Membrane filtration plays a major role in milk processing. Apart from normal fat standardised pasteurised drinking milk it is used for nearly all dairy products as a cost effective and gentle process, which uses no heat.
Microfiltration (MF) has two different applications for liquid milk, both with skim milk as feed product:
Reverse Osmosis (RO) is used for initial concentration before other processes or before transportation, thereby reducing volume/transport cost.
Ultrafiltration (UF) is used to concentrate all large molecules like fat and protein and reduce the content of lactose and minerals by selecting a membrane with a specific pore size between fat/protein and lactose/minerals. These products are normally referred to as Milk Protein Concentrate (MPC) or Milk Protein Isolates (MPI). Diafiltration is often used here to increase protein to dry basis ratio in order to reach for example MPC 85.
UF is also used for concentration of whole or standardized milk for production of various cheeses, fully or partially concentrated.
The diluted streams from MF and UF processes are often concentrated by RO or nanofiltration (NF) or a combination of both in order to remove/recover water or by NF to remove monovalent ions and thereby demineralise.
Two types of membrane filtration processes, namely ultrafiltration and microfiltration, are used for protein standardization in milk. While ultrafiltration is used to standardize the total protein, microfiltration is used to standardize the casein.
Using ultrafiltration, it is possible to concentrate raw or pasteurized whole or skim milk. Ultrafiltration of skim milk can produce different milk protein concentrate (MPC) and milk protein isolate (MPI) products with polymeric spiral membranes. Microfiltration of skim milk can produce casein concentrate with polymeric spiral membranes or ceramic membranes depending on the specific application.
The by-products (permeate stream) from ultrafiltration and microfiltration of milk are also valuable. The permeate stream from the ultrafiltration process is primarily lactose, and this stream is an ideal product for standardization of milk powders.
The permeate stream from the microfiltration process contains native whey proteins that can be further processed into higher value protein products such as whey protein isolate or other whey protein concentrates.