Selective membrane separation process, driven by a pressure gradient, in which suspended sol- U ids, colloids, emulsified solids such as fat-protein complexes, and dissolved macromolecules with molecular weight in the range 10,000-100,000 Da are retained by the membranes. Molecules that do not pass through the membranes constitute the retentate. Lower molecular weight dissolved materials that pass through the membrane under a driving force of relatively low hydrostatic pressure (1-10 bar) are the permeate. Ultra-filtration is generally used in the concentration and fractionation of large molecules from materials such as cheese whey and milk.