Contamination
Food can become contaminated at any stage during its production (growth), processing or cooking. Contamination can occur "at any time from farm to fork", as one expert has put it.
Food poisoning can be caused by:
- not cooking food at the right temperature and/or for the right length of time,
- not chilling food at the correct temperature,
- the food being handled by someone who has not washed their hands properly,
- people eating food after it has passed its ‘use by’ date, and
- cross-contamination.
Cross-contamination
Cross-contamination is a cause of food poisoning that is often overlooked. It occurs when harmful bacteria are spread between food, surfaces and equipment.
For example, if you prepare raw chicken on a chopping board and then do not wash the board before preparing a ready-to-eat meal such as a salad or sandwiches, harmful bacteria can be spread from the chopping board to the ready-to-eat meal.
Cross-contamination can also occur if you store raw meat above ready-to-eat meals. The meat juices can drip on to the meals and contaminate them.
Sources of contamination
Bacteria
Food contamination is usually caused by bacteria. Some common types are described below.
Campylobacter
Campylobacter is the most common cause of food poisoning in England. Campylobacter bacteria are usually found in raw meat and poultry, unpasteurised milk and untreated water.
Salmonella
Salmonella bacteria are often found in raw meat and poultry. They can also be passed into dairy products such as eggs and unpasteurised milk.
Listeria
Listeria bacteria are common and have been found in a range of chilled ready-to-eat food including:
- pre-packed sandwiches,
- pâté,
- butter,
- soft cheeses, such as brie, camembert or others with a similar rind,
- soft blue cheese,
- cooked sliced meats, and
- smoked salmon.
Escherichia coli
Escherichia coli, known as E. coli, are bacteria found in the digestive system of many animals, including humans. Most strains are harmless but the strain called E. coli 0157 can cause serious illness.
Most cases of E. coli food poisoning occur after eating undercooked beef or drinking unpasteurised milk.
Viruses
Two common causes of viral food poisoning are:
- the rotovirus, and
- the norovirus.
The rotovirus is more common in children than adults because most adults develop immunity to it. Noroviruses can affect people of any age.
People infected with either virus can contaminate food if they do not wash their hands properly after going to toilet and then handle food.
Parasites
In England, food poisoning from parasites is rare. It is much more common in the developing world.
Toxoplasmosis is the most likely cause of parasitical food poisoning in England. It is caused by a parasite that is found in the digestive systems of many animals, particularly cats.
People can develop toxoplasmosis by consuming undercooked contaminated meat, or food or water contaminated with the faeces of infected cats.
Toxins
There is a small risk that oily fish could be contaminated by toxins such as a chemical called polychlorinated biphenyls (PCBs).
The levels of toxins in contaminated fish are thought to be very low but they could still pose a risk to unborn babies. Pregnant women and women who might have a baby one day are advised not to eat more than two portions of oily fish a week.
Examples of oily fish include:
- fresh tuna (not canned tuna, which does not count as oily fish),
- mackerel,
- sardines,
- trout, and
- salmon.
Some fish contain a high level of mercury, which can also damage an unborn baby's nervous system. Therefore, pregnant women should avoid eating:
- shark,
- swordfish,
- marlin, and
- more than two tuna steaks a week (or four medium cans of tuna a week).
