Food Preservation Principles and Methods (thermal, drying, refrigeration, chemical)
What the exam tests
This domain asks you to match a preservation principle to a food and to explain the microbial or chemical basis. Expect thermal processing terms, drying and water activity, refrigeration and freezing, and permitted chemical preservatives with their limits and restrictions.
Core rules and concepts
- Heat kills microorganisms in a logarithmic order. The D-value is the time at a stated temperature to reduce a population by 90%; the z-value is the temperature change needed to change D tenfold.
- For low-acid canned foods (pH above 4.6, aw above 0.85), the target is a 12D process for Clostridium botulinum; F0 is the equivalent time at 121 degrees Celsius.
- Drying lowers water activity: most bacteria need aw above 0.91, most moulds above 0.80, and dried foods below about 0.60 are shelf-stable.
- Refrigeration slows growth; freezing to minus 18 degrees Celsius stops growth but does not sterilise. Blanching before freezing inactivates enzymes that cause off-flavour.
- Chemical preservatives are permitted only for specified foods and limits: benzoic acid and sodium benzoate (INS 210/211), sulphur dioxide (INS 220), nitrite (INS 250). They supplement, not replace, hygiene.
Worked example
A low-acid canned vegetable has D121 = 0.2 min for C. botulinum. A 12D process needs 12 x 0.2 = 2.4 min at 121 degrees Celsius. At 2.0 min the reduction is 10D, so a hypothetical load of 10^12 spores would leave 10^2 survivors; that is why the scheduled process is not shortened. If the retort has slow come-up and cooling, the integrated F0 must be calculated, not just the holding time.
Traps
Confusing D and z; treating commercial sterility as absolute sterility; adding a preservative above the permitted limit; assuming freezing kills all microbes; ignoring pH and aw when choosing a process.