The invention relates to a process for treating animal skim milk and sweet whey and/or acid whey, having: (a) ultrafiltration (UF1) of a first liquid composition including animal skim milk with 70-90 wt % casein and 10-30 wt % whey proteins, based on total protein, over a first ultrafiltration membrane having a molecular weight cut-off of 2.5-25 kDa using a volume concentration factor of 1.5-6 to obtain a retentate (UFR1) and a permeate (UFP1); (b) ultrafiltration (UF2) of a second liquid composition including sweet whey and/or acid whey over a second ultrafiltration membrane having a molecular weight cut-off of 2.5-25 kDa using a volume concentration factor of 2-15 to obtain a retentate (UFR2) and a permeate (UFP2); and (c) mixing the UF retentate originating from step (a) with the UF retentate originating from step (b) to obtain a mixture of UF retentates.