Dairy Membrane Filtration

The Four Membrane Processes

Microfiltration (MF) — Removing Bacteria, Fat or Casein

Bacterial removal MF (0.6–1.4 µm)

Removes bacteria, spores and somatic cells from skim milk while retaining the casein and whey proteins. Used to produce extended-shelf-life (ESL) milk, sometimes called "filtered fresh" milk. Typical efficiency: 99.9% bacterial removal. Combined with mild HTST pasteurisation, gives 30–60 day chilled shelf life.

Casein-from-whey separation MF (0.1–0.2 µm)

Retains casein micelles (~100 nm) while passing whey proteins through. Creates two product streams: casein-enriched retentate (for cheese or casein production) and "ideal whey" permeate (for WPI/WPC manufacturing). Particularly valuable for producing native whey protein without sweet cheese whey character.

Fat / cream MF

0.8–1.4 µm membranes can separate fat globules from skim. Less common; typically separation is done by centrifugal separator.

Ultrafiltration (UF) — The Workhorse

UF is the most widely deployed membrane process in dairy. It retains proteins while passing lactose, salts and water. Membranes are typically rated by molecular weight cutoff (MWCO):

Whey UF (the classic application)

Sweet or acid cheese whey (~6.5% TS, 12% protein on TS) is concentrated by UF to produce WPC:

• Volume reduction 5:1 → WPC 34 (34% protein on dry weight)
• Volume reduction 10:1 + diafiltration → WPC 60
• Volume reduction 20:1 + diafiltration → WPC 80

Milk UF for cheese

Concentrating cheese milk to ~5:1 (i.e. 50% retentate, 50% permeate) before cheese making delivers 15–25% yield gain by retaining whey proteins in the cheese. Used for UF feta and increasingly for cream cheese and quark.

Milk UF for milk protein concentrate (MPC)

Skim milk concentrated to 5:1, 10:1 or 20:1 produces MPC 50, MPC 70 or MPC 80–85 powders. High-value protein ingredients for nutrition, beverages and cheese fortification.

Nanofiltration (NF) and Reverse Osmosis (RO)

NF — selective de-mineralisation

NF passes monovalent salts (NaCl) but retains divalent salts (Ca, Mg) and lactose. Used to partially de-mineralise whey for infant formula applications (where lactose is required but minerals must be reduced). Also used to de-salt salty whey streams.

RO — water removal and pre-concentration

RO essentially removes water, producing very concentrated retentate (up to 25% TS for skim, 30% TS for whey). Applications:

• Pre-concentration before evaporation (saves evaporator energy)
• Whey or milk concentration for transport (reduce volume to ship)
• Producing low-volume concentrates for fortification (e.g. yogurt SNF boost)
• CIP rinse water recovery (filter rinse water back for reuse)

Membrane Equipment

Membrane materials

Module configurations

• Spiral-wound — dominant for UF, NF, RO. Compact, low capital cost. Sensitive to particulates and fat. Typical: 8 inch diameter, 38–40 inch length per element.
• Plate-and-frame — older design; expensive; still used for UF in some operations
• Tubular ceramic — for MF and difficult duties (high fat, abrasive). High capital, low fouling, very long life (10+ years).
• Hollow fibre — common for water/wastewater; less common in dairy

Diafiltration — The Purification Step

Diafiltration is the addition of clean water (or another diluent) to the retentate during UF to "wash through" small molecules that don't pass quickly enough at high concentration. Process:

1. Concentrate by UF to e.g. 5:1 ratio
2. Add water to bring volume back to original; continue UF
3. Repeat 1-3 times until target lactose / mineral level achieved

Diafiltration is essential for producing WPC 80+ (cannot be achieved by concentration alone) and for high-purity MPC powders. It consumes significant clean water; modern operations use NF or RO permeate to reduce demineralisation requirements.

Fouling and Cleaning

Membrane fouling is the gradual decline in flux due to deposition on or within the membrane structure. Caused by:

• Protein adsorption — whey proteins, particularly β-lactoglobulin, adsorb to UF surfaces
• Mineral precipitation — calcium phosphate scaling, especially in concentrated retentate
• Fat deposition — even small fat amounts foul UF rapidly
• Microbial growth — biofilm on permeate side particularly

Cleaning is critical to membrane performance and lifespan:

Operating Metrics

Frequently Asked Questions

What is the difference between MF, UF, NF and RO?

They differ by pore size: MF (0.1–10 µm) removes bacteria, fat and casein; UF (1–100 nm, 1–500 kDa) retains proteins and passes lactose; NF (~1 nm) retains lactose and divalent salts but passes monovalent salts; RO (<1 nm) retains essentially everything dissolved, producing water. Selection depends on which fraction you want to retain vs pass.

What is WPC and how is it made?

Whey Protein Concentrate (WPC) is made by ultrafiltering whey to concentrate the protein. WPC 34 (34% protein on dry basis) is made by 5:1 UF concentration; WPC 80 (80% protein) requires UF + 2-3 diafiltration steps. WPC is then spray dried to produce the final powder.

Can milk be concentrated by RO before evaporation?

Yes. RO can concentrate skim milk to ~25% solids, more than doubling the inlet solids to the downstream evaporator. This saves significant evaporation energy and is increasingly used in milk powder plants. Limited by RO membrane pressure tolerance and concentrate viscosity.

What is the typical lifespan of a UF membrane?

Polymeric (PES) UF membranes last 1–3 years in dairy service with good operating practice. Ceramic membranes last 10+ years — higher capex but lower lifecycle cost for harsh duties. Lifespan is shortened by chemical exposure (high pH, oxidising chemicals), thermal cycling, and physical abrasion.

What is fouling and how is it controlled?

Fouling is the gradual decline in membrane flux due to deposit accumulation on the membrane surface. Controlled by: high cross-flow velocity (sweep boundary layer), proper feed pre-treatment (remove fat for UF), pulsing or backflushing (where supported), and regular CIP with alkaline + acid cycles. Fouling is the primary cause of operational performance issues.

What is diafiltration?

Diafiltration is the addition of water (or other diluent) to the UF retentate during processing to "wash through" small molecules (lactose, salts) that can't be efficiently concentrated past a certain ratio. Essential for high-purity products (WPC 80+, MPC 80+). Done by repeated cycles of concentration + dilution.

Is membrane filtration replacing evaporation?

Complementing, not replacing. RO is often used as a pre-concentration step before evaporation to save energy. UF replaces evaporation for protein concentration (e.g. WPC production). But for converting concentrated milk into powder, spray drying still requires high-solids feed that only evaporation can deliver. The two technologies work together.

Further reading: John Watson publishes articles on dairy industry topics on LinkedIn. Browse all articles by John Watson on LinkedIn →

See related: Dairy Membrane Filtration consultancy, Milk powder & infant formula, Feta cheese (incl. UF feta), Cheese making, Milk pasteurisation, Cream production, Dairy quality control, all dairy science information, consultancy services.