Ultrafiltered milk, also known as UF milk, UF skim, or diafiltered milk, is a subclassification of milk protein concentrate that is produced by passing milk under pressure through a thin, porous membrane to separate the components of milk according to size. Specifically, ultrafiltration allows the smaller lactose, water, mineral, and vitamin molecules to pass through the membrane, while the larger protein and fat molecules are retained and concentrated. The removal of water and lactose reduces the volume of milk, and thereby lowers its transportation and storage costs.