Milk Processing: Filtration, Clarification, Homogenisation, Pasteurisation and Sterilisation
Milk Processing tests the purpose and control of filtration, clarification, homogenisation, pasteurisation and sterilisation. Know the sequence, temperature-time combinations and the tests that prove each process.
Core processes
Filtration removes sediment and somatic cells; clarification uses centrifugal force for fine dirt. Homogenisation forces milk through a narrow valve at 15-20 MPa and 60-70 C, reducing fat globules below about 2 micrometres and preventing cream separation. Pasteurisation uses LTLT 63 C for 30 min or HTST 72 C for 15 s, followed by rapid cooling. UHT treatment is about 135-150 C for 2-4 s with aseptic packaging. Sterilisation in containers is a separate heat process.
Worked example
An HTST plant runs at 10,000 L/h with a 15 s holding time. Convert flow: 10,000 L/h = 10 m3/h = 0.002778 m3/s. Holding tube volume = flow × time = 0.002778 × 15 = 0.04167 m3 = 41.7 L. If the tube is smaller, residence time is shorter and the plant must divert flow.
Traps
Pasteurisation does not sterilise milk. A negative phosphatase test indicates adequate pasteurisation, not freedom from post-process contamination. Do not confuse homogenisation with pasteurisation. For HTST market milk, homogenisation is normally placed before or within the pasteurisation flow, not after it without aseptic protection. Flow diversion valves must work or pasteurised milk may be under-processed.