Claimed is the preparation of an infant formula base product, comprising: (a) microfiltration (ME) of animal skim milk over a membrane having a porosity of 0.10 - 0.35 micrometer and operating with a volume concentration factor of 1.5 - 8, resulting in a retentate (MFR) and a permeate (MFP)； (b) ultrafiltration (UF) of the MEP originating from step (a) over a membrane having a molecular weight cut-off of at most 25 kDa and operating with a volume concentration factor of 1.5 - 8, resulting in a retentate (UFR) and a permeate fUEP)； (c) combining the MFR from step (a) and the UFR from step (b), to obtain a defined casein/whey ratio. Alternatively, the product of step (a) is used as a protein source. Claimed is also a composition comprising 40 - 50 g casein, 30 - 40 g lactose and 5 - 16 g ash per 100 g dry weight.
