Very high molecular weight cut-off (MWCO) membranes were used in ultrafiltration of apple, apricot, peach and pear pulps in order to increase their consistency for industrial purposes. Sugars and acids were not rejected by the membranes and therefore the pulps mantained characteristics very similar to the original ones while the sterile permeates were suitable additives for beverages and yogurts. In this working condition, pectins form a gel layer on the membranes whose resistance controls the process: the permeate flux J can be correlated with the transmembrane pressure drop Delta P by the equation J = (1/a)Delta P1-n since the gel layer behaves as a compressible filter cake. On the other hand, the average permeate flux does not follow the gel-polarization approach since, instead of being controlled by the accumulation of pectins, it is strongly dependent on the total acidity of the fluid. A correlation is suggested that takes into account the effect of the total acidity and supposes the existence of a limit value.