Cost-effective and gentle process
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.
- bacteria/spore removal
- protein/casein standardisation
- concentration and segregation of specific components for very specific, high value milk based ingredients
- a whole range of cultured and cheese products (produced by partial or full concentration)
Processes and applications
Microfiltration (MF) has two different applications for liquid milk, both with skim milk as feed product:
- Removal of bacteria and spores. The purpose could be for cheese milk or milk for milk powder preparation, but could also be for Extended Shelf Life (ESL) milk. Membrane pore size is selected to retain bacteria and spores, allowing all other nonfat milk components to pass.
- Separation of milk proteins into a casein stream and a serum protein stream for manufacture of milk based dairy ingredients. The membrane pore size is selected to retain the large casein micelles, letting the smaller serum proteins pass. This process is often combined with a washing step (diafiltration) in order to refine product streams further and improve yield of serum proteins.
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 standardised 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.
For milk processing by membrane filtration, water is an important aspect to consider – for two reasons. Approximately 86% of whole milk is water, and the equivalent figure for skim milk is approximately 91%. At the same time, many milk filtration applications require water to wash out minor components, and for all applications, filtration requires large volumes of water for cleaning.
The key technology for water recovery is RO, often in combination with an additional polishing step (ROP). RO can be applied to initial whole or skim milk streams or later in the process on MF or UF permeate. The resulting water is of excellent quality and can be used again for both processing water and cleaning water.