Pasteurisation, UHT and sterilisation of milk
Why and how milk is heat treated: batch and HTST pasteurisation, ESL, UHT and sterilisation, plant design, the phosphatase test, and regeneration sums.
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Raw milk is an excellent medium for bacteria, including some that cause serious disease, such as those of tuberculosis, brucellosis, listeriosis and food poisoning. Heat treatment made milk one of the safest foods available. Pasteurisation, named after Louis Pasteur, who in the 1860s showed that gentle heating prevented wine and beer from spoiling, was applied commercially to milk from the late nineteenth century and dramatically reduced milk-borne disease, especially among children. Today, different combinations of time and temperature give milks that keep for days in the refrigerator, weeks, or months on a shelf.
Milk heat treatments, processing equipment and quality tests are core topics in dairy technology and food science exams.
Purposes of heat treatment
- Safety: destroy pathogenic microorganisms.
- Keeping quality: reduce spoilage bacteria and inactivate enzymes such as lipases.
- Product functions: for example, denaturing whey proteins to give yoghurt a firmer texture.
The main heat treatments
| Treatment | Typical conditions | Result |
|---|---|---|
| Thermisation | about 57–68 °C for about 15 s | reduces spoilage bacteria in milk held before processing; not a safety step |
| Batch (LTLT) pasteurisation | 63 °C for 30 min | small-scale plants, village dairies |
| HTST pasteurisation | 72 °C for 15 s (71.7 °C in the US standard) | most fresh milk; refrigerated shelf life about 1–3 weeks |
| Higher-heat or ESL milk | often about 125–138 °C for a few seconds, or microfiltration plus pasteurisation, with very clean filling | extended shelf life of several weeks, refrigerated |
| UHT | about 135–150 °C for 1–10 s, then aseptic packaging | commercially sterile; months at room temperature |
| In-container sterilisation | about 110–120 °C for 15–20 min in bottles or cans | long shelf life, stronger cooked flavour |
Pasteurisation standards were set to destroy the most heat-resistant common non-spore-forming pathogens, historically Mycobacterium tuberculosis and later Coxiella burnetii (the Q fever agent), with a safety margin. Pasteurisation does not kill bacterial spores, so pasteurised milk must be kept cold. UHT and sterilisation inactivate spores to the level needed for commercial sterility.
The HTST pasteuriser
A continuous plate pasteuriser has:
- Balance tank, keeping a constant supply.
- Regeneration section: incoming cold raw milk is warmed by outgoing hot pasteurised milk, which is cooled at the same time, saving energy; regeneration commonly recovers around 90 per cent of the heat.
- Heating section, using hot water.
- Holding tube, sized so that the fastest-moving milk is held for at least the required time.
- Flow diversion valve, which automatically returns milk to the balance tank if its temperature falls below the set point.
- Cooling sections, bringing milk to about 4 °C.
Homogenisation and fat standardisation are usually carried out in the same line.
Homogenisation
Most market milk is also homogenised: forced at high pressure (often about 10 to 25 MPa) through a narrow gap, which breaks fat globules into much smaller ones, typically below about 1 micrometre. Small globules, coated with milk proteins, no longer rise to form a cream layer, and milk tastes creamier and whiter. Homogenisation is usually done hot, just before or after the holding section.
Checking pasteurisation
- The alkaline phosphatase test: this natural milk enzyme is destroyed by conditions slightly milder than pasteurisation, so its absence shows milk was properly pasteurised and not mixed with raw milk.
- Recording thermometers and charts for every batch.
- Microbiological tests: total plate counts and coliforms detect post-pasteurisation contamination.
- Lactoperoxidase and turbidity tests distinguish higher heat treatments.
Effects on milk
- Pasteurisation causes little change in flavour or nutritional value; a small loss of some heat-sensitive vitamins.
- UHT denatures more whey protein, produces cooked flavours from sulphur compounds, and can cause slight Maillard browning; long storage can lead to gelation and sediment.
- Sterilisation in container gives stronger cooked and caramel flavours and a darker colour.
- Heat treatment does not significantly change calcium content or protein nutritional value.
Regeneration and holding tube
Regeneration efficiency. Raw milk enters at 4 °C, is heated to 72 °C, and leaves the regeneration section at 64 °C before final heating.
- Regeneration = (64 − 4) ÷ (72 − 4) × 100 = 60 ÷ 68 ≈ 88 per cent.
- The heating section only has to raise milk from 64 to 72 °C, an 8 °C rise instead of 68 °C.
Holding tube. A plant processes 10,000 L per hour (2.78 L per second) with a holding time of 15 s.
- Volume of the holding tube = 2.78 × 15 ≈ 41.7 L (0.0417 m3).
- With a pipe of 76 mm internal diameter (cross-section π × 0.0382 ≈ 0.00454 m2), length = 0.0417 ÷ 0.00454 ≈ 9.2 m based on average velocity.
Because flow is faster at the pipe centre than average, designers lengthen the tube so the fastest particle is still held 15 s, often using a factor of about 1.2 for turbulent flow, giving about 11 m.
Key points
- Heat treatment makes milk safe and extends shelf life; pasteurisation targets pathogens such as Coxiella burnetii but not spores.
- LTLT 63 °C for 30 min; HTST 72 °C for 15 s; ESL around 125–138 °C for seconds with clean filling; UHT 135–150 °C for 1–10 s with aseptic packing; in-container sterilisation about 110–120 °C for 15–20 min.
- HTST plant: balance tank, regeneration, heating, holding tube, flow diversion valve, cooling.
- Alkaline phosphatase test confirms adequate pasteurisation.
- Regeneration efficiency = (temperature after regeneration − raw) ÷ (pasteurisation − raw).
What the exams ask
- School-leaving and entrance papers: why milk is pasteurised and the common methods.
- Degree and research-fellowship entrances: time–temperature standards, HTST plant components, phosphatase test, UHT and aseptic packaging, and effects on milk constituents.
- Dairy technology examinations: plant design calculations, regeneration, holding tube sizing and quality control.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Is raw milk healthier than pasteurised milk?
Pasteurisation causes only small nutritional losses, while raw milk can carry dangerous pathogens. Public health authorities widely recommend pasteurised milk.
Why does UHT milk taste different?
The high temperature creates sulphur and other cooked flavour compounds and changes some whey proteins.
Why must pasteurised milk be refrigerated?
Pasteurisation does not kill all bacteria and spores, and the survivors grow at room temperature. Cold storage keeps them in check until the use-by date.
Sources
- Walstra, P., Wouters, J. T. M. and Geurts, T. J. Dairy Science and Technology, 2nd edition. CRC Press, 2006.
- Tetra Pak. Dairy Processing Handbook. Tetra Pak Processing Systems, online edition.
- US Food and Drug Administration. Grade "A" Pasteurized Milk Ordinance. FDA, latest revision.
- Codex Alimentarius Commission. Code of Hygienic Practice for Milk and Milk Products (CXC 57-2004).
- Deeth, H. C. and Lewis, M. J. High Temperature Processing of Milk and Milk Products. Wiley-Blackwell, 2017.
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