Describe the causes of food spoilage and food poisoning and the conditions bacteria need to grow

What this dot point is asking

SEAB wants you to describe what causes food spoilage and food poisoning and the conditions bacteria need to grow. The big idea is that micro-organisms, especially bacteria, spoil food and cause illness, and they need particular conditions to multiply. If we deny them those conditions (keep food cold, cook it hot, keep it clean), we stay safe. The marks come from the four growth conditions, the danger zone, and the signs and symptoms.

The answer

What causes food to spoil

Food spoilage is when food goes off and is no longer fit to eat. It is caused mainly by micro-organisms (bacteria, moulds and yeasts), and also by natural enzymes in the food. Signs of spoilage include a bad or sour smell, a slimy surface, mould, a change in colour, and an off taste. Spoiled food usually warns us through these signs.

Food poisoning and harmful bacteria

Food poisoning is illness caused by eating food containing harmful bacteria or their toxins. The danger is that, unlike spoilage, harmful bacteria often cannot be seen, smelt or tasted, so food can look fine and still make you ill. Symptoms include stomach pain, vomiting, diarrhoea, nausea and fever. It is especially dangerous for young children, the elderly, pregnant women and people who are already ill.

The four conditions bacteria need to grow

Harmful bacteria multiply quickly when they have:

• Warmth: they grow fastest at warm temperatures, especially around body temperature.
• Moisture: they need water, so moist foods are higher risk.
• Food (nutrients): high-protein, high-moisture foods such as meat, fish, eggs, dairy and cooked rice are favourites.
• Time: given enough time, a few bacteria become millions.

Remove any one of these and growth slows or stops, which is the basis of safe storage and cooking.

The temperature danger zone

The danger zone is the temperature range in which bacteria multiply most quickly, roughly $5\,^\circ\text{C}$ to $60\,^\circ\text{C}$. To stay safe:

• Keep cold food below $5\,^\circ\text{C}$ (in the fridge), where growth is slowed.
• Keep hot food above $60\,^\circ\text{C}$, where growth is stopped.
• Do not leave food in the danger zone for long, and in Singapore's warm climate this matters even more because room temperature sits inside the danger zone.

Examples in context

Example 1. Cooked rice left out overnight. Cooked rice left on the counter overnight in Singapore is warm, moist and nutrient-rich, and sits for many hours, all the conditions bacteria need. Bacteria such as Bacillus cereus and their toxins can grow in it, which is why leftover rice should be cooled quickly and refrigerated, a common real cause of food poisoning.

Example 2. Chilled storage of fresh fish. A fishmonger keeps fresh fish on ice and a home cook puts it straight into the fridge because cold below $5\,^\circ\text{C}$ slows bacterial growth. Removing the warmth condition keeps the fish safe and fresh for longer, showing how denying bacteria one of their four needs controls them.

Try this

Q1. State the four conditions that harmful bacteria need to grow. [4 marks]

• Cue. Warmth, moisture, food (nutrients) and time.

Q2. Explain what is meant by the temperature danger zone. [2 marks]

• Cue. The range (roughly 5 to 60 degrees Celsius) in which bacteria multiply fastest; food should not be left in it for long.

Q3. Give one reason food poisoning can be more dangerous than spoilage. [1 mark]

• Cue. Harmful bacteria often cannot be seen, smelt or tasted, so the food looks safe but can still make you ill.