Principles of Ultrafiltration and the Concentration and Fractionation of Cow's Milk

The principle of ultrafiltration (UF) is filtration of solutions or suspensions under pressure through a semipermeable membrane. The membrane has pores that allow the solvent and small molecules to pass through and the larger molecules to be retained. Ultrafiltration may therefore be considered as both a concentration and a fractionation process according to the particular components of interest. Ultrafiltration is often associated with reverse osmosis (RO), sometimes called hyperfiltration, and UF and RO are known as membrane processes. The association exists because both are pressure-driven processes, the apparatus and plants for each look very similar, and originally cellulose acetate was the base material of the membranes for both processes. But RO membranes do not have pores and are permeable only to water, so that RO is purely a concentration process. The principle of RO is quite different from that of UF. Water passes through an RO membrane by a solution/ diffusion process and is opposed by the osmotic pressure of the solution being concentrated so that far higher pressures are used for RO than for UF. Ultrafiltration has been used for a long time in the laboratory. For example, a most readable review of UF on this scale was