DAIRY TECHNOLOGY: MILK PROCESSING
Diagram
Pasteurized milk and UHT milk side by side
Pasteurized milk
Milk heated gently, just enough to kill pathogens. It must stay cold and lasts days.
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UHT milk
Milk heated very hot for a few seconds, then packed sterile. It keeps for months without a fridge.
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The core difference
Heat level and packing. UHT is sterile and shelf stable; pasteurized milk is safe but still perishable.
Figure 1: Pasteurized milk compared with UHT milk. Drawn in our own style from A. K. M. Humayun Kober, Milk and Dairy Foods: Nutrition, Processing and Healthy Aging, CRC Press, 2024, Ch. 2 and Ch. 8.
Introduction
Pasteurized milk and UHT milk are both heat treated milks, and both are safe to drink. The difference is how hot they are heated and how long they keep.
The core difference is shelf life and flavour. Pasteurized milk is heated gently, keeps its fresh taste, and must be kept in the fridge for a week or two. UHT milk is heated to about 135 to 150 degrees C for a few seconds, sealed in a sterile pack, and keeps for months without refrigeration, at the cost of a cooked flavour.
Quick comparison
Table 1: Pasteurized Milk compared with UHT Milk
| Aspect | Pasteurized Milk | UHT Milk |
|---|---|---|
| Heat treatment | 72 degrees C for 15 seconds (HTST) or 63 degrees C for 30 minutes | 135 to 150 degrees C for 2 to 5 seconds |
| Microbes left | Pathogens destroyed, some spoilage microbes survive | Commercially sterile, no microbes able to grow in the pack |
| Packing | Filled into clean packs, not sterile | Filled aseptically into sterile packs |
| Storage | Must be kept cold at all times | Keeps at room temperature until opened |
| Shelf life unopened | About 1 to 2 weeks in the fridge | About 3 to 6 months at room temperature |
| Flavour | Fresh, close to raw milk | Cooked flavour, from the high heat |
| Colour | White, normal | Slightly browner, from Maillard browning at high heat |
| Nutrition | Almost unchanged | Small losses of heat-sensitive vitamins; protein, fat and calcium nearly unchanged |
| After opening | Use within a few days, keep cold | Use within a few days, keep cold, like pasteurized milk |
| Main use | Daily drinking milk where cold chains work | Drinking milk where refrigeration is weak, plus long-life supply |
Scientific explanation
Definition
Pasteurized milk is milk heated to destroy pathogenic microbes and then cooled quickly. It is safe but still perishable.
UHT milk is milk heated to ultra-high temperature, 135 to 150 degrees C for a few seconds, then packed aseptically into sterile containers. It is commercially sterile and shelf stable.
Working principle
Both treatments use heat to kill microbes, but UHT uses far more heat for a far shorter time. The very high temperature kills bacterial spores that survive pasteurization, and the sterile pack keeps new microbes out.
The short time limits the damage to flavour and nutrients: a few seconds at 140 degrees C harms the milk less than many minutes at that temperature would. The cooked flavour comes mainly from changes in the whey proteins and from Maillard browning.
Apparatus
A pasteurizer with a plate heat exchanger, holding tube and cooler, plus a cold store.
A UHT plant with a steam injection or indirect heating section, a holding tube, a cooling section, and an aseptic filler that fills sterile packs in a sterile zone.
Measurement procedure
For UHT milk, heat the milk to 135 to 150 degrees C, hold it for 2 to 5 seconds, cool it at once, then fill it aseptically into pre-sterilized packs and seal them.
For pasteurized milk, heat to 72 degrees C, hold for 15 seconds, cool below 10 degrees C, and pack into clean containers. Check pasteurization with the phosphatase test and check sterility of UHT milk by incubation of sample packs.
Factors affecting readings
The fat and solids of the milk affect how fast heat moves through it and how much cooked flavour forms.
The holding time and the cooling speed decide the result: too short a hold leaves spores alive, too long a hold adds cooked flavour. A leak in the aseptic pack lets microbes in and spoils the whole batch.
Formulas and calculations
The books give no formula for this pair. The time and temperature pairs for pasteurization and for UHT treatment are set by experiment and by law.
Shelf life is not calculated from a formula here: it is measured by storage trials, keeping packs at set temperatures and testing them over weeks and months.
Industrial applications
- Pasteurized milk: daily drinking milk in places with a reliable cold chain.
- Pasteurized milk: the base milk for yoghurt, cheese and cultured products made fresh each day.
- UHT milk: drinking milk in places where refrigeration is weak or where milk travels long distances.
- UHT milk: emergency and institutional supply, schools and hospitals that need long-life stock.
- UHT milk: cream and flavoured milks sold as long-life packs.
Advantages and limitations
- Pasteurized milk advantage: fresh flavour, close to raw milk.
- Pasteurized milk limitation: short shelf life, and it needs refrigeration from factory to home.
- UHT milk advantage: keeps for months without a fridge until opened.
- UHT milk limitation: a cooked flavour and a slightly browner colour, and some people dislike the taste.
FAQ
Is UHT milk safe to drink?
Yes. It is commercially sterile, so no microbes can grow in the sealed pack.
Why does UHT milk taste different?
The very high heat changes whey proteins and causes a little Maillard browning, which gives a cooked flavour.
Does UHT milk need a fridge?
Not until it is opened. Once opened, keep it cold and use it within a few days, like pasteurized milk.
Is UHT milk less nutritious?
Only slightly. Protein, fat, lactose and calcium are nearly unchanged; small amounts of heat-sensitive vitamins are lost.
Which milk should I buy?
Pasteurized milk if you have a fridge and like fresh flavour; UHT milk if you need milk that keeps without refrigeration.
References and further reading
- A. K. M. Humayun Kober, Milk and Dairy Foods: Nutrition, Processing and Healthy Aging, CRC Press, 2024, Ch. 2 and Ch. 8 (Google Books page read)
- FTN books on disk: Market Milk, Dairy Processing Handbook by Bylund, Condensed and Dried Milk (agrimoon dairy series).
