Within the dairy industry, four different membrane filtration processes are used: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). The figure illustrates which milk and whey components can be concentrated by means of each process, depending on the density of the membrane.
RO is the tightest possible membrane process in liquid separation. It concentrates the total solids, and only water can pass through the membrane; all dissolved and suspended material is rejected.
NF separates a range of minerals from a liquid, allowing only the fluid and certain monovalent ions to pass through the membrane.
The ultrafiltration (UF) membrane separates the feed (e.g. skim milk) into two streams, allowing water, dissolved salts, lactose, and acids to pass through it in either direction, while retaining (and thereby concentrating) proteins and fat.
Microfiltration uses the most open type of membrane, which is used to separate bacteria, spores, and fat globules from the stream, and for fractionation of skim milk.