Processing operations: filtration, clarification, separation, homogenisation, pasteurisation, sterilisation
What this domain tests
This section tests the purpose and control of filtration, clarification, separation, homogenization, pasteurization and sterilization. You should know what each operation removes or changes and the key time-temperature conditions.
Core concepts
- Filtration removes visible sediment; clarification uses centrifugal force to remove fine particles and some somatic cells.
- Separation uses density difference to produce cream and skim milk; standardization adjusts fat to a target.
- Homogenization forces milk through a narrow valve at high pressure, reducing fat globules so cream separation is slowed.
- Pasteurization kills vegetative pathogens but not all spores: LTLT is 63°C/30 min; HTST is 72°C/15 s.
- UHT treatment is about 135–150°C for 1–4 s and requires aseptic packaging.
- In-container sterilization uses about 115–121°C and gives a shelf-stable product; it changes colour and flavour more than pasteurization.
Worked example
Standardize 1000 L of 4.5% fat milk to 3.5% fat using skim milk at 0.05% fat. Let x = litres of skim milk. Fat balance: 1000×4.5 + x×0.05 = (1000+x)×3.5. This gives 4500 + 0.05x = 3500 + 3.5x, so 1000 = 3.45x and x = 289.9 L. Add about 290 L skim milk.
Traps
Pasteurization is not sterilization; pasteurized milk still needs refrigeration. Separation is not filtration. Homogenization does not make milk sterile. The phosphatase test is a check on pasteurization, not a test for added water.