Milk Processing and Preservation (Chilling, Pasteurisation, Storage)
Domain 4 covers the cold chain, pasteurisation time-temperature combinations and storage. The exam expects you to match a process to its purpose and recognise post-process contamination.
Core rules
- Chill raw milk to about 4°C quickly and keep it cold before processing.
- LTLT: 63°C for 30 minutes; HTST: 72°C for 15 seconds; UHT: 135-150°C for 2-4 seconds plus aseptic packaging.
- Phosphatase test is negative after adequate pasteurisation; it does not prove sterility.
- Pasteurised milk still needs refrigeration; UHT milk is shelf-stable only in a sterile package. Homogenisation reduces cream line but is not a substitute for pasteurisation.
Worked example
Cool 1000 L milk from 35°C to 4°C. Mass ≈ 1000 × 1.03 = 1030 kg; ΔT = 31 K; cp ≈ 3.93 kJ/kg·K. Q = 1030 × 3.93 × 31 = 125,485 kJ; divide by 3600 = 34.9 kWh. The reasoning is that cooling load depends on mass, specific heat and temperature drop, not on volume alone.
Traps
- Pasteurising already-sour milk does not make it safe or good.
- A negative phosphatase test does not excuse poor post-pasteurisation hygiene.
- UHT is not the same as boiling or ordinary pasteurisation.
- Storing pasteurised milk above 7°C shortens shelf life.