Microfiltration of concentrated milk protein dispersions: The role of pH and minerals on the performance of silicon nitride microsieves

The flux behavior of 0.8 and 0.5 μm silicon nitride microsieves was studied in the pH range from 6 to 9 during microfiltration of milk protein dispersions supplemented with a mineral complex. The physicochemical properties of these dispersions were assessed by particle size distribution (PSD), zeta potential and rheological measurements. Microfiltration was carried out in a pilot cross-flow system where back pulsing was used. Results showed that increasing protein concentration from 30 to 90 g L−1 at pH ≈ 7 has a negligible effect on charge and PSD but increased viscosity and strongly reduced performance of both 0.5 and 0.8 μm microsieves. Adding minerals reduced further the 0.8 μm membrane flux and avoid filtration through the 0.5 μm membrane, despite the fact that PSD was ≈0.3 μm in all solutions. Raising the pH of dispersions at 9.0, which is higher than the isoelectric pH of both micelles and microsieves, drastically increased permeate flux of both membranes. Results from this work point out that even under quite high ionic strength conditions, the flux behavior of microsieves is mainly controlled by repulsive electrostatic protein-membrane interactions, hence, the pH of the dispersion and the membrane charge are critical when processing complex dairy products.