Impact of colloidal interactions on the flux in cross-flow microfiltration of milk at different pH values: A surface energy approach

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 build-up 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.