In this work, the membrane technique of nanofiltration (NF) was used to carry out partial demineralization of whey and milk ultrafiltration permeate (MUP). The experiments were performed in a completely automated NF pilot plant. An aromatic polyamide spiral wound membrane supplied by Osmonics (USA) was selected (DK2540C model). Batch and discontinuous diafiltration (DF) configurations were compared. Permeate fluxes were higher for MUP due to the lower amount of proteins, which can cause severe membrane fouling. The degree of demineralization for the divalent cations was negligible for both feed streams. Protein and lactose permeation were also observed to be very low. Ion permeation was higher for MUP probably due to the lower amount of proteins. When discontinuous DF was performed, ion removal increased in the case of whey, but it did not significantly improve in the case of MUP. The degree of ion removal was higher than 30%. Finally, the Donnan Steric Partitioning pore Model was used to predict lactose and ion rejection. A good agreement between predicted and experimental data was observed.

Ultrafiltration (UF) and reverse osmosis (RO) have been used to increase the total solids of skim-milk in order to produce a protein-enriched yoghurt. Milk concentrated by either process can be fermented adequately, but RO does not change the proportions of milk components. Ultrafiltration with 10 kDa cut-off membranes, allows us to obtain concentrated milk with different protein/lactose ratios. Ratios of 1.2-1.3 lead to a good textured yoghurt with up to 35% more proteins than commercial products.

Experimental data are reported for the reverse osmosis (RO) of reconstituted whey from ultrafiltrated whey powder, using the following conditions: Temperature 15°C, differential pressure range 2.0–5.0 MPa and pH=5.1–9.0. Using the osmotic-pressure model, mass transfer coefficients and solid concentration at the membrane surface are determined for the RO process.