Why foods spoil
Foods spoil because of microorganisms (bacteria, yeasts and moulds), enzymes in the food itself that cause ripening and decay, and chemical changes such as rancidity of fats. Spoilage is fastest when there is moisture, warmth, air and the right pH. Preservation stops or slows spoilage so that food lasts longer.
Methods of storage and preservation
| Method | Principle | Examples |
|---|---|---|
| Drying (sun, solar, oven) | Removes water; microbes cannot grow | Dried fish, meat (nyama kavu), cassava, fruit, grains |
| Salting and curing | Salt draws water out of cells and microbes | Salted fish, bacon |
| Smoking | Smoke dries and has antimicrobial chemicals | Smoked fish and meat |
| Cooling and refrigeration (0–5 °C) | Cold slows growth | Milk, meat, vegetables |
| Freezing (−18 °C) | Stops growth; water is frozen | Meat, fish, vegetables |
| Canning / bottling | Heating destroys microbes in a sealed container | Tinned food, bottled juice |
| Pickling | Acid (vinegar) stops growth | Pickled onions, mango pickle |
| Sugar (jam, syrup) | High sugar concentration stops growth | Jams, marmalade |
| Pasteurisation | Heating to kill microbes | Milk, juice |
| Fermentation | Good microbes make acid | Yoghurt, sour milk (mtindi) |
| Irradiation, chemicals | Radiation or preservatives | Spices, juices |
Simple preservation technologies
For small farmers, simple technologies are the solar dryer (a box with a glass or plastic cover that traps the heat), the drying rack for fish and vegetables, hermetic bags for grain, evaporative coolers (zeer pots, charcoal coolers), smoking kilns, solar cookers and improved clay stores. These cut losses after harvest and improve food security.
Drying: how much weight is lost?
10 kg of fresh fish with 75% moisture is dried to 15% moisture. Find the mass of the dried fish.
- Use the formula.
Answer: About 2.9 kg of dried fish.
Choose the fresh mass, the starting moisture and the final moisture.
Key terms
- Preservation
- Treating food so that it does not spoil.
- Pasteurisation
- Heating to kill microbes without cooking.
- Pickling
- Preserving in vinegar or brine.
- Fermentation
- Change of food by microbes, such as yoghurt-making.
Summary
- Food spoils because of microorganisms, enzymes and chemical changes.
- Preserve it by drying, salting, smoking, cooling, canning, pickling or using sugar.
- Simple technologies include solar dryers and hermetic bags.
- Dry mass = fresh mass × (100 − m₁) ÷ (100 − m₂).