The essentials
Food processing is any method used to turn raw agricultural produce into food that is safe, convenient, longer-lasting and more attractive. Food preservation prevents spoilage, which is caused mainly by micro-organisms (bacteria, yeasts, moulds), enzymes in the food, insects and rodents, and chemical changes such as rancidity. All preservation methods work by killing or inactivating microbes and enzymes, or by creating conditions in which they cannot grow.
Processing adds value, reduces wastage of seasonal produce, provides variety through the year and creates employment.
Key points
- High temperature: blanching (brief heating of vegetables to inactivate enzymes before freezing or drying); pasteurisation (mild heat that kills disease-causing microbes, e.g. milk at about 63 °C for 30 minutes or about 72 °C for 15 seconds); sterilisation and canning (destroy all microbes).
- Low temperature: refrigeration slows microbial growth; freezing stops it.
- Removal of water: sun drying, dehydration, spray drying (milk powder); microbes need water to grow.
- Preservatives: salt (pickles), sugar (jams, jellies, squash) — both act by osmosis; acids such as vinegar; chemical preservatives such as sodium benzoate and potassium metabisulphite (KMS). KMS is not used for naturally coloured products like some red fruit juices because it can bleach colour; sodium benzoate is used for coloured products.
- Fermentation: curd, idli, dosa, bread, vinegar.
- Irradiation: controlled exposure to ionising radiation to kill microbes and insects or delay sprouting.
- Packaging: protects food; examples include aseptic (tetra) packaging and vacuum packing.
Exam tips
For every method, be ready to state its principle. Sugar and salt both preserve by drawing water out of microbial cells (osmosis).
Common mistakes
Thinking pasteurisation kills all microbes — that is sterilisation.