Pasteurisation and heat treatment of milk
From raw milk to safe milk — the physics of heat treatment and control points
Raw milk as it comes from the cow can contain disease-causing bacteria such as listeria, salmonella and campylobacter. As a dairy farmer, heat treatment is therefore one of the most important tools you have to make milk safe to drink and use in production — without destroying the nutrients and flavor that make milk milk. Pasteurization is not just a machine setting you set and forget. It is a process you must understand in depth because it is a critical control point in the entire production.
§Why do we heat-treat milk?
The purpose of pasteurisation is to reduce the number of disease-causing microorganisms to a level where the milk is safe to drink while extending shelf life. According to the dairy industry's own information on mejeri.dk milk is heated to at least 72 degrees for at least 15 seconds thereby killing harmful bacteria without affecting the milk's nutritional composition. This combination of temperature and time — not temperature alone — is what determines whether pasteurisation is sufficient.
§HTST — the method you encounter most often
The most widespread method is called HTST, High Temperature Short Time. The milk is passed through a plate heat exchanger, heated to at least 72 °C and held at that temperature for at least 15 seconds in a so-called holding tube, before it is cooled quickly again. HTST-pasteurized milk, which you find in the refrigerated section, typically keeps cool for two to three weeks.
| Method | Temperature | Holding time | Typical shelf life |
|---|---|---|---|
| HTST (short-time pasteurization) | At least 72 °C | At least 15 seconds | 2-3 weeks chilled |
| UHT (ultra-high temperature). | Ca. 135-138 °C | 2-5 seconds | Several months unopened at room temperature |
§UHT—when milk needs to last longer.
If milk is to be able to sit on the shelf for a long time without cooling UHT treatment is used. Here the milk is heated briefly to around 135-138 degrees in a few seconds. The stronger heat treatment also kills bacterial spores which gives a much longer shelf life but it can give a slight change in taste compared to HTST milk. As a dairy technician you need to know which method a given product requires and why.
§Critical control points in practice
Pasteurisation is typically one of the critical control points (CCP) in the dairy's HACCP-based self-monitoring programme. This means that temperature and holding time must be monitored and documented continuously — often automatically via the plant's control, but you must be able to read, understand and react to deviations.
- 01Temperature must be measured continuously immediately after the holding pipe
- 02The holding time depends on the pipe length and flow rate — if flow is increased, temperature must be compensated
- 03A flow diversion valve (bypass valve) automatically sends under-heated milk back so it does not reach the market as finished product
- 04All deviations must be logged and investigated as part of self-control
§What happens if pasteurization fails?
If temperature or holding time is not high enough, pathogenic bacteria can survive and end up in products that reach the consumer. Therefore, automatic diversion of under-treated milk, frequent calibration of sensors and a clear understanding of why the combination of time and temperature matters is a fixed part of the work as a dairy technician.
“Pasteurisation means the milk is heated to 72 degrees for 15 seconds, which kills any disease-causing bacteria without affecting the milk's nutritional composition.”
— mejeri.dk