Milk Homogenisation

What Homogenisation Does

Raw whole milk has fat globules ranging from ~0.1 to ~10 µm in diameter, with an average around 3–4 µm. These globules are less dense than the surrounding water phase, so they rise to the surface forming a cream layer (Stokes' Law: see Stokes' Law page). For homogenised retail milk this is undesirable — consumers expect uniform appearance and no cream-line separation.

Homogenisation works by forcing milk through a narrow gap (the homogenising valve) at very high pressure. The combination of high velocity, sudden pressure drop, turbulence and cavitation breaks fat globules into much smaller particles. Average size after homogenisation is typically 0.4–0.8 µm, with a much narrower distribution.

The Two-Stage Homogeniser

A modern dairy homogeniser is a high-pressure piston pump (typically 3, 5 or 7 pistons in series) discharging through one or two homogenising valves. The two-stage design is the industry standard:

The second stage is crucial because immediately after the first stage, the freshly-disrupted fat globules tend to re-aggregate into clusters because their newly-exposed surfaces have insufficient phospholipid and protein coverage. A second pressure drop disperses these clusters before they can re-form.

Single-stage operation

Some applications use single-stage homogenisation: for products that benefit from cluster formation (e.g. some cream products), or where the cost savings justify the slightly inferior product quality. Most fluid milk and yogurt operations run two-stage.

Where Homogenisation Sits in the Process

Homogenisation is normally placed between the regenerator and the heating section of the pasteuriser — at around 60–75°C. This temperature window matters because:

• Above 40°C — the fat is fully liquid, allowing efficient disruption
• Below 80°C — whey proteins haven't yet denatured significantly
• At pasteurisation temperature — aseptic homogenisation post-pasteurisation requires aseptic equipment (used for UHT)

For UHT milk and aseptic products, homogenisation can be either upstream (before sterilisation) or downstream (aseptic homogeniser after sterilisation). Downstream is preferred for product stability but requires significantly more expensive aseptic equipment.

The Milk Fat Globule Membrane (MFGM)

Native milk fat globules are surrounded by a complex membrane (the MFGM) of phospholipids, glycoproteins, cholesterol and enzymes — about 8–10 nm thick. The MFGM stabilises the fat-in-water emulsion in milk and influences flavour, digestibility and nutritional properties.

Homogenisation disrupts the native MFGM. The newly-exposed fat surfaces are coated by adsorbed milk proteins (mainly caseins and whey proteins). The new coating is thinner and less complex than native MFGM, with consequences:

• Increased susceptibility to lipolysis — the new surface is less protective against lipase action, potentially causing rancid flavour if any active lipase is present. Always homogenise downstream of (or simultaneously with) pasteurisation to inactivate lipases.
• Altered creaming and rheology — smaller globules don't cream; viscosity changes
• Improved emulsion stability — resists creaming during long shelf life (essential for UHT)
• Cheese yield impact — not typically used for hard cheese milk (weakens curd); used for some cream cheese and soft cheese to improve yield via casein binding

Pressure Selection by Product Type

Energy Consumption and Operating Cost

Homogenisation is significant energy consumer in a dairy plant. Power requirement is approximately:

For a 50,000 L/day plant, homogenisation energy is around 2% of total plant electrical consumption. This is meaningful at scale.

Maintenance and Common Issues

• Valve wear — tungsten carbide or ceramic valves last typically 2,000–5,000 hours. Worn valves give erratic pressure and increased fat globule size.
• Piston/plunger seals — replaced every 1,000–2,000 hours. Leaking seals reduce pressure delivery and contaminate the crankcase.
• CIP fouling — protein and mineral build-up on plunger ceramic surfaces; CIP regime must include hot caustic and acid steps.
• Noise and vibration — high-pressure pulsing pumps; dampeners and isolation mounts essential for plant noise control.
• Cavitation damage — under-supplied inlet pressure causes cavitation in the cylinders; ensure positive suction pressure (typically 2–4 bar gauge).

Frequently Asked Questions

What is homogenisation in milk processing?

Homogenisation is the high-pressure process that reduces milk fat globule size from ~3–4 µm to under 1 µm by forcing milk through a narrow gap at 150–250 bar. This prevents cream layer formation and gives milk uniform appearance, taste and shelf life.

Why are most homogenisers two-stage?

After the first-stage pressure drop, newly-disrupted fat globules tend to re-aggregate into clusters because their freshly-exposed surfaces have insufficient protein coverage. A second-stage pressure drop (~30–50 bar) disperses these clusters. Two-stage operation also provides back-pressure that stabilises the first stage.

What pressure is milk homogenised at?

Standard pasteurised whole milk runs at 180–220 bar total (first stage typically 150–200 bar, second stage 30–50 bar). UHT milk and yogurt milk go higher (up to 250 bar). Cream is gentler (50–100 bar). Whipping cream is normally not homogenised at all.

Where in the pasteuriser is homogenisation done?

Almost always between the regenerator and the heating section, at 60–75°C. This temperature ensures the fat is fully liquid for efficient disruption while whey proteins remain native. Aseptic post-pasteurisation homogenisation is used for some UHT operations but requires more expensive aseptic equipment.

Can hard cheese be made from homogenised milk?

Not satisfactorily. Homogenisation breaks the milk fat globule membrane (MFGM) and the smaller fat globules become entrained with casein during coagulation, weakening the curd network. The result is a soft, crumbly cheese with poor cutting and ageing properties. Hard cheese always uses unhomogenised cheese milk.

Why isn't whipping cream homogenised?

Whipping depends on partial coalescence of intact fat globules around air bubbles. Homogenised cream has fat globules that are too small and too well-stabilised — they don't coalesce when whipped, so the cream doesn't hold air. Whipping cream is always non-homogenised or only very gently treated.

How does homogenisation affect milk shelf life?

Two effects: (1) it eliminates cream-line separation, making the product visually acceptable for longer; (2) it disrupts the native MFGM, increasing susceptibility to lipolytic rancidity if any active lipase is present. Always homogenise downstream of (or with) pasteurisation to inactivate lipase. Net effect for properly-processed milk is improved sensory shelf life.

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

See related: Milk pasteurisation, Milk separator, Cream production, Stokes' Law, UHT & aseptic processing, Cheese making, Ice cream production, Yogurt production, all dairy science information, consultancy services.