Food Processing Unit Operations (Thermal, Drying, Freezing, Irradiation)
This domain tests matching unit operations to their purpose: heat transfer, mass transfer, size reduction, separation, freezing and irradiation. Know typical conditions and what each operation does and does not achieve.
Core rules and concepts
- Blanching at 70-95°C for 1-5 minutes inactivates enzymes, fixes colour and removes air; it is not sterilization.
- Pasteurization: HTST 72°C/15 s or LTLT 63°C/30 min. It kills vegetative pathogens but not spores.
- Sterilization/retorting typically uses 121°C for about 15 minutes; UHT uses 135-150°C for a few seconds plus aseptic filling.
- Drying lowers water activity; freeze drying removes water by sublimation; freezing removes heat quickly to form small ice crystals.
- Irradiation doses are measured in kGy: sprout inhibition about 0.05-0.2, insect disinfestation about 0.2-1.0, shelf-life extension about 1-3, pathogen reduction at higher validated doses.
- Evaporation concentrates liquids; centrifugation and membrane separation split components.
Worked example
Green peas are blanched at 90°C for 2 minutes, cooled, and tested for peroxidase using guaiacol and hydrogen peroxide. No colour development means the enzyme is denatured and blanching is adequate. Colour means extend blanching; over-blanching causes texture and vitamin loss.
Traps that cost marks
- Blanching is not a sterilization step.
- Pasteurization does not inactivate all spores.
- Irradiation does not make food radioactive.
- Freezing does not stop enzymes; vegetables need blanching first.
- Higher irradiation dose is not automatically better; it must match the purpose and legal limit.