Food Preservation Principles and Methods
Principles
Preservation works by one or more of four principles: asepsis (keeping microbes out), removal of microorganisms (filtration, centrifugation), inhibition of growth (low temperature, low water activity, low pH, preservatives, modified atmosphere) and destruction (heat, irradiation, high pressure). Shelf life is a system property: packaging, storage temperature and formulation all count.
Thermal and low-temperature methods
- Blanching at 70–100 °C for 1–5 min inactivates enzymes such as peroxidase and polyphenol oxidase before drying, freezing or canning.
- Pasteurisation kills vegetative pathogens; it is not sterilisation. UHT plus aseptic packing gives ambient shelf life.
- Commercial sterility for low-acid canned foods uses the 12D botulinum cook at 121 °C with F0 of about 3 min.
- Chilling slows microbes; freezing stops growth but does not kill them. Fast freezing gives small ice crystals and better texture.
Water activity, drying and chemicals
Drying lowers aw; most bacteria stop growing below aw 0.90 and most moulds below 0.70. Osmotic dehydration uses sugar or salt syrup. FSSAI permits Class I preservatives (salt, sugar, vinegar, spices, honey) generally, and Class II preservatives (benzoic acid, sorbic acid, sulphur dioxide, nitrites) only within prescribed limits for specified foods. Irradiation is permitted for potatoes, onions, spices and certain other items within set doses.
Worked example
Mango pulp at pH 3.6 is hot-filled at 90 °C into pre-sterilised jars. Reasoning: acid plus heat kills yeasts and moulds, and a pressure retort is unnecessary because Clostridium botulinum cannot grow below pH 4.6. If the same pulp had pH 5.8, the process would be unsafe and a retort would be mandatory.
Traps
Saying refrigeration kills bacteria; treating pasteurisation as sterilisation; exceeding Class II preservative limits for supposed safety; ignoring aw when a product is only dried to a leathery texture; assuming blanching is only for colour.