Milk Pasteurisation

What is Pasteurisation?

Pasteurisation is a heat treatment applied to milk and milk products to destroy pathogenic (disease-causing) microorganisms while minimising changes to flavour, colour and nutritional value. The reference pathogen historically used to set the time-temperature standard is Mycobacterium tuberculosis; today Coxiella burnetii (the cause of Q fever) is the practical benchmark, being the most heat-resistant non-spore-forming pathogen of concern in milk.

Pasteurisation has two functions:

• Public health — eliminate pathogens to a level at which they pose no significant health hazard
• Keeping quality — reduce the total bacterial count and inactivate enzymes that would otherwise spoil the product

Pasteurisation does not sterilise. Heat-resistant spores survive (notably Bacillus cereus), which is why pasteurised milk has a chilled shelf life of days to weeks rather than months.

Pasteurisation Methods

Several time-temperature combinations are recognised internationally. The same thermal lethality (F-value) can be delivered by many combinations of time and temperature, but the impact on flavour, colour and protein denaturation differs significantly.

HTST — the workhorse

HTST at 72°C/15s is the dominant commercial method worldwide for fresh milk. It delivers approximately a 5-log reduction in Coxiella burnetii, which provides a safety margin against more heat-sensitive pathogens (Salmonella, Listeria, E. coli, Campylobacter). The 15-second hold is achieved by sizing a holding tube downstream of the heating section based on the product velocity.

UK regulations require milk to be heated to not less than 71.7°C for not less than 15 seconds (or equivalent). EU Regulation (EC) No 853/2004 specifies the same baseline. The US FDA Pasteurised Milk Ordinance specifies 161°F (71.7°C) for 15s, or 145°F (63°C) for 30 minutes for LTLT.

Higher-temperature pasteurisation for special purposes

Some products call for higher temperatures than HTST:

• Yogurt milk — 85–90°C for 5–10 minutes denatures whey proteins and improves gel structure
• Soft cheese — 85–90°C for 30 seconds increases yield by retaining whey proteins in the curd
• Cream for butter — ~80°C is common to overcome the protective effect of fat on bacteria

F-value and z-value — the Mathematics of Thermal Death

Pasteurisation is not just about reaching a target temperature; it is about delivering enough thermal lethality to achieve the required log-reduction in target organisms. This is captured by two parameters:

z-value — the temperature sensitivity

The z-value is the temperature change (in °C) required to change the decimal reduction time by a factor of 10. Each organism (and each spore type) has its own z-value:

F-value — integrated thermal lethality

The F-value is the equivalent time at a reference temperature that delivers the same lethality as a real time-temperature history. For pasteurisation, the reference is usually F₇₂^z=4.4: the equivalent time at 72°C with z=4.4.

The Bigelow model converts any time-temperature combination to its F-value equivalent:

For HTST 72°C/15s with z = 4.34: the same lethality is delivered at 63°C in approximately 30 minutes — matching the LTLT standard. This is not a coincidence; the LTLT and HTST standards were intentionally calibrated to deliver equivalent thermal death.

Plate Heat Exchanger (PHE) Design

HTST pasteurisation is almost universally done in a plate heat exchanger. A modern HTST PHE has four sections:

Key process controls

• Flow rate — sets the residence time in the holding tube. Lower flow = longer hold = higher F-value
• Outlet temperature from heating section — recorded continuously by the chart recorder or DCS
• Flow diversion valve (FDV) — automatic safety valve that diverts under-temperature milk back to the raw side for reprocessing
• Pressure differential — pasteurised side must be at higher pressure than raw side to prevent cross-contamination through a plate leak
• Alkaline phosphatase test — the standard endpoint verification (see below)

Verification — The Alkaline Phosphatase Test

Alkaline phosphatase (ALP) is a naturally-occurring milk enzyme with a thermal death curve almost identical to Mycobacterium tuberculosis. If milk has been correctly pasteurised, ALP activity will be effectively eliminated. The ALP test is therefore the standard regulatory test to verify pasteurisation:

• Pass criterion: ALP activity below 350 mU/L (EU) or "negative" by the phenol method
• Detection method: Fluorimetric or colorimetric assay using a phenol-based substrate
• Limitations: ALP can be regenerated by some heat-resistant microbial enzymes during cold storage, so testing must be done shortly after pasteurisation

Other Microbiocidal Methods

Pasteurisation is the dominant technology, but several other methods can complement or partially replace it:

Bactofugation & centrifugal separation

High-speed centrifuges (bactofuges) remove the densest particles — bacterial spores and somatic cells — from milk. Used in ESL milk and high-quality cheese milk to remove spores that would otherwise survive pasteurisation. Typical efficiency: 90–99% spore removal.

Microfiltration

Membrane filters with pore size 0.6–1.4 µm remove most bacteria, somatic cells and clumps. Used to produce extended-shelf-life ("filtered fresh") milk, often combined with mild heat treatment. The technology preserves more native protein character than HTST alone.

UV treatment

High-intensity UV light (200–280 nm wavelength) destroys microorganisms by damaging DNA. Effective only on surfaces and transparent fluids — not suitable for whole milk due to lack of penetration. Used for packaging sterilisation, water treatment and biofilm prevention.

High-pressure processing (HPP)

Hydrostatic pressures of 400–600 MPa damage microbial cell membranes without significant heat. Used commercially for some premium dairy products in Europe and the US. Cost-effective only for high-value products.

Pulsed electric field (PEF)

Application of short, high-voltage electrical pulses damages microbial membranes. Still largely R&D-stage for dairy in 2026, with limited commercial adoption.

Pasteurisation in the Wider Process

Pasteurisation interacts with several other process steps:

• Homogenisation — usually placed between regenerator and heating section. See our homogenisation page
• Separation / standardisation — typically before pasteurisation. See our milk separator page and milk standardisation calculator
• Cheese milk pasteurisation — same HTST process; some specialty cheeses skip it. See cheese making
• Cream pasteurisation — higher temperature (~80°C) due to fat protecting bacteria. See cream production
• UHT processing — see our UHT & aseptic processing page

Frequently Asked Questions

What temperature is milk pasteurised at?

The standard is HTST: 72°C (161°F) for 15 seconds, then rapidly cooled to 4°C. LTLT (batch pasteurisation) uses 63°C for 30 minutes. Higher temperatures (85–90°C) are used for some specific products like yogurt milk and soft cheese.

What's the difference between pasteurisation and UHT?

Pasteurisation (HTST 72°C/15s) destroys pathogenic bacteria and gives milk a ~10-14 day chilled shelf life. UHT (ultra-high temperature, ~135–150°C for 1–4 seconds) destroys virtually all microorganisms including spores, allowing ambient shelf life of 6–9 months when aseptically packaged. UHT also denatures more whey protein and develops "cooked" flavour notes.

Does pasteurisation kill all bacteria?

No. Pasteurisation destroys pathogens and significantly reduces total bacterial count, but heat-resistant spores (notably Bacillus cereus) survive. This is why pasteurised milk must be kept chilled and has a limited shelf life. Full sterilisation requires UHT or in-bottle sterilisation.

What is the alkaline phosphatase test?

Alkaline phosphatase (ALP) is a naturally-occurring milk enzyme that is destroyed by proper pasteurisation. The ALP test measures residual enzyme activity in pasteurised milk; a negative result confirms correct pasteurisation. EU regulations require ALP activity below 350 mU/L.

How is the holding time achieved in HTST?

By sizing a holding tube downstream of the heating section based on the product flow rate. For 15-second hold at a given flow rate, the tube length is calculated to give 15 seconds of residence at minimum flow. A flow-recorder controller monitors flow, and a flow diversion valve automatically diverts milk back for reprocessing if temperature or flow falls outside spec.

What is the F-value of pasteurisation?

F₇₂^z=4.4 for HTST 72°C/15s is 15 seconds by definition. For comparison, LTLT 63°C/30min has F₇₂^z=4.4 of approximately 14–16 seconds — very close, confirming the two methods deliver equivalent thermal lethality. See our F-value calculator for any other combinations.

Why is cream pasteurised at a higher temperature than milk?

Fat globules physically protect bacteria from heat. Cream at 40% fat requires higher temperatures (typically 80°C+) to deliver the same thermal lethality as 72°C in milk. Similar logic applies to concentrated milk and infant formula reconstituted from concentrate.

Can pasteurised milk make you ill?

Properly pasteurised milk that has been correctly cooled and stored is one of the safest foods. The main risks are post-pasteurisation contamination (during filling, packaging or storage) and spore-forming bacteria like Bacillus cereus that survive pasteurisation and grow during prolonged refrigerated storage. Modern PHE design, aseptic filling and cold-chain integrity address both.

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

Related Downloads

Related calculator: See our Pearson's Square calculator for milk fat standardisation calculations before pasteurisation.

See related: Pasteurisation F-value calculator, Milk pasteuriser training, Homogenisation, Milk separator, Cheese making, Cream production, UHT & aseptic processing, Dairy laboratory testing, all dairy science information, or browse all consultancy services.