Food Processing, Preservation and Technology
Processing questions test whether you know why a step exists and what it costs in quality. Learn each operation as principle, typical conditions, and effect on nutrients and microbes.
Thermal processes
- Blanching: 80–100 °C; inactivates peroxidase and polyphenol oxidase, fixes colour, expels tissue gases; over-blanching leaches water-soluble vitamins.
- Pasteurisation: LTLT 63 °C/30 min, HTST 72 °C/15 s - kills vegetative pathogens, still needs refrigeration.
- UHT: 135–150 °C for 2–4 s plus aseptic packing; ambient shelf life of months.
- Canning/retorting: 12D cook for low-acid foods (F0 of about 3 min at 121.1 °C); high-acid foods (pH below 4.6) need milder heat.
Non-thermal and other methods
- HPP, pulsed electric field, cold plasma, ultrasound, irradiation (low doses for sprout inhibition, higher for insect disinfestation).
- Chilling (0–4 °C), freezing (−18 °C), IQF, cryogenic freezing.
- Drying: sun, hot air, spray, drum, freeze drying; osmotic dehydration; concentration by evaporation and reverse osmosis.
- Chemical preservation: Class I (salt, sugar, vinegar, oil) and Class II preservatives such as benzoic, sorbic and propionic acids and sulphur dioxide, within permitted limits.
- Hurdle technology combines water activity, pH, temperature and preservatives so no single hurdle need be severe.
Worked example
A juice at pH 3.6 is required to be ambient-stable. Botulinum control is not the driver because pH is below 4.6, so a hot fill or modest pasteurisation with a sealed container can suffice; a low-acid beverage would demand a full 12D retort process. Reasoning from pH decides the process, not the product name.
Traps
- Believing freezing kills bacteria - it only stops growth.
- Confusing D-value (time for 90% reduction at a temperature) with z-value (temperature change for a tenfold change in D).
- Forgetting that drying raises solute concentration and can cause case hardening.