Food poisoning and contamination
1 Food poisoning:
Everybody at one time or another have had the experience of eating food and some time later becoming sick. This is called food poisoning. The symptoms may include:
- stomach pains
- feeling weak
- fever or chills/sweating
Fig. 3.26: Food poisoning comes from harmful bacteria on food.
Food poisoning can be caused by eating food contaminated with bacteria, viruses, chemicals or poisonous metals such as lead or cadmium. Most food poisoning, however, is caused by bacteria and because of this, only bacteria will be discussed in this section.
Food which has become contaminated with harmful bacteria does not always taste bad. Most of the time it looks, smells and tastes like it normally does.
Some food poisoning diseases are more common than others. For example, disease caused by Staphylococcus aureus occurs a lot more often than disease caused by Clostridium botulinum.
Some foods cause food poisoning more than others and need to be cooked properly and/or kept in the refrigerator. These include chicken, meat, seafood, eggs, cooked rice, ham, salami, milk and all dairy foods. It is important chicken is cooked properly to the bone and then kept in the fridge for no more than 2 days. If reheating chicken, or left-overs, make sure it is steaming hot and only reheat it once.
It is important to remember that the same food handling practices are used to prevent all food poisoning diseases. Washing your hands with soap and drying them on a paper towel or with a clean cloth is the best way to stop the spread of bad bacteria.
The four most common types of food poisoning bacteria are discussed below.
These bacteria are found on the skin, in sores, infected eyes and in the nose, throat, saliva and bowel of humans. There may be many of these bacteria in the yellow mucus (slimy substance) which comes from the nose or is coughed up when a person has a cold or a lung infection.
Staphylococci do not cause illness until they get onto food and grow and multiply. While they are doing this they produce a toxin (poison). It is the toxin which causes the illness. The toxin is not destroyed by cooking the food.
Symptoms of staphylococcus food poisoning usually appear between 1 and 8 hours after eating the infected food.
There are hundreds of different types of salmonella bacteria but not all are harmful to humans. They are found mainly in the intestines, bowels and faeces of humans and other animals. It is the salmonella bacteria themselves which can cause salmonella food poisoning.
Fig. 3.27: Bacteria on food.
People can get salmonella food poisoning from:
- poor food handling practices in the home or in food outlets
- seafood caught in polluted water or eggs with dirty shells
- meat or poultry which has been contaminated by poor food handling before it gets to the food outlet, such as at the abattoir
Salmonella food poisoning takes up to 48 hours to develop after the food is eaten. Symptoms include nausea, stomach cramps, diarrhoea, fever and headache, and may last between 3 and 21 days. It can cause death in very young, weak or very old people. People who have cancer or are taking medication for serious health conditions such as heart, kidney or liver problems need to also be particularly careful that they eat safe food.
These bacteria are found in the soil and in the intestines of animals, including cattle, poultry, fish and humans. Food poisoning caused by clostridium bacteria is important to know about because these bacteria are common in the environment.
People can get clostridium food poisoning from poor food handling practices in the home, in the factory or in a food outlet, especially relating to cooking and storage/refrigeration temperatures.
Clostridium food poisoning symptoms occur about 12 hours after eating the contaminated food and are similar but usually less severe than the other types.
Symptoms include stomach pains, diarrhoea and sometimes nausea and vomiting. Symptoms last about 24 hours.
One type of clostridium bacteria produces a very serious food poisoning disease called botulism. This disease is caused by eating food which is contaminated with an extremely poisonous toxin produced by the bacteria Clostridium botulinum. Unless properly treated about one-third of people who get this disease die within 3-7 days.
These bacteria are found in many animals including dogs, cats, cattle and poultry. The sources of infection from these bacteria are usually contaminated food and water.
People can get campylobacter from:
- ingestion of contaminated food or water (especially undercooked chicken & creek or river water)
- contact with infected animals (especially puppies or kittens with diarrhoea)
- poor food handling (especially by using the same chopping boards, knives and plates for raw and cooked chicken)
Campylobacter food poisoning symptoms usually last from 2 to 5 days. These include diarrhoea, severe abdominal pain, vomiting and fever. It is a serious disease in Indigenous communities because of the possibility of dehydration from diarrhoea.
2 How bacteria grow and multiply
Bacteria reproduce (breed) by splitting in half. When they do this they are said to multiply. In the right conditions, bacteria multiply at a very fast rate.
Fig. 3.28: Bacteria can multiply very quickly.
Disease causing bacteria grow best when there is:
- warmth (37°C-38°C) (Note: human body temperature is 37°C)
- food supply
In ideal conditions, bacteria double their numbers every 20 minutes. For example, if a piece of kangaroo meat infected with 100 food poisoning bacteria is left lying on a kitchen bench on a warm day, the bacteria will double their number every 20 minutes, and in 3 hours, the 100 bacteria will multiply to over 50,000 bacteria.
The following table shows how the bacteria will multiply on the meat over 3 hours:
||Number of bacteria|
It is important to note that once inside a person’s intestine the bacteria can continue to multiply. This means that a person may eat contaminated food having only a few bacteria on it, but eventually suffer from food poisoning.
Ways food can become contaminated through incorrect food handling
Food can become contaminated with disease-causing bacteria anywhere the food is handled or stored. These places include:
- in a factory where it is processed ready for sale
- in a truck in which it is taken from the factory to the shop
- in a shop
- in a food outlet such as a school canteen or take-away shop
- between the shop and home
- in a home
Most food has to be prepared in some way before it is eaten. During this preparation the food is handled by people. There are many ways in which unhygienic practices can cause food poisoning bacteria to be deposited on the food while it is being handled. Some examples are:
- Leaving food uncovered. Pets, flies, cockroaches and other insects carry germs, including food poisoning bacteria, which contaminate the food
- Touching parts of the body while handling food. While preparing food a food handler might scratch a pimple, touch a sore, push back hair, scratch an ear or rub or pick the nose. Every one of these activities contaminates the fingers with bacteria. If the person’s hands are not washed before handling food again, these bacteria will be passed to the food.
Fig. 3.29: Rubbing the nose while preparing food helps spread germs.
- Sneezing or coughing near food. If a food handler, or anyone else, sneezes or coughs near uncovered food, then the food almost will certainly be sprayed with bacteria laden droplets.
Fig. 3.30: Sneezing over food spreads germs.
- Licking fingers while handling food. Human saliva carries staphylococcus bacteria and licking the fingers could result in these bacteria being passed to the food.
Fig. 3.31: Licking fingers while handling food spreads germs.
- Not washing hands after going to the toilet during food handling. If a person goes to the toilet during food handling activities and does not wash his/her hands afterwards food poisoning bacteria may be passed onto the food.
Fig. 3.32: Washing hands after going to the toilet helps stop the spread of germs.
- Poor handling of high risk foods. High risk foods are those which generally need refrigeration and have a high moisture content. Poor handling of high risk foods is a common cause of food poisoning. High risk foods include:
- chicken, duck and other poultry
- fish and shellfish
- raw meat products
- dairy products (milk, cheese, cream)
- unpasteurized cow or goats milk
- eggs and egg products
Cross contamination. Certain foods will always contain some bacteria. Poor handling of these foods may result in cross contamination. Cross contamination is the passing of bacteria from contaminated food to uncontaminated food. Cross contamination can occur when storing or handling food.
An example of cross contamination during storage is:
A high risk food, such as a raw chicken thawing in a refrigerator, is placed in contact with cooked meat. The bacteria from the raw chicken contaminates the cooked meat. Since the cooked meat is not heated again before eating, the bacteria from the chicken pass to the person who eats the meat.
An example of cross contamination during handling is:
Before cooking a fish which is contaminated with salmonella bacteria, a person uses a knife and cutting board to cut it up. Bacteria from the fish will be left on the knife and cutting board. The person slices cooked ham using the same knife and board without washing them first. The bacteria are transferred to the ham.