What is food contamination?
Contamination means something harmful has got into food that should be clean. It can happen on the farm, at sea, in the factory, in the shop or in the kitchen.
Two groups matter here: harmful substances (chemicals and mould poisons) and parasites (living things that live on us).
Harmful substances and prevention
Harmful substances in food come from three places:
- Farm chemicals: pesticide and weed-killer that stay on crops. Prevention: use only the allowed amount, wait the set days before harvest, wash and peel produce. Limit set by law is called the maximum residue limit.
- Heavy metals: mercury, lead and cadmium from factory waste or old pipes. They do not break down. They build up along the food chain: small fish eat tiny plants, big fish eat many small fish, so big fish hold the most. This build-up is called bioaccumulation. Prevention: treat waste before it reaches rivers, check fish, do not eat too much large fish.
- Mould toxins (mycotoxins): mould grows on damp grain, nuts and bread. Aflatoxin from mould on groundnuts and maize can harm the liver. Cooking does not remove it. Prevention: dry grain fully, store in a dry, airy place, throw away mouldy food.
Other sources: dioxins from burning plastic, chemicals leaking from poor packaging, and harmful residue of animal medicine in milk or meat.
Parasitic diseases and prevention
A parasite lives in or on another living thing (the host) and takes food from it. Parasites reach us through food and water.
- Tapeworm: tiny forms live in raw or half-cooked pork and beef. In the gut they grow into long flat worms.
- Fish worms (Anisakis, liver flukes): live in raw or half-cooked fish and can cause pain in the stomach and liver.
- Roundworm and others: tiny eggs travel in soil, dirty water and on raw vegetables. They hatch in the gut.
- Amoeba and Giardia: very small one-celled parasites in dirty water.
Prevention:
- Cook meat and fish until hot all through (75 °C in the centre). Freezing fish at -20 °C for several days also kills worms.
- Buy meat checked by an inspector.
- Wash vegetables in clean water, peel or cook them.
- Boil or filter drinking water.
- Use toilets. Wash hands with soap. Cut nails.
- Take medicine for worms (deworming) as the doctor says.
Try it
In the 3D, pick each case. Watch the orange cubes in the fish grow, then press Prevent. At home: look at the grain and nut containers. Is there any damp or mould smell? Are the vegetables washed in clean water? Write one thing you will change.
Key formulas and definitions
- Contamination = harmful thing gets into clean food
- Parasite lives on a host and takes food from it
- Cook centre ≥ 75 °C; freeze fish at -20 °C to kill worms
- Heavy metals build up: small fish → big fish (bioaccumulation)
Worked examples
1. A small fish has 1 unit of mercury. A big fish eats 10 small fish. About how much mercury could the big fish hold at least?
About 10 units (10 × 1), and more over time because mercury is not removed. This is bioaccumulation.
2. Why can cooking not make mouldy groundnuts safe?
The aflatoxin made by the mould is a chemical poison. Heat kills the mould but does not break the poison.
3. A family eats half-cooked pork and later finds a flat worm. Which parasite, and how to prevent it?
Tapeworm. Cook pork until hot all through (75 °C) and buy inspected meat.
4. Spinach grown with sewage water is eaten raw. Which risk? Give two fixes.
Worm eggs (roundworm and others). Fixes: wash in clean water and cook it; stop using sewage water on crops.
5. Fish is frozen at -20 °C for 7 days before sushi. Why is this done?
The cold kills fish worms (like Anisakis), so raw fish is safer.
Common mistakes
- Thinking cooking removes every poison. Heavy metals and mould toxins stay.
- Eating raw vegetables washed in dirty water.
- Cutting off the mouldy part of bread and eating the rest. Mould threads go deep.
- Thinking a parasite is the same as a germ. Parasites are bigger and live on a host, such as worms.