Food poisoning

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"Food poisoning" is a general name given to illnesses contracted by consuming contaminated food or water. The micro-organisms responsible for illness are bacteria, viruses and fungi, commonly called "germs: or "bugs". But illness can also be caused by chemical contaminants (such as heavy metals), toxins produced by the growth of some micro-organisms (eg. Staphylococci bacteria) and by a variety of organic substances that may be present naturally in foods (such as certain mushrooms and some seafood).

Generally food poisoning results from contamination of food and the subsequent growth of food poisoning micro-organisms. Food poisoning outbreaks are often recognised by the sudden onset of illness within a short period of time among many individuals who have eaten or drunk one or more foods in common. Single cases are difficult to identify unless, as in Botulism for example, there are distinct symptoms.

Food poisoning may be one of the most common causes of acute illness; yet cases and outbreaks are generally under-recognised and under-reported.


Bacteria are microscopic organisms that are found everywhere--in air, soil, water, plants, animals and the human body. You can't see, taste or smell most bacteria. If the environmental conditions are favourable, just about any material will support the growth of some bacteria. Most bacteria are harmless and some are helpful, like those that change milk into cheese or yoghurt. But others cause food spoilage and some known as pathogens are harmful and can cause illness and sometimes death.

The number of bacteria present in food may be used to determine whether or not the food has been handled correctly.

The diseases that are spread by bacteria that enter the body in food, can multiply at an amazing rate when they are provided with warmth and moisture, (especially at room temperature).

Our food can become an ideal home for them.


Clean food can be contaminated by bacteria from four main sources- The people present in the workplace and their clothing. Other food that is already contaminated. Dirty kitchen or work premises and equipment. Insects and vermin. Sometimes, harmful bacteria pass directly from the source to high risk food, but usually they rely on other things to transfer them to food. These things are called Vehicles. Indirect contamination using an intermediate vehicle is the most common, eg.- the movement of bacteria from the intestine of a food handler to food via their hands, after using the toilet.

Where contamination is passed from raw food to high risk food via for example, a cutting board, this is known as Cross Contamination.

The path that bacteria use to move from the source to the food, is known as the Route.



Foods involved Contaminated poultry meat and meat products, especially stews, gravies and pies. Main Symptoms Abdominal pain, diarrhoea and nausea. Onset of Illness 8 to 22 hours, (usually 10 to 12 hours). Source This organism is found in the waste of animals and man, and often in raw meat and in soil. It thrives in airless conditions and survives ordinary cooking.


Foods involved Contaminated meat and meat products, especially poultry. Custard, cream, milk and egg products, and salads. Main Symptoms Fever, headache, aching limbs, abdominal pain, nausea, diarrhoea, and sometimes vomiting. Onset of Illness 6 to 72 hours (usually 12 to 36 hours). Source Salmonella bacteria are often present in the waste of man and animals, (especially rodents and poultry). This illness is infectious and can be spread to other people.


Foods involved Contaminated moist protein foods. Meat, eggs and fish products. Main Symptoms Abdominal pain, severe vomiting, diarrhoea, abdominal cramps, and sometimes collapse. Onset of Illness 1 to 6 hours (usually 2 to 4 hours). Source Staphylococcal bacteria may come from infected sores, nasal secretions and skin (perspiration and hair). The toxin that causes illness can survive ordinary cooking.


Foods involved Contaminated meat and meat products, especially poultry. Contaminated water, and raw milk. Main Symptoms Diarrhoea, abdominal pain, fever, nausea, and vomiting. Onset of Illness 1 to 10 days (usually 3 to 5 days). Source Campylobacter bacteria are often present in the waste of man and animals (especially domestic animals and poultry). This illness is infectious and can be spread to other people.


Foods involved All foods can be affected; (eg. soap powders/rat poison getting into dry food mixes; garden poison residues in soft drink bottles). Main Symptoms Abdominal pain, nausea, at times vomiting and diarrhoea. These symptoms may not be present for a lot of poisons-- in these cases often the first symptom is of collapse. Onset of Illness Usually less than half an hour.

Other food-borne diseases of note are Listeria, Yersinia and Cryptosporidium.


Foods involved Contaminated processed meats and meat products, raw milk, seafood, poultry and vegetables etc (eg coleslaw). Main Symptoms Normal host Acute/mild fever, influenza-like symptoms. At risk host Fever, intense headache, nausea, meningeal irritation and vomiting. Infection of the foetus, septicemia, meningitis, and still-birth. Onset of Illness 3 days to 3 weeks. Source Listeria bacteria are commonly found in soil, water, vegetation, domestic animals, and man. The illness, though infectious, is relatively rare. At Risk Hosts Pregnant women, the elderly, and those with lowered immune systems.


Foods involved Contaminated meat and meat products, especially pork mince and tongue. Contaminated water, seafood and raw milk. Main Symptoms Under 5 yrs diarrhoea, (sometimes bloody). Over 5yrs abdominal pain (like appendicitis), also fever, joint pain sore throat and rash. Onset of Illness 12 hrs to 11 days (usually 24 to 48 hours). Source Yersinia bacteria are often present in the waste of farm animals (especially pigs) and infected pets-(puppies and kittens) and man. This illness is infectious and can be spread to other people.


Foods involved Contaminated food and water, unpasteurised milk or fruit juices. Main Symptoms Diarrhoea (often watery), abdominal cramps/pain, and anorexia. Fever, nausea, and vomiting occur less often. Onset of Illness 1 to 12 days (usually 7 days). Source Cryptosporidium parasites are often present in the waste of farm animals, poultry, pets and man. This illness is infectious and can be spread to other people.


Inadequate cooling/refrigeration, food left at room temperature. Too long between preparation and consumption. Inadequate reheating. Inadequate cooking. Cross-contamination from raw to high risk/ready to eat foods. Infected food handlers. Inadequate hot holding temperatures. Inadequate hand washing. Contaminated raw foods and ingredients. Improper cleaning of equipment and utensils. PREVENTION OF FOOD POISONING

In most cases of food poisoning a chain of events takes place, and if we are to reduce the incidence of illness, this chain must be broken.


There are three main ways of breaking the food poisoning chain -


Inspecting all food and washing fruit and vegetables before preparation. Separating raw and high risk/ready to eat foods at all stages of preparation, storage, display and distribution. The same equipment, utensils and working surfaces must not be used to handle raw and high risk/ready to eat foods. Only handling food when unavoidable. Gloves, tongs and other utensils, plates and trays should be used in preference to hands,(but must be washed or changed frequently). Keeping food covered as much as possible. Preventing insects, animals and birds from entering food rooms. Not using unsuitable, defective, or dirty equipment. Using good personal hygiene practices - always. Not coughing or sneezing over or around food. Not handling the food contact surfaces of crockery, cutlery or utensils. All food handlers wearing suitable protective clothing. Using the correct cleaning procedures. Promptly removing unfit or waste food and refuse from food areas.


Keeping high risk foods at temperatures that inhibit the growth of bacteria (ie. out of the danger zone). Food should be kept below 4°C in a refrigerated unit, or above 70°C in a suitable warming unit. Ensuring that during preparation, food is in the danger zone for as short a time as possible. High risk foods must not be left sitting out at room temperature. Using suitable preservatives such as salt and sugar. Using various packing methods like gas flushing or vacuum packing. Not allowing dried foods to absorb moisture.


Adequately cooking food, ensuring that a minimum internal cooking temperature of 80°C is reached. Heat processing such as pasteurisation, sterilisation or canning. A combination of a suitable temperature and sufficient time is always required to destroy bacteria. The time and temperature required will depend on the particular organism, (eg. spores of Clostridium perfringens are much more heat resistant than Salmonella bacteria).


Good personal hygiene reduces the chance of contamination of food.

Hands must be washed before and after handling food. If unwell, do not handle food until cleared by a doctor. The hair, nose and mouth must not be touched during food preparation. Suitable light coloured protective clothing should be worn. Cuts and abrasions should be covered with waterproof bandages and if on the hands suitable gloves worn. Rings and other jewellery should not be worn as they can harbour dirt and bacteria and could themselves fall into the food being prepared.


Food decays or goes off, due to the micro-organisms that always exist in food;- they are not necessarily the bacteria that cause food poisoning. The signs that food is spoiling are:

Taints due to flavour change may also occur.

Moulds may also form slimy whiskers.

Some moulds have coloured spores that give the food a distinctive colour, for example, black pin mould on bread, or blue and green mould on citrus fruit and cheese.


Micro-organisms are often called bugs. This is a little too simple however and food handlers should know a little more about them. They differ from one another in appearance and activity, and looking at those found in food as a whole we find that provided suitable nutrients are available growth occurs - At temperatures between -7 to around 70°C. Over a pH range from 0 to 11. In the presence or absence of oxygen. At water activities above about 0.6. Spoilage of any particular food will be by those organisms most suited to the conditions in and around that food. The three main groups of concern are -


Bacteria are the main cause of food-borne illness or food poisoning and are an important cause of food spoilage. They thrive where food and water are present and the temperature is suitable, as in the nose, throat, skin, bowel and lower urinary tract of man and animals. They are single cell organisms usually having a definite outer envelope or capsule for protection. They multiply by dividing into two, which can occur very quickly, (eg. every 20 minutes). They can actively move and some link themselves together in chains or in bunches. To resist harm, some bacteria can form spores (tough reproductive cells that are able to survive under adverse conditions), that can resist damage by heat (as in cooking), by cold (as in freezing) and by chemicals such as disinfectants. A spore can survive in dust, on vegetation and in soil for weeks, months or even years until it finds itself in a suitable environment for growth.


Viruses are organisms much smaller than bacteria. In their pre-infective stage they are just like a chemical with none of the requirements for life, but once in a living cell they take over and begin to multiply. They can grow only in living tissue, but can be carried in food from one person to another.



Yeasts are single cell organisms much larger than bacteria and can be found in the soil, on plants and on the skin and body of man. They multiply by forming offspring as buds which grow and then detach themselves.

Some can produce disease, some cause skin infections in man and others cause diseases in plants. Some yeasts spoil food, but beneficial uses are in the making of beer, wine and bread.


Moulds grow as single cell filaments that can branch together making a strongly knit structure like a mat, that can often be seen with the naked eye. Usually they look fluffy, being a familiar sight on foods like jam, cheese and bread. They multiply by producing clusters of dry spores which are blown by the air like seeds.

Many moulds spoil food and a few can cause disease in plants and man, but beneficial uses are in the ripening of cheeses and production of antibiotics.


There are certain environmental conditions that must be met for micro-organisms to grow and multiply and when these conditions exist they can very quickly increase in number. These conditions are -

Anything less than optimum conditions will lead to a slowing down or a stopping of growth and then possibly their death.


Time is needed for the organism to grow and reach maturity. In most cases we try to prevent an organism from maturing by making its environment unsuitable for growth. FOOD

All organisms need food for growth and energy.


Each micro-organism has an optimum temperature where it grows most rapidly and a maximum and minimum temperature at which it will grow. Outside this range it will grow very slowly, or not at all.



The numbers on the pH scale, as shown in the following diagram, indicate the acidity or alkalinity of a fluid. Micro-organisms can grow and multiply only within a certain pH range. Most prefer to live in a neutral environment around pH 7. A small group of micro-organisms prefer an acid environment and do not grow in the neutral range. Low pH generally inhibits microbial growth. Yeasts and moulds are the most capable of growth at low pH. Other acid-producing bacteria such as lactic acid bacteria also predominate at low pH.

Approximate pH Growth Ranges for some Food-borne Illness Causing Micro-Organisms


Without water, Dehydration (loss of moisture) occurs and the life and growth processes of micro-organisms slow down and may stop. The micro-organisms might not be destroyed however. The use of salt or syrups (sugar) in various foods is a way of activating this process. These salts and sugars are crystals that compete with the micro-organisms for the available water that they need for survival.

It is now generally accepted that the water requirements of micro-organisms should be defined in terms of the water activity (aw) in the environment. This is a measure of the availability of water to micro-organisms for metabolism (the processes of life). The (aw) of pure water is 1.00, - a 22% salt solution has an (aw) of 0.86 and a saturated salt solution is 0.75. The (aw) value for most fresh foods is above 0.99.

Approximate Minimum (aw) Values for Growth



Micro-organisms respire. That is, they get energy by breaking down chemicals, usually sugars, inside the cell. Aerobic organisms must use oxygen obtained from their environment (usually air) before they can produce energy for life and growth. Anaerobic organisms can produce this energy only in the absence of oxygen. Facultative organisms can respire in either aerobic or anaerobic conditions.


This is the availability of oxygen to micro-organisms, and can be controlled by packaging, eg. by gas flushing.


Control of micro-organisms is needed to prevent -

Yeast and mould spores, and most other bacteria are destroyed at temperatures between 70 - 100°C for 5 to 10 minutes exposure.

Bacterial spores however, are very difficult to destroy. Some for example, need at least 10 minutes at 100 to 120°C.

The following terms are commonly used in cleaning -

Sterilisation - The process of destroying or removing all microbial life. Disinfection - The killing of disease causing bacteria as well as other living micro-organisms, but not usually bacterial spores. Disinfection does not necessarily kill all micro-organisms, but reduces them to a level not usually harmful to health. In this group are the fungicides (kills fungi), bactericides (kills bacteria) and virucides (kills viruses). Sanitising - A term meaning that an article or surface is visibly clean and is free of disease producing organisms. IMPORTANT TEMPERATURES IN FOOD SAFETY

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