Study session 8  Food Contamination and also Spoilage


All food should be safe and complimentary from contamination and spoilage at all points in its journey from its resource until the reaches the consumers. However, food contamination is a significant public health trouble in Ethiopia, causing foodborne conditions that influence many human being every year. Hence, awareness the potential resources of food pollution is crucial component of an excellent nutrition and good health. In this study session we are going to concentrate on food contamination by microorganisms, chemicals and also physical factors.

Food might be contaminated by different microorganisms or by chemicals the can cause health troubles for anyone that eats it. In study Session 9 friend will discover in detail about foodborne diseases. But first you will be introduced to the an easy principles that food microbiology in this examine session, and around the ways in i beg your pardon food i do not care contaminated by different microorganisms, chemicals and also physical objects. Friend will likewise learn around the causes of food spoilage and its after-effects for health.

Learning Outcomes for research Session 8

When you have studied this session, you have to be may be to:

8.1 Define and also use correctly all of the key words printed in bold. (SAQ 8.1)

8.2 define the factors that contribute to bacterial growth and multiplication. (SAQ 8.2)

8.3 define the resources and types of food contamination. (SAQ 8.2)

8.4 explain the key routes that food contamination. (SAQ 8.3)

8.5 perform the types of food spoilage and describe the qualities of food spoilage in different food items. (SAQ 8.4)

8.1  Infectious agents and also foodborne diseases

8.1.1  An overview

Infectious agents are organisms that can be happen to, and between, human being in the procedure of infection transmission. Those that reason diseases are frequently referred to together pathogens (‘pathogenic’ method disease-causing). Many transmittable agents (bacteria, viruses, fungi and protozoa) room microorganisms that space too tiny to it is in seen except with a microscope; the adult stages of disease-causing parasites (e.g. Worms) may be seen v the naked eye, however their eggs and immature stages room microscopic. Microbiology is the science that encounters the examine of microorganisms.

Although epidemic often an outcome in disease, it is possible to be infected with a microorganism and still appear healthy. This is either due to the fact that the disease has not yet had time to develop, or due to the fact that the who immune device is keeping it under control. However, the transmittable agent can still it is in passed on come others, for instance by spreading into food taken on by the infected person.

You are watching: Contamination of food items by other living organisms

The majority of foodborne illness (those caused by contagious agents transmitted to world in the food we eat) are because of bacteria, however as girlfriend will watch in research Session 9, viruses, parasites and also toxins can also cause foodborne diseases.

8.1.2  Bacteria

Bacteria are the many abundant of all organisms. Bacteria room unicellular organisms (made of one cell) and are very small in size, ranging from 0.5 come 5.0 micrometres (µm).

1.0 micrometre (µm) = 0.001 millimetres (mm); 1000 micrometres (µm) = 1mm. The symbol for the micrometre is µm. µ is a Greek letter express ‘mu’.

Bacteria reproduce asexually. This way that castle don’t need a partner to reproduce, however simply divide into two, producing two new bacteria. There space pathogenic bacteria qualified of causing human being illness and also food spoilage, yet there are also beneficial varieties of bacteria that are essential to an excellent health and also a healthy and balanced environment. Because that example, useful bacteria live in our gut and help us digest our food; part bacteria are used to create foods such as yoghurt and cheese; and also others break down wastes in the environment.

Some bacteria are capable of forming highly resistant and endurable structures referred to as spores. Bacter spores are resistant come heat, freezing, drying, chemicals and also other adverse environments. This method the spores deserve to survive the normal processes of food storage and preparation. Two instances of spore-forming bacteria vital in food pollution are Bacillus and Clostridium.

Temperature, humidity, oxygen and water are essential for bacteria to grow and also multiply. Under favourable conditions a farming bacterial population can double at regular intervals varying from around 15 minute to numerous hours. This means that the numbers of bacteria in food have the right to increase rapidly and also soon come to be hazardous to health, specifically if the food has actually a favourable temperature and water content. In the following section, us look in detail at determinants that have the right to promote or hold-up bacterial growth in ours food.

The growth of microorganisms in food assets can be influenced by extrinsic factors and intrinsic factors, together you will see below. By knowledge the determinants affecting the growth of microorganisms in food we can know how to store food for sure to eat. This knowledge can also assist us to work out how to maintain food for longer.

8.2.1  Extrinsic factors

Extrinsic factors are factors in the atmosphere external come the food, which impact both the microorganisms and also the food itself during processing and also storage. Extrinsic determinants include temperature, humidity and also oxygen.


Different microorganisms thrive over a wide selection of temperatures. Some microorganisms favor to prosper in the cold, some choose to flourish at room temperature and others prefer to grow at high temperatures. This is of paramount prominence in food safety, due to the fact that if you understand the temperature development ranges for dangerous microorganisms it helps you to choose the ideal temperature because that food warehouse to do them less able come grow and reproduce.


The humidity of the storage atmosphere is critical factor for the development of microbe at the food surfaces. If you save food in a dry atmosphere, microbe are much less able to grow than if the food is stored in a humid (moist) environment. Therefore, dry conditions are far better for food storage than moist conditions.


Many microorganisms require oxygen in order come develop and reproduce: this are dubbed aerobic microorganisms. A good example is Escherichia coli, a faecal bacter which grows conveniently on countless foods. If you keep food in a short oxygen environment, aerobic bacteria cannot grow and also multiply. Conversely, there are some microorganisms that grow without oxygen, dubbed anaerobic microorganisms. An example of this is Clostridium botulinum, the bacterium causing botulism, which can survive in very low oxygen settings such as tinned foods.

8.2.2  Intrinsic factors

Intrinsic factors exist as part of the food product itself. Because that example, meat has certain characteristics that might promote the development of details microorganisms. The following typical intrinsic factors impact the growth and also multiplication of microorganisms in foods.

The scientific term pH is a measure of exactly how acidic or alkaline an setting is, top top a range that has actually ‘neutral’ (neither acid nor alkaline) in ~ pH7. Environments that room acidic have pH values below 7; those that room alkaline have actually pH values over 7. Most microorganisms grow finest at close come the neutral pH value (pH 6.6 to 7.5). Just a few microorganisms prosper in really acid conditions below a pH that 4.0. Bacteria prosper at a relatively specific pH for each species, however fungi prosper over a broader range that pH values. Because that example, most meats naturally have actually a pH of around 5.6 or above. At this pH meat is at risk to spoilage by bacteria, moulds and yeasts; yet the pH of meat have the right to be lowered by pickling, which provides it much less favourable together an environment for microorganisms to thrive in.

Moisture content (water activity, aw)

Microorganisms require a moist atmosphere to flourish in. The water needs of microbe are described in regards to water task (represented through the symbol aW), a measure of how much water is present. The water task of pure water is aW = 1.00. Many foodborne pathogenic bacteria need aW to be better than 0.9 for growth and also multiplication; however, Staphylococcus aureus may grow with aW together low together 0.86. But even Staphylococcus aureus cannot grow and multiply in drier food prefer bread, which has aW = 0.7, back fungi deserve to (Figure 8.1).


Figure 8.1 Bread is as well dry because that bacterial growth, but fungi can grow in it an extremely quickly. (Photo: Basiro Davey)

Think of some foods items that keep well when they are dry but come to be contaminated conveniently when they are wet.

You may have actually thought of different examples: the one that we thought of is rice. When rice is dried it will store for a lengthy time, but when that is cooked and wet it will go bad quite quickly and cause food poisoning.

Nutrient content

In order to grow, main point and duty normally, microorganisms require a range of nutrient such together nitrogen, vitamins and also minerals. Microorganisms therefore grow fine on nutrient-rich foods.

Structure that food items

The organic covering of some foods provides excellent protection versus the entry and subsequent damage by spoilage organisms. Instances of together protective structures room the skin that fruits and vegetables such as tomatoes and bananas (Figure 8.2).


Figure 8.2 plenty of fruits room protected versus spoilage by microorganisms, but only if their skin is intact. (Photo: Basiro Davey)

8.3  Routes the microbial pollution of food

Bacteria space a major source that microbial pollution of food, i.e. The undesired existence in food that harmful microorganisms or the harmful substances they produce. Viruses, parasites and also fungi are likewise able to contaminate food and also cause foodborne illnesses in humans. Microbe can go into food through different routes.

Look at figure 8.3, which was presented in examine Session 1, and notice that eat contaminated food is a ingredient of numerous transmission routes. The most common routes of entrance are disputed below.


8.3.1  Air and dust

Microorganisms are discovered everywhere in ours environment. Many types can be found in air and also dust, and can contaminate food at any type of time throughout food ready or when food is left uncovered (Figure 8.4). Imagine a kitchen whereby food is prepared and also stored in countryside communities, and think how conveniently microorganisms in the air and also dust can contaminate the food.


8.3.2  Soil, water and also plants

Many microorganisms present in soil and also water may contaminate foods. Microorganisms likewise grow on plants and can contaminate food if treatment is not taken to remove them by washing or inactivate them by cooking. Floor is a specifically rich source of Clostridium bacteria. Water may be contaminated through faeces. Tree may likewise be contaminated through faeces if untreated sewage has actually been supplied as a fertiliser.

8.3.3  Gastrointestinal tract

The intestines of every humans and also animals are full of microorganisms, several of which are valuable but others room pathogenic. Bacterial pathogens such as Salmonella, Campylobacter and Escherichia coli (strain O157:H7) are common examples. Pollution of foodstuffs by faecal product is the major cause of food poisoning events. This has indirect contamination, for example from people’s hand if they prepare food there is no washing their hands after ~ visiting the latrine/toilet (see below).

Escherichia coli (abbreviated to E.coli) exist in numerous harmless arrays or ‘strains’, but some strains space pathogenic. The strain called E.coli O157:H7 reasons a perhaps fatal foodborne disease in humans.

8.3.4  Animals

Many foodborne microorganisms are current in healthy pets raised because that food, typically in their intestines, hides, feathers, etc. Meat and also poultry carcasses deserve to be contaminated throughout slaughter by contact with small amounts of minister contents. For example, in pets slaughtered in rural communities without any safety measures, microorganisms current in the animals’ intestines can conveniently contaminate the meat.

Animal hides are critical source of pollution of the basic environment, the hand of meat workers, and also skinned meat carcasses. Hides room a primary source of E.coli O157:H7 and also Salmonella species, both of which cause sickness and also diarrhoea. Hides end up being contaminated either due to the fact that the outside of the hide is dirty, or since once eliminated from the animal, the inside of the hide is a good breeding place for microorganisms.

8.3.5  Animal feeds

Animal feeds room a source of microorganisms, specifically Salmonella, which have the right to contaminate poultry and other farm yard animals. The organisms in dry pet feed spread out throughout the regional environment and may gain on to pet hides, hair and also feathers, and also on people who manage the feeds.

8.3.6  Food handlers

The ax food handler deserve to be used to anyone who touches or handle food, and this includes civilization who process, transport, prepare, cook and serve food. The existence of microbe on the hands and outer garments of food handlers mirrors the standard of restroom in the environment and also the individuals’ personal hygiene (as friend learned in previously study sessions). The microbe transmitted to foodstuffs by food handlers may come indigenous the hides that animals, soil, water, dust, gastrointestinal tracts and other environmental sources. In food preparation at home, foodborne microorganisms deserve to be presented from the unwashed hands of world who are infected by bacteria and also viruses, and also who cook and also serve the food to household members.

8.3.7  Food utensils

Food utensils space cutting boards, knives, spoons, bowls and other tools used in food preparation, i m sorry may become contaminated throughout food processing and preparation. For example, in family members where over there is no accessibility to to run water, the food utensils might not be appropriately cleaned, stored and also handled, and may come to be a significant route that food contamination.

8.3.8  Cross-contamination

Cross-contamination of food is the move of harmful microorganisms in between food items and food contact surfaces. Prepared food, utensils and also surfaces may end up being contaminated by life food products and microorganisms. These have the right to be moved from one food to another by using the same knife, cutting plank or other utensil without washing it between uses. A food the is totally cooked can become re-contaminated if it touches raw foodstuffs or contaminated surfaces or tools that save pathogens. For example, you must never:

allow raw meat to touch cooking meatput cook meat on a cutting plank that has actually just been provided for raw meat without cleaning it an initial (Figure 8.5)store life meat on a shelf over cooked meat so the it could leak blood and also raw juices on to the cooking meat below.

Figure 8.5 never ever put cooking meat on an unclean cutting plank that has been provided to prepare life meat. (Photo: Basiro Davey)
8.3.9  Unsafe temperature

An unsafe temperature because that food storage is a major factor in food contamination. Many microorganisms should multiply come a very huge number prior to enough are current in food come cause condition in someone that eats it. However, if bacteria have the right to have warm, moist conditions and an ample supply of nutrients, one bacterium can reproduce by splitting (on average) every fifty percent an hour and can create 17 million bacteria in 12 hours! So, if you leave lightly contaminated food out overnight, it will certainly be very contaminated and also infectious by the following day.

8.3.10  Poor personal hygiene

Poor personal hygiene the food handlers is another major factor in food contamination. The most necessary contaminants that food are the microorganisms excreted v faeces indigenous the intestinal street of humans. These pathogens space transferred to the food native faecal matter present on the hands.

We have currently mentioned failure to wash hands ~ visiting a toilet together a source of food contamination. Can you suggest other times once food handlers should wash their hands?

Hands must be wash before starting work on preparing food, and after touching any food, surface or tools that may be contaminated (e.g. After handling raw meat).

Bad personal habits like scratching your hair and nose through your fingers likewise contributes come food contamination. Sneezing and also coughing diffusion contaminants and also microorganisms with the air and onto uncovered food, and onto surfaces and also hands that have the right to transfer the infectious agents into food.

8.3.11  Pests

Foods have the right to be damaged and also contaminated by pests. Plenty of stored seed are lost through the damage done by pests, consisting of termites (mist), beetles, locusts, cockroaches, flies and rodents such together rats and also mice. Pests deserve to damage and contaminate foods in assorted ways, such as boring into and also feeding on the insides that grains, or tunnelling into stems and roots of food plants. For example, weevils cause huge losses of stored grains, particularly in warm and also humid problems such as in lowland locations of Ethiopia.

Pests additionally damage the protective skin that foods allowing microorganisms to gain inside the food and also causing it to rot much more quickly. Pests deserve to pollute food with their excreta, and also with bodies and also body pieces when lock die. They likewise transfer microorganisms on to food while walking on that (Figure 8.6). Flies and also cockroaches easily move between wastes and also foods, moving microorganisms v them as they go.

Figure 8.6 Insects have the right to leave dirt, excreta and also possibly pathogenic microorganisms if lock are enabled to crawl on food. (Photo: Basiro Davey)

8.4  Avoiding food contamination

You now understand that food can be contaminated from sources in the herbal environment, people, food preparation surfaces and utensils, raw and also uncooked food, animals, pests, and waste material. To protect against contamination, food production and preparation operations need to be carefully controlled.

8.4.1  Microbial food contamination

Prevention of microbiological (often abbreviated to ‘microbial’) pollution is an essential function in food preparation, together summarised in crate 8.1.

Food handlers should follow these strategies:

Thorough handwashing before and also during food preparation, especially after utilizing the toilet, and also handling life food or waste.Soap/ash sanitiser and also clean water should be available for handwashing at practically locations.Sick food handlers need to not prepare food! One sick person can reason a foodborne an illness outbreak, an especially where people are in crowded or unsanitary life conditions.Raw and cooked foods should it is in separated, due to the fact that raw foodstuffs are a resource of microorganisms and can recontaminate ready foods.
8.4.2  Chemical air pollution of food

Attention likewise needs to be offered to possible chemical air pollution of food. Food deserve to be contaminated through the misuse or mistaken taking care of of chemicals, consisting of pesticides, bleach and also other clean materials. All chemicals (detergent, disinfectant, sanitiser) provided in the food preparation area should be removed prior to food ready begins, come prevent any type of chemical pollution of the food.

Other possible sources of chemical pollution are:

reusing containers which have been supplied for chemicals (Figure 8.7)using chemistry sprays (e.g. To kill cockroaches) in locations where food is exposedaccidentally including chemicals which have a texture comparable to table salt or sugar throughout food preparation; lock should constantly be stored separately.
Figure 8.7 The deserve to that is being supplied to scoop salt indigenous this sack has actually previously been offered for insecticide. (Photo: Pam Furniss)
8.4.3  Physical contamination of food

Physical contaminants include stones, piece of glass, and also metal. Physical pollution can occur at any kind of stage the the food chain: because that example, stones, bones, twigs, pieces of shell or foreign objects can get in food throughout handling and preparation. These products should it is in removed, if possible, for instance by sieving or picking the end the items through clean fingers.

8.5  Food spoilage

Food spoilage is the procedure of readjust in the physical and chemical properties of the food so the it becomes unfit because that consumption. Food spoilage is any kind of undesirable change in food. Many natural foods have a limited life: because that example, fish, meat, milk and also bread space perishable foods, which means they have actually a short storage life and they conveniently spoil. Various other foods additionally decompose eventually, even though they keep for a considerably longer time. The main reason of food spoilage is invasion by microorganisms such as fungi and also bacteria.

8.5.1  Microbial spoilage

Microbial spoilage is caused by microorganisms prefer fungi (moulds, yeasts) and also bacteria. Lock spoil food by growing in it and producing building materials that adjust the colour, texture and also odour of the food. Eventually the food will certainly be unfit for person consumption.

When food is extended with a furry growth and becomes soft and also smells bad, the spoilage is resulted in by the growth of moulds and also yeasts (look back at figure 8.1). Microbial spoilage by moulds and also yeasts includes souring that milk, expansion of mould on bread and also rotting of fruit and vegetables. This organisms are rarely harmful come humans, however bacterial contamination is often an ext dangerous since the food walk not always look bad, also if it is severely infected. As soon as microorganisms get access to food, lock utilise the nutrients uncovered in it and their numbers promptly increase. They readjust the food’s flavour and also synthesise brand-new compounds that deserve to be harmful to humans. Food spoilage directly affects the colour, taste, odour and also consistency or structure of food, and it may end up being dangerous come eat. The existence of a bad odour or odor coming indigenous food is an indication that it might be unsafe. Yet remember that not all unsafe food smells bad.

What is the difference between food contamination and food spoilage?

Food contamination is when food is contaminated v microorganisms or substances and eating the could result in foodborne disease. Food spoilage is any kind of undesired change in the herbal colour, taste or texture of food items that makes it unfit for consumption due to the fact that it has actually lost its quality and also nutritional value.

The term contact spoilage is provided when microbial spoilage is the an outcome of direct call or touching in between the food and also any contaminated or unclean surface ar such together shelves, food ready boards or unwashed hands. It also includes food-to-food contact, for example in between cooked meat and also raw meat or in between rotting fruit and sound fruit.

8.5.2  Physical spoilage

Physical spoilage is as result of physical damages to food during harvesting, handling or distribution. The damages increases the chance of chemistry or microbial spoilage and also contamination due to the fact that the protective outer layer that the food is bruised or broken and also microorganisms can go into the foodstuff more easily. For instance you may have noticed that as soon as an to apologize skin is damaged, the apologize rots more quickly.

8.5.3  Chemical spoilage

Chemical reactions in food room responsible for changes in the colour and also flavour of foods throughout processing and storage. Foods items are of ideal quality as soon as they space fresh, but after fruits and also vegetables are harvested, or animals are slaughtered, chemical changes begin instantly within the foods and lead to degradation in quality. Fats failure and end up being rancid (smell bad), and naturally-occurring enzyme promote major chemical alters in foods items as lock age.

Enzymic spoilage (autolysis)

Every living organism offers specialised proteins referred to as enzymes to drive the chemistry reactions in that cells. After ~ death, enzymes play a duty in the decomposition of once-living tissue, in a procedure called autolysis (self-destruction) or enzymic spoilage. Because that example, some enzymes in a tomato help it come ripen, however other enzymes cause it to decay (Figure 8.8). As soon as enzymic spoilage is under way, that produces damage to the tomato skin, for this reason moulds can start to can strike it as well, speeding the procedure of decay.

Figure 8.8 duty of enzymes in tomato spoilage: the tomato ~ above the ideal has additionally been assaulted by fungi (moulds), speeding its decay.
Enzymic browning

When the cells of fruits and also vegetables such together apples, potatoes, bananas and also avocado space cut and also exposed to the air, enzymes present in the cell bring about a chemistry reaction in i m sorry colourless compounds space converted into brown-coloured compounds. This is called enzymic browning. If the food is cooked very soon after cutting, the enzymes are damaged by heat and the browning does no occur. Because that example, apples space prone to discolouration if reduced as soon as raw, however when cooking they do not walk brown.

8.5.4  Appearance of spoiled food

Spoiled food is generally an ext a trouble of appearance than a problem of condition causing. In food spoilage, the alters in illustration or structure of the food, such together rottenness, softness and change in colour, taste or odour are usually obvious, conversely, in contaminated food such features may not be noticed. A huge majority the the microbe responsible because that food spoilage space not pathogenic to humans. However, you have to advise human being in your neighborhood that they have to not eat food that is spoiled since it is no nutritious and may do them ailing (cause vomiting).

8.6  Factors influence food spoilage

Finally, we turn to the factors that have the right to increase or hold-up the procedure of food spoilage. They include its water content, ecological conditions, packaging and storage.

8.6.1  Water content

The quantity of water easily accessible in a food deserve to be explained in terms of the water task (aw).

The water activity of most fresh foods items is 0.99. This way that they have actually a really high water content and also can assistance a many microbial growth.

Meat is traditionally dried by adding table salt come it. Can you indicate why salting enables the meat to it is in stored because that a long time?

You will learn an ext about salting, smoking, refrigeration and also other food preservation approaches in study Session 10.

The salt draws out water indigenous the meat for this reason the aW is reduced. This renders the problems too dry for harmful microorganisms to main point in the salted meat.

8.6.2  Environmental conditions

No issue whether food is fresh or processed, the price of its degradation or spoilage is affected by the atmosphere to which it is exposed. The exposure of food come oxygen, light, warmth or even tiny amounts of moisture can often trigger a series of damaging chemistry and/or microbial reactions. An altering the atmosphere can help to delay spoilage. For example, storing foodstuffs at short temperatures reduces spoilage due to the fact that both microbial and also enzymic degeneration is much faster at higher temperatures.

8.6.3  Packaging and storage

Packaging is a method of safeguarding food once it is raw, or ~ it has been processed or prepared. It help to safeguard food against harmful contaminants in the atmosphere or conditions that promote food spoilage consisting of light, oxygen and moisture. The kind of packaging is a crucial factor in ensuring that the food is protected. Packaging of foodstuffs in cans, jars, cartons, plastics or paper also serves to for sure food security if that is intact, due to the fact that it provides protection against the entry of microorganisms, dust, dirt, insects, chemicals and also foreign material.

Summary of research Session 8

In research Session 8, you have learned that:

One of the main, and also most effective, methods of protecting food consumers is to prevent food from coming to be contaminated through pathogenic microorganisms.Cross-contamination is a process whereby pathogens room transferred from one food resource to another, e.g. When pathogens in raw food are transferred come cooked foods which will certainly not it is in cooked again, so any pathogens castle contain will endure to epidemic the consumer.Food may become contaminated through food handlers, contaminated surfaces and also utensils, pests, and contaminated water supplied in food preparation.Separate storage and also preparation areas, and also separate utensils, should always be supplied for raw foods.Foods have the right to be contaminated with the mishandling of chemicals such as pesticides, bleach and other clean materials.Food can be contaminated by physical contaminants such as stones, glass, bones and also feathers at any stage of the food chain.Food spoilage is the procedure of transforming the physical and chemical properties of the food, making it unfit because that consumption.Food spoilage is led to by living microorganisms and also by enzymic activity (autolysis). Spoilage can also be brought about by physics or chemical factors.

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Self-Assessment inquiries (SAQs) for research Session 8

Now the you have completed this research session, you deserve to assess exactly how well friend have achieved its learning Outcomes through answering these questions. Compose your answers in your examine Diary and discuss them v your tutor at the next Study assistance Meeting. Girlfriend can inspect your answers with the notes on the Self-Assessment concerns at the finish of this Module.