The effect of gas/liquid two-phase flows was investigated on a dairy application which deals with the separation of casein micelles from soluble proteins. Particular attention was paid to the influence of the two-phase flow on permeate flux, protein transmission and energy consumption. Commercial ultra high temperature (UHT) skimmed milk and reconstituted milk were microfiltered with a 0.1 mum mineral membrane under constant transmembrane pressure experiments. After the application of a high TMP, hysteresis was observed and gas/liquid two-phase flows, as well as single-phase flows, failed to disrupt the cake of micelles, due to the irreversible modifications of its structure. No hysteresis was found with the variation of the wall shear stress. When comparing performance with and without gas sparging at the same tau(WT), permeate fluxes were similar for both fluids below a critical value of the shear stress, showing that the wall shear stress was the major hydrodynamic parameter involved in the flux improvement. When the wall shear stress exceeded the critical value, two different behaviors were observed: for two-phase flows, permeate flux kept increasing whereas, for single-phase flows, it tended to level off and then decrease. This was explained by the non-uniform pressure conditions along the membrane, due to the increasing pressure drop for higher liquid flow rates. During microfiltration of the reconstituted milk, the transmissions of soluble proteins remained constant with increasing tau(WT) and were very similar: 69 and 65% for alpha-lactalbumin (alpha-LA) and 20 and 25% for beta-lactoglobulin (beta-LG) for single- and two-phase flow experiments, respectively. (C) 2003 Elsevier Science B.V. All fights reserved.