Food Preservation
Food preservation extends shelf life by slowing or stopping microbial growth, enzyme activity and chemical deterioration. The exam tests thermal processing, low-temperature preservation, drying, irradiation, chemical preservatives, fermentation and hurdle technology.
Core rules
- Blanching inactivates enzymes before freezing or drying but is not a sterilization step.
- Pasteurization reduces vegetative pathogens and is usually combined with refrigeration; sterilization destroys all viable microorganisms and their spores.
- D-value is the time at a given temperature to reduce a microbial population by 90%. z-value is the temperature change needed to change D-value tenfold.
- For low-acid canned foods, the reference process is a 12D reduction of Clostridium botulinum spores.
- Water activity, pH, redox potential, temperature and preservatives act as hurdles; combining hurdles allows milder individual treatments.
Worked example
A Clostridium botulinum spore suspension has D121 = 0.21 min. A 12D process at 121 °C requires 12 × 0.21 = 2.52 min. The reasoning uses the definition of D-value: each D reduces the population by one log, so 12D gives a 12-log reduction.
Traps
- Confusing D-value with z-value.
- Calling pasteurization a sterilization process.
- Ignoring pH when selecting a thermal process.
- Assuming freezing kills all microorganisms.
- Using preservatives to mask poor hygiene.