This work constitutes a first approach to determine the critical zone of stability during gas-sparged crossflow microfiltration (0.1 mum) of skimmed ultra-high temperature (UHT) and reconstituted milks for the separation of casein micelles from soluble proteins. Conditions for stable operation were investigated with and without air sparging by imposing, at a constant wall shear stress, different levels of permeate flux while monitoring the variation in the transmembrane pressure (TMP). The determination of the critical fluxes allowed to assume a common domain of stability for single- and two-phase flows conditions, thus confirming the relevance of the wall shear stress value during 0.1 mum microfiltration of skimmed milk whichever way it is generated (standard crossflow filtration or unsteady gas/liquid flow). Whatever the filtration conditions (single-phase flows/two-phase flows), during the phase of increasing flux, a significant decrease in soluble protein transmission was observed: for reconstituted milk, under two-phase flow conditions, the transmission decreased from 80% to 60% for alpha-lactalbumin (alpha-LA) and from 50% to 30% for beta-lactoglobulin (beta-LG). This was due to the sharp increase in TMP when the flux was close to the limiting flux. During the phase of decreasing J, separation performance was strongly altered: for the same J, the TMP was significantly higher and lower soluble protein transmissions were observed, especially for the beta-LG. These results showed the transition to an irreversible fouling, which led to a more tightly packed, thus less porous, cake structure. Unsteady filtration conditions, as well as standard ones, failed to disrupt it. (C) 2004 Published by Elsevier Ltd.