Principles of Food Preservation and Processing
Principles of Food Preservation and Processing
This domain tests how heat, cold, drying, irradiation, chemicals and packaging extend shelf life. Numerical questions on D, z and F values are common.
Core rules
- Thermal processing: pasteurization (LTLT 63 °C/30 min; HTST 72 °C/15 s) kills vegetative pathogens but not all spores. Sterilization aims at commercial sterility.
- Kinetics: D value is the time at a given temperature for a 1-log reduction; z value is the temperature change for a 10-fold change in D; F0 is equivalent time at 121 °C.
- Canning: low-acid foods need a 12D process for C. botulinum; acid foods need less heat.
- Cold: chilling slows growth; freezing stops growth but not all enzymes; blanching inactivates enzymes before freezing.
- Drying: removes water and lowers aw; rate depends on air temperature, humidity and velocity; case-hardening slows drying.
- Hurdle technology: combines low aw, low pH, preservatives and chilled storage for safety.
Worked example
A spore has D121 = 0.2 min. A 12D process needs 12 × 0.2 = 2.4 min at 121 °C. If the retort comes up slowly, the actual F0 must be integrated over the whole come-up and cooling period, not just holding time. Short holding at 121 °C may therefore still be inadequate.
Traps
Do not confuse D and z; pasteurization is not sterilization; freezing does not guarantee safety; drying depends on equilibrium moisture, not time alone.