Causes of food spoilage
- Enzymes: natural proteins in food that speed up ripening and cause enzymic browning, such as a cut apple turning brown. Acid (lemon juice) or blanching slows this.
- Moulds: fungi that grow on food, especially bread, cheese and fruit, producing visible furry patches.
- Yeasts: fungi that spoil sugary foods, such as fruit and jam, by fermenting them.
- Bacteria: some spoil food; pathogenic bacteria cause food poisoning without changing how food looks or smells.
What micro-organisms need
- Warmth: bacteria grow fastest at around 37°C. The danger zone is 5°C to 63°C.
- Moisture, food (especially high-protein foods) and time: in ideal conditions bacteria can double every 10 to 20 minutes.
- A suitable pH: most prefer neutral conditions, so acid (vinegar, as in pickling) slows them.
- Cold (below 5°C) slows growth; freezing (−18°C) stops it but doesn't kill bacteria; heat (75°C and above) kills most.
Food poisoning bacteria
- Campylobacter: raw poultry. The most common cause of food poisoning in the UK.
- Salmonella: raw eggs, raw poultry and meat.
- E. coli: undercooked beef (such as burgers) and unwashed vegetables.
- Staphylococcus aureus: from people's skin, nose and cuts, so good hygiene prevents it.
- Listeria: soft cheese, pâté and chilled ready meals; risky in pregnancy.
- Bacillus cereus: cooked rice left warm.
Useful micro-organisms
- Yeast makes bread rise. Bacteria turn milk into yoghurt and cheese. Moulds give blue cheeses their flavour.
Key terms
- Enzymic browning
- Browning of cut fruit and vegetables caused by enzymes.
- Danger zone
- The temperature range of 5°C to 63°C where bacteria grow fastest.
- Pathogenic bacteria
- Bacteria that cause illness.
- Mould
- A fungus that grows on food.
- Cross-contamination
- Bacteria spreading from one food or surface to another.
- High-risk food
- A moist, high-protein food where bacteria grow easily, such as cooked chicken.