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Food contamination and spoilage
Methods of food contamination: physical, chemical, bacterial
1. Physical
Physical contamination is when physical objects are allowed to be mixed into or added to
food either during preparation, processing or storage. These physical objects are called
foreign bodies.
- Glass should not be used in food preparation, production and processing areas or
even storage areas if the food the food is not covered or protected. In areas where
food is at risk even light bulbs or filaments should have plastic coverings.
- Wood especially wooden pallets or pieces of flaking wood should be kept out of food
production and preparation areas to avoid food being contaminated by pieces or
splinters of wood. Wooden chopping boards should be avoided and old or damaged
chopping boards should be thrown away.
- The decorations of walls and ceilings should be carefully maintained to avoid food
being contaminated by flaking paint or plaster.
- All machinery and equipment should be properly maintained to avoid parts falling off
into the food i.e. Rubbers and plastic parts, nuts and bolts, oil
- Food handlers can be a source of physical contamination if they do not follow good
personal hygiene. Hair especially long hair should be covered with a net or hat and
tied up. Jewellery should not be worn as items of jewellery such as rings or gems could
contaminate food if they fall in. Nails should be kept short and no varnish used as this
could break off into the food. Smoking is banned in food production, preparation, and
storage areas to prevent cigarette ends and ash contaminating food.
- Soil from the ground when harvesting
© Food – a fact of life 2009
2. Chemical
a) Addition of chemicals: This is when chemicals, such as cleaning fluids, sterilants, or
pesticides are added to food by mistake. Sometimes contamination can arise when
preservatives are added in quantities far above permitted levels. In industry the
chemicals used for cleaning plants (industrial equipment) or in kitchens can contaminate
food or be packed instead of the food product.
Chemical contamination can be avoided by always storing chemicals in labelled
containers away from food. All addition of preservatives and chemicals should be
carefully monitored.
b) Migration of chemicals:
This is where chemicals such as metals or chemical toxins migrate from equipment such
as pipes or saucepans or storage containers such as tin cans, into food.
Only plastics which are safe for food use, should be used to store foods. Food should not
be stored in open tin cans and care should be taken with aluminium saucepans when
used to cook high acid foods.
c) Absorption of undesirable odours:This is when a large range of unwanted smells are
'picked up' by foods. Foods with high fat content are most prone to odour contamination.
The odours 'picked up' can be anything from perfume or aftershave from handlers to
paint from the walls. Chemicals used in food preparation and storage areas and even
the odour of other foods can be picked up. For example eggs can pick up fish odours if
stored near to each other.
© Food – a fact of life 2009
3. Bacterial
This is when food is contaminated by matter of a biological origin. As soon as food is harvested,
slaughtered or manufactured into a product it starts to change. This is caused by two main processes:
autolysis – self destruction, caused by enzymes present in the food
microbial spoilage – caused by the growth of bacteria, yeasts and moulds
Spoilage - Changes in food, either through enzyme deterioration of food or micro-organism
growth, will eventually lead to the food becoming inedible or unsafe if eaten.
The rate of deterioration depends on a variety of factors which must
be controlled carefully.
Contaminants may be already present in the food, e.g. salmonella in
chicken or transferred to the food by humans, flies, rodents and other
Poisoning - Micro-organisms occur naturally in the environment, on cereals, vegetables, fruit,
animals, people, water, soil and in the air. Most bacteria are harmless, but a small number can
cause illness. Food which is contaminated with food poisoning micro-organisms can look, taste
and smell normal.
The most common type is human hair from handlers who have failed to adequately cover all hair
on the scalp or face. Hair from pets or pests can also be found if they are allowed to contaminate
Rodent droppings can also be found in uncovered and unprotected food.
Dead insects or parts of their bodies when sufficient care has not been taken over the storage and
preparation of food.
A rarer type of biological contamination is when poisonous plants are mixed with similar looking
food; e.g. poisonous berries mixed with redcurrants or blackberries or toadstools contaminating
© Food – a fact of life 2009
Desirable food changes
Autolysis and micro bacterial changes are sometimes
desirable (and are not referred to as spoilage), for
example enzymes cause fruit to ripen.
Here are some positive micro bacterial
changes below.
Bacteria in yoghurt
© Food – a fact of life 2009
Mould in some
cheeses, e.g. Stilton.
Yeast in bread
1. Autolysis - enzymes
Enzymes are chemicals that are found in food.
These chemicals have important uses in food. They can cause food to
deteriorate in three main ways:
• ripening – this will continue until the food becomes inedible, e.g.
banana ripening;
• browning – enzymes can react with air causing the skin of certain foods,
e.g. potatoes and apples discolouring;
• oxidation – loss of certain nutrients, such as vitamins A, C and thiamin
from food, e.g. over boiling of green vegetables.
2. Microbial spoilage - bacteria
These are single celled micro-organisms (they cannot be seen by the
naked eye) which are present naturally in the environment.
There are many different kinds, some are useful, e.g. in the production of
yogurt, and some harmful.
The presence of bacteria in food can lead to digestive upset.
Some bacteria produce toxins which can lead to this also.
Spores can also be produced by some bacteria leading to toxins being
© Food – a fact of life 2009
3. Microbial spoilage - yeast
Yeasts are single celled fungi which can reproduce by ‘budding’. This
means that a small offshoot or bud separates from the parent yeast
cell. Yeasts can also form spores which can travel through the air.
These are easily killed by heating to 100ºC.
In warm, moist conditions in the presence of sugar, yeasts will cause
foods like fruit to ferment producing alcohol and carbon dioxide gas.
Yeast is used in the production of bread and wine.
4. Microbial spoilage - mould
Moulds are fungi which grow as filaments in food. They reproduce by
producing spores in fruiting bodies which can be seen on the surface of
These fruiting bodies sometimes look like round furry blue-coloured
growths, e.g. mould on bread.
Some moulds can be seen by the naked eye, e.g. on bread.
© Food – a fact of life 2009
Conditions for bacterial growth
Micro-organisms need conditions to survive and
reproduce. These can include:
• temperature
• moisture
• food
• time
• pH level
• oxygen
© Food – a fact of life 2009
Conditions for bacterial growth
1. Temperature
Bacteria need warm conditions to grown and multiply.
The ideal temperature for bacterial growth is 30ºC – 37ºC.
Some bacteria can still grow at 10ºC and 60ºC.
Most bacteria are destroyed at temperatures above 63 ºC.
Bacterial growth danger zone in 5ºC - 63ºC.
At very cold temperatures, bacteria become dormant – they do not
die, but they cannot grow or multiply.
100ºC Water boils
82ºC Core temperature of hot food
5ºC - 63ºC danger zone for rapid growth of micro-organisms
1ºC - 4ºC temperature of fridge
0ºC Freezing point of water
-18ºC temperature of freezer
© Food – a fact of life 2009
2. Moisture
Where there is no moisture bacteria cannot grow. However, bacteria and
moulds can both produce spores which can survive until water is added
to the food. Dried foods, or those with high sugar or salt content will not
support bacterial growth. Bacteria remain dormant.
3. Food
Bacteria need a source of food to grow and multiple, these food usually contain
large amounts of water and nutrients. High protein food are normally a good source
i.e. meat, poultry, dairy products (except butter and hard cheese).
4. Time
One bacterium can divide into two every 20 minutes. Food where bacteria rapidly
multiple in are called perishable foods.
5. pH level
An acidic or alkaline environment can promote or inhibit microbial growth. Most
bacteria prefer a neutral pH (6.6 – 7.5). Moulds and yeasts can survive at pH levels of 11/5 (very acidic), food spoilage usually occurs by yeast and moulds.
6. Oxygen
Some bacteria need oxygen to grow and multiply. These are called aerobic bacteria.
Other bacteria grow well when there is no oxygen present, these are known as
anaerobic bacteria.
© Food – a fact of life 2009
The Ten Main Reasons for Food Poisoning
Food prepared too far in advance and stored at
room temperature.
2. Cooling food too slowly prior to refrigeration.
3. Not reheating food to a high enough temperature
to destroy food poisoning bacteria.
4. The use of cooked food contaminated with food
poisoning bacteria.
5. Undercooking.
6. Not thawing frozen poultry for sufficient time.
7. Cross-contamination from raw food to cooked
8. Storing hot food below 63ºC.
9. Infected food handlers.
10. Use of leftovers.
© Food – a fact of life 2009
High, medium and low risk foods
High – Some foods are high-risk, as they provide the ideal conditions needed
for micro-organisms to grow. If these foods become contaminated with
food-poisoning micro-organisms and conditions allow them to multiply, the
risk of food-poisoning increases. foods more prone to bacterial infection,
e.g. raw or cooked meats, raw or cooked fish and shellfish/seafood, eggs,
cooked rice and lentils, gravies and soups, milk, tofu, fresh filed pasta
Medium – foods that may contain pathogenic organisms but will not normally
support their growth; or which are unlikely to contain pathogenic
organisms but may support the formation of them or growth of toxins, e.g.
fruits and vegetables, canned meat, dairy products,orange juice,
pasteurised milk, dairy products, ice cream, peanut butter, and milk-based
Low – foods unlikely to contain pathogenic micro-organisms and will not
normally support their growth due to food characteristics, e.g. grains and
cereals, bread, alcohol, carbonated beverages, sugar-based
confectionery and fats and oils
© Food – a fact of life 2009
Food poisoning and bacteria
There are thousands of cases of food poisoning each year, many of which
are not reported or recorded in official statistics.
Food poisoning may result from poor domestic food preparation, or poor
food processing in industry.
This may result in loss of business and people’s jobs if it is a serious outbreak.
Micro-organisms occur naturally in the environment, on cereals, vegetables,
fruit, animals, people, water, soil and in the air.
Most bacteria are harmless but a small number can cause illness.
Food which is contaminated with food poisoning micro-organisms can look,
taste and smell normal.
People at high risk
Elderly people, babies and anyone who is ill or pregnant needs to be extra
careful about the food they eat. For example, pregnant women or
anyone with low resistance to infection should avoid high risk foods such
as unpasteurised soft cheese.
© Food – a fact of life 2009
Symptoms of food poisoning
Food poisoning can be mild or severe.
The symptoms will be different depending on
what type of bacteria is responsible.
Common bacteria causing food poisoning include:
• Campylobacter
• E Coli 0157
• Salmonella
• Staphylococcus aureus
• Listeria Monocytogenes
• Bacillus cereus
© Food – a fact of life 2009
Chicken, Eggs, Pies, Sausages,
Faeces, Sewage, Unwashed
Headache, Fever, Vomiting,
Abdominal pain, Aching of limbs,
Escherichia Coli (E
Coli) 0157
Raw meat, dairy products
Vomiting, Diarrhoea (may contain
blood), Fever, Headache
Nose, Skin, Cuts, Cooked meat,
Pies, Dairy products (Custard,
Ice Cream)
Severe vomiting, Diarrhoea,
Bacillus Cereus
Rice, Gravy, Cream, Sausages,
Cured meat, Faeces
Vomiting, Diarrhoea, Abdominal
Poultry, Raw meat,
Unpasteurised milk
Profuse diarrhoea (may contain
blood), Abdominal pain, Nausea,
Water, Soil, Manure, Milk and its
products, Soft cheeses, Pate,
Cook-chill foods, Ready made
Flu-like symptoms, Miscarriage,
Blood poisoning, Pneumonia
© Food – a fact of life 2009