Terms and Principles

See Figure 5.1 and Membrane Processing in the Dairy Science and Technology Education website.

Reverse Osmosis (RO).

A pressure driven process where small molecules (molecular weight less than 100 daltons, eg., water) are separated from larger molecules by a semi-permeable membrane. In practice the term describes a concentration process where water is removed to increase total solids content of a liquid. For example desalination can be accomplished using (RO). It is appropriate to think of RO and the related membrane processes, UF and nanofiltration, as chemical filters where the separation characteristics are determined by the pore size of the membrane and the chemical interactions between the product and the membrane. The most common RO membrane material is cellulose acetate with operating pH of 5 -7. Dairy applications include supplementation of milk evaporators, whey concentration, and waste treatment.

Ultra filtration (UF). 

A membrane process similar to RO where semi-permeable membranes are used to separate large molecules (molecular weight greater than 10,000) such as proteins from small molecules such as sugars. Common membrane materials are polysulfone (operating pH 2 - 12) and ceramic (pH 2-12, retort sterilizable).


A membrane process with separation characteristics intermediate between RO and UF. It is designed to separate small minerals and ions from larger molecules such as sugars. It is used to demineralize cheese whey as an alternative to ion exchange and electrodialysis processes.


A membrane filtration process designed to separate particles greater than .2 µM. Principal dairy applications are spore removal from milk to prevent late gas defect in cheese and to extend the shelf life of pasteurized milk.

Permeate and Retentate. 

Material passing through the membrane is permeate while material retained by the membrane is retentate. For example UF milk permeate is composed of water, sugar, some minerals and non-protein nitrogen compounds. UF retentate is a concentrate of milk fat and protein, including both caseins and whey proteins.

Component processing. 

Developments in membrane processing combined with other conventional and emerging technologies make it possible to isolate and recombine milk components in new ways to produce new products, process conventional products more efficiently, or reduce waste.