Food Preservation and Processing: Spoilage, Methods and Home Products
Food Preservation and Processing examines why food spoils, which principle each preservation method uses, and how home-scale processing (jam, squash, pickle) and food standards keep food safe.
Causes of spoilage
Microorganisms (bacteria, moulds, yeasts), enzymes, insects and rodents, oxidation of fats, and gain or loss of moisture.
Methods and the principle behind each
- High temperature: pasteurization (63 °C for 30 minutes or 72 °C for 15 seconds) for milk and juice; canning/sterilization at about 121 °C under pressure for low-acid foods.
- Low temperature: refrigeration slows microbes; freezing stops growth but does not kill all bacteria.
- Drying and dehydration: removes the water microbes need.
- Sugar and salt: create osmotic pressure that draws water out of microbial cells - jam, jelly, murabba, squash; pickles and cured foods.
- Acid: vinegar or lemon lowers pH - pickles, chutney, ketchup.
- Chemical preservatives: sodium benzoate for acidic foods and squashes, potassium metabisulphite for drying and juices; permitted limits matter.
- Smoking, irradiation and vacuum packing are also used.
Worked example
Why does jam keep for months? Jam is boiled to about 68% soluble solids, so water activity falls and the sugar, with fruit acid and pectin, blocks microbial growth and sets the gel. If you add water or cut sugar to 50%, water activity rises, mould appears and the jam fails to set. That reasoning - not just "sugar preserves" - is what earns marks.
Traps
- Writing that freezing sterilises food.
- Mixing pasteurization with sterilization.
- Confusing a permitted preservative with an adulterant.