresponding to increase of solute concentration to about 151% of its initial value, the dominant mechanism changes from fouling to concentration polarization. It was confirmed that at every moment of ultrafiltration actual experimental flux follows that model (fouling or concentration polarization) which predicts lower flux. (C) 2010 Elsevier B.V. All rights reserved.