The effect of colloidal interactions between casein micelles on the flux of a tubular ceramic membrane at cross-flow microfiltration (MF) of skim milk was studied. Filtration experiments were performed at a pH range of 6.8-5.9. Compared to filtration of milk at its native pH (6.8), the flux was reduced and membrane fouling proceeded faster when acidified milk was filtered. To explain the observed flux behavior, a new interaction model for casein micelles was developed, which incorporates on the basis of the extended DLVO (xDLVO) theory hydrophilic and hydrophobic interactions in the form of Lewis acid-base forces, which were derived from protein surface energies. It could be shown that deposit layer buildup is strongly influenced by the charge-dependent protein surface hydrophilicity, whereas electrostatic interactions between proteins can be neglected in high ionic strength fluids like milk. (C) 2010 Elsevier B.V. All rights reserved.