All food should be safe and free from contamination and spoilage at all points in its journey from its source until it reaches the consumers. However, food contamination is a serious public health problem in Ethiopia, resulting in foodborne diseases that affect many people every year. Hence, awareness of potential sources of food contamination is an important component of good nutrition and good health. In this study session we are going to concentrate on food contamination by microorganisms, chemicals and physical factors.
Food may be contaminated by different microorganisms or by chemicals that can cause health problems for anyone who eats it. In Study Session 9 you will learn in detail about foodborne diseases. But first you will be introduced to the basic principles of food microbiology in this study session, and about the ways in which food becomes contaminated by different microorganisms, chemicals and physical objects. You will also learn about the causes of food spoilage and its consequences for health.
When you have studied this session, you should be able to:
8.1 Define and use correctly all of the key words printed in bold. (SAQ 8.1)
8.2 Describe the factors that contribute to bacterial growth and multiplication. (SAQ 8.2)
8.3 Explain the sources and types of food contamination. (SAQ 8.2)
8.4 Describe the main routes of food contamination. (SAQ 8.3)
8.5 List the types of food spoilage and describe the characteristics of food spoilage in different food items. (SAQ 8.4)
Infectious agents are organisms that can be passed to, and between, people in the process of infection transmission. Those that cause diseases are often referred to as pathogens (‘pathogenic’ means disease-causing). Many infectious agents (bacteria, viruses, fungi and protozoa) are microorganisms that are too small to be seen except with a microscope; the adult stages of disease-causing parasites (e.g. worms) may be seen with the naked eye, but their eggs and immature stages are microscopic. Microbiology is the science that deals with the study of microorganisms.
Although infections often result in disease, it is possible to be infected with a pathogen and still appear healthy. This is either because the disease has not yet had time to develop, or because the person’s immune system is keeping it under control. However, the infectious agent can still be passed on to others, for example by spreading into food handled by the infected person.
The majority of foodborne diseases (those caused by infectious agents transmitted to people in the food we eat) are due to bacteria, but as you will see in Study Session 9, viruses, parasites and toxins can also cause foodborne diseases.
Bacteria are the most abundant of all organisms. Bacteria are unicellular organisms (made of one cell) and are very small in size, ranging from 0.5 to 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 pronounced ‘mu’.
Bacteria reproduce asexually. This means that they don’t need a partner to reproduce, but simply divide into two, producing two new bacteria. There are pathogenic bacteria capable of causing human illness and food spoilage, but there are also beneficial species of bacteria that are essential to good health and a healthy environment. For example, beneficial bacteria live in our gut and help us digest our food; some bacteria are used to produce foods such as yoghurt and cheese; and others break down wastes in the environment.
Some bacteria are capable of forming highly resistant and endurable structures called spores. Bacterial spores are resistant to heat, freezing, drying, chemicals and other adverse environments. This means the spores can survive the normal processes of food storage and preparation. Two examples of spore-forming bacteria important in food contamination are Bacillus and Clostridium.
Temperature, humidity, oxygen and water are important for bacteria to grow and multiply. Under favourable conditions a growing bacterial population can double at regular intervals ranging from about 15 minutes to several hours. This means that the numbers of bacteria in food can increase rapidly and soon become hazardous to health, particularly if the food has a favourable temperature and water content. In the next section, we look in detail at factors that can promote or delay bacterial growth in our food.
You will learn more about food preservation methods in Study Session 10.
The growth of microorganisms in food products can be affected by extrinsic factors and intrinsic factors, as you will see below. By understanding the factors affecting the growth of microorganisms in food we can know how to keep food safe to eat. This knowledge can also help us to work out how to preserve food for longer.
Extrinsic factors are factors in the environment external to the food, which affect both the microorganisms and the food itself during processing and storage. Extrinsic factors include temperature, humidity and oxygen.
Different microorganisms grow over a wide range of temperatures. Some microorganisms like to grow in the cold, some like to grow at room temperature and others like to grow at high temperatures. This is of paramount importance in food safety, because if you know the temperature growth ranges for dangerous microorganisms it helps you to select the proper temperature for food storage to make them less able to grow and reproduce.
The humidity of the storage environment is an important factor for the growth of microorganisms at the food surfaces. If you store food in a dry atmosphere, microorganisms are less able to grow than if the food is stored in a humid (moist) environment. Therefore, dry conditions are better for food storage than moist conditions.
Many microorganisms need oxygen in order to develop and reproduce: these are called aerobic microorganisms. A good example is Escherichia coli, a faecal bacterium which grows readily on many foods. If you keep food in a low oxygen environment, aerobic bacteria cannot grow and multiply. Conversely, there are some microorganisms that grow without oxygen, called anaerobic microorganisms. An example of this is Clostridium botulinum, the bacterium causing botulism, which can survive in very low oxygen environments such as tinned foods.
Intrinsic factors exist as part of the food product itself. For example, meat has certain characteristics that may promote the growth of certain microorganisms. The following common intrinsic factors affect the growth and multiplication of microorganisms in foods.
pH is pronounced ‘pee-aitch’.
The scientific term pH is a measure of how acidic or alkaline an environment is, on a scale that has ‘neutral’ (neither acid nor alkaline) at pH7. Environments that are acidic have pH values below 7; those that are alkaline have pH values above 7. Most microorganisms grow best at close to the neutral pH value (pH 6.6 to 7.5). Only a few microorganisms grow in very acid conditions below a pH of 4.0. Bacteria grow at a fairly specific pH for each species, but fungi grow over a wider range of pH values. For example, most meats naturally have a pH of about 5.6 or above. At this pH meat is susceptible to spoilage by bacteria, moulds and yeasts; however the pH of meat can be lowered by pickling, which makes it less favourable as an environment for microorganisms to grow in.
Microorganisms need a moist environment to grow in. The water requirements of microorganisms are described in terms of water activity (represented by the symbol aW), a measure of how much water is present. The water activity of pure water is aW = 1.00. Most foodborne pathogenic bacteria require aW to be greater than 0.9 for growth and multiplication; however, Staphylococcus aureus may grow with aW as low as 0.86. But even Staphylococcus aureus cannot grow and multiply in drier food like bread, which has aW = 0.7, although fungi can (Figure 8.1).
Think of some foods that store well when they are dry but become contaminated quickly when they are wet.
You may have thought of different examples: the one that we thought of is rice. When rice is dry it will store for a long time, but when it is cooked and wet it will go bad quite quickly and cause food poisoning.
In order to grow, multiply and function normally, microorganisms require a range of nutrients such as nitrogen, vitamins and minerals. Microorganisms therefore grow well on nutrient-rich foods.
The natural covering of some foods provides excellent protection against the entry and subsequent damage by spoilage organisms. Examples of such protective structures are the skin of fruits and vegetables such as tomatoes and bananas (Figure 8.2).
Bacteria are a major source of microbial contamination of food, i.e. the undesired presence in food of harmful microorganisms or the harmful substances they produce. Viruses, parasites and fungi are also able to contaminate food and cause foodborne illnesses in humans. Microorganisms can enter food through different routes.
Look at Figure 8.3, which was introduced in Study Session 1, and notice that eating contaminated food is a component of many transmission routes. The most common routes of entry are discussed below.
Microorganisms are found everywhere in our environment. Many types can be found in air and dust, and can contaminate food at any time during food preparation or when food is left uncovered (Figure 8.4). Imagine a kitchen where food is prepared and stored in rural communities, and think how easily microorganisms in the air and dust could contaminate the food.
Many microorganisms present in soil and water may contaminate foods. Microorganisms also grow on plants and can contaminate food if care is not taken to remove them by washing or inactivate them by cooking. Soil is a particularly rich source of Clostridium bacteria. Water may be contaminated by faeces. Plants may also be contaminated by faeces if untreated sewage has been used as a fertiliser.
The intestines of all humans and animals are full of microorganisms, some of which are beneficial but others are pathogenic. Bacterial pathogens such as Salmonella, Campylobacter and Escherichia coli (strain O157:H7) are common examples. Contamination of foods by faecal material is the major cause of food poisoning events. This includes indirect contamination, for example from people’s hands if they prepare food without washing their hands after visiting the latrine/toilet (see below).
Escherichia coli (abbreviated to E.coli) exists in many harmless varieties or ‘strains’, but some strains are pathogenic. The strain called E.coli O157:H7 causes a potentially fatal foodborne disease in humans.
Many foodborne microorganisms are present in healthy animals raised for food, usually in their intestines, hides, feathers, etc. Meat and poultry carcasses can be contaminated during slaughter by contact with small amounts of intestinal contents. For example, in animals slaughtered in rural communities without any safety measures, microorganisms present in the animals’ intestines can easily contaminate the meat.
Animal hides are an important source of contamination of the general environment, the hands of meat workers, and skinned meat carcasses. Hides are a primary source of E.coli O157:H7 and Salmonella species, both of which cause sickness and diarrhoea. Hides become contaminated either because the outside of the hide is dirty, or because once removed from the animal, the inside of the hide is a good breeding place for microorganisms.
Animal feeds are a source of microorganisms, especially Salmonella, which can contaminate poultry and other farm animals. The organisms in dry animal feed spread throughout the local environment and may get on to animal hides, hair and feathers, as well as on people who handle the feeds.
The term food handler can be applied to anyone who touches or handles food, and this includes people who process, transport, prepare, cook and serve food. The presence of microorganisms on the hands and outer garments of food handlers reflects the standard of hygiene in the environment and the individuals’ personal hygiene (as you learned in earlier study sessions). The microorganisms transmitted to foods by food handlers may come from the hides of animals, soil, water, dust, gastrointestinal tracts and other environmental sources. In food preparation at home, foodborne microorganisms can be introduced from the unwashed hands of people who are infected by bacteria and viruses, and who cook and serve the food to family members.
Food utensils are cutting boards, knives, spoons, bowls and other equipment used in food preparation, which may become contaminated during food processing and preparation. For example, in families where there is no access to running water, the food utensils may not be properly cleaned, stored and handled, and may become a major route of food contamination.
Cross-contamination of food is the transfer of harmful microorganisms between food items and food contact surfaces. Prepared food, utensils and surfaces may become contaminated by raw food products and microorganisms. These can be transferred from one food to another by using the same knife, cutting board or other utensil without washing it between uses. A food that is fully cooked can become re-contaminated if it touches raw foods or contaminated surfaces or utensils that contain pathogens. For example, you should never:
An unsafe temperature for food storage is a major factor in food contamination. Many microorganisms need to multiply to a very large number before enough are present in food to cause disease in someone who eats it. However, if bacteria can have warm, moist conditions and an ample supply of nutrients, one bacterium can reproduce by dividing (on average) every half an hour and can produce 17 million bacteria in 12 hours! So, if you leave lightly contaminated food out overnight, it will be highly contaminated and infectious by the next day.
Poor personal hygiene of food handlers is another major factor in food contamination. The most important contaminants of food are the microorganisms excreted with faeces from the intestinal tract of humans. These pathogens are transferred to the food from faecal matter present on the hands.
We have already mentioned failure to wash hands after visiting a toilet as a source of food contamination. Can you suggest other times when food handlers should wash their hands?
Hands should be washed before starting work on preparing food, and after touching any food, surface or equipment that may be contaminated (e.g. after handling raw meat).
Bad personal habits like scratching your hair and nose with your fingers also contributes to food contamination. Sneezing and coughing spreads contaminants and microorganisms through the air and onto uncovered food, and onto surfaces and hands that can transfer the infectious agents into food.
Foods can be damaged and also contaminated by pests. Many stored grains are lost through the damage done by pests, including termites (mist), beetles, locusts, cockroaches, flies and rodents such as rats and mice. Pests can damage and contaminate foods in various ways, such as boring into and feeding on the insides of grains, or tunnelling into stems and roots of food plants. For example, weevils cause large losses of stored grains, especially in warm and humid conditions such as in lowland areas of Ethiopia.
Pests also damage the protective skin of foods allowing microorganisms to get inside the food and causing it to rot more quickly. Pests can pollute food with their excreta, and with bodies and body fragments when they die. They also transfer microorganisms on to food while walking on it (Figure 8.6). Flies and cockroaches readily move between wastes and foods, transporting microorganisms with them as they go.
You now know that food can be contaminated from sources in the natural environment, people, food preparation surfaces and utensils, raw and uncooked food, animals, pests, and waste material. To prevent contamination, food production and preparation operations need to be carefully controlled.
Prevention of microbiological (often abbreviated to ‘microbial’) contamination is an important function in food preparation, as summarised in Box 8.1.
Food handlers should follow these strategies:
Attention also needs to be given to possible chemical contamination of food. Food can be contaminated through the misuse or mistaken handling of chemicals, including pesticides, bleach and other cleaning materials. All chemicals (detergent, disinfectant, sanitiser) used in the food preparation area should be removed before food preparation begins, to prevent any chemical contamination of the food.
Other possible sources of chemical contamination are:
Physical contaminants include stones, pieces of glass, and metal. Physical contamination can occur at any stage of the food chain: for example, stones, bones, twigs, pieces of shell or foreign objects can enter food during handling and preparation. These materials should be removed, if possible, for example by sieving or picking out the items with clean fingers.
Food spoilage is the process of change in the physical and chemical properties of the food so that it becomes unfit for consumption. Food spoilage is any undesirable change in food. Most natural foods have a limited life: for example, fish, meat, milk and bread are perishable foods, which means they have a short storage life and they easily spoil. Other foods also decompose eventually, even though they keep for a considerably longer time. The main cause of food spoilage is invasion by microorganisms such as fungi and bacteria.
Microbial spoilage is caused by microorganisms like fungi (moulds, yeasts) and bacteria. They spoil food by growing in it and producing substances that change the colour, texture and odour of the food. Eventually the food will be unfit for human consumption.
When food is covered with a furry growth and becomes soft and smells bad, the spoilage is caused by the growth of moulds and yeasts (look back at Figure 8.1). Microbial spoilage by moulds and yeasts includes souring of milk, growth of mould on bread and rotting of fruit and vegetables. These organisms are rarely harmful to humans, but bacterial contamination is often more dangerous because the food does not always look bad, even if it is severely infected. When microorganisms get access to food, they utilise the nutrients found in it and their numbers rapidly increase. They change the food’s flavour and synthesise new compounds that can be harmful to humans. Food spoilage directly affects the colour, taste, odour and consistency or texture of food, and it may become dangerous to eat. The presence of a bad odour or smell coming from food is an indication that it may be unsafe. But 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 with microorganisms or substances and eating it could result in foodborne disease. Food spoilage is any undesired change in the natural colour, taste or texture of food items that makes it unfit for consumption because it has lost its quality and nutritional value.
The term contact spoilage is used when microbial spoilage is the result of direct contact or touching between the food and any contaminated or unclean surface such as shelves, food preparation boards or unwashed hands. It also includes food-to-food contact, for example between cooked meat and raw meat or between rotting fruit and sound fruit.
Physical spoilage is due to physical damage to food during harvesting, processing or distribution. The damage increases the chance of chemical or microbial spoilage and contamination because the protective outer layer of the food is bruised or broken and microorganisms can enter the foodstuff more easily. For example you may have noticed that when an apple skin is damaged, the apple rots more quickly.
Chemical reactions in food are responsible for changes in the colour and flavour of foods during processing and storage. Foods are of best quality when they are fresh, but after fruits and vegetables are harvested, or animals are slaughtered, chemical changes begin automatically within the foods and lead to deterioration in quality. Fats break down and become rancid (smell bad), and naturally-occurring enzymes promote major chemical changes in foods as they age.
Every living organism uses specialised proteins called enzymes to drive the chemical reactions in its cells. After death, enzymes play a role in the decomposition of once-living tissue, in a process called autolysis (self-destruction) or enzymic spoilage. For example, some enzymes in a tomato help it to ripen, but other enzymes cause it to decay (Figure 8.8). Once enzymic spoilage is under way, it produces damage to the tomato skin, so moulds can begin to can attack it as well, speeding the process of decay.
When the cells of fruits and vegetables such as apples, potatoes, bananas and avocado are cut and exposed to the air, enzymes present in the cells bring about a chemical reaction in which colourless compounds are converted into brown-coloured compounds. This is called enzymic browning. If the food is cooked very soon after cutting, the enzymes are destroyed by heat and the browning does not occur. For example, apples are prone to discolouration if cut open when raw, but when cooked they do not go brown.
Spoiled food is generally more a problem of appearance than a problem of disease causing. In food spoilage, the changes in appearance or texture of the food, such as rottenness, softness and change in colour, taste or odour are usually obvious, whereas in contaminated food such characteristics may not be noticed. A large majority of the microorganisms responsible for food spoilage are not pathogenic to humans. However, you should advise people in your community that they should not eat food that is spoiled because it is not nutritious and may make them sick (cause vomiting).
Finally, we turn to the factors that can increase or delay the process of food spoilage. They include its water content, environmental conditions, packaging and storage.
The amount of water available in a food can be described in terms of the water activity (aw).
Can you recall the aW of pure water?
Pure water has aW = 1.0.
The water activity of most fresh foods is 0.99. This means that they have a very high water content and can support a lot of microbial growth.
Meat is traditionally dried by adding table salt to it. Can you suggest why salting enables the meat to be stored for a long time?
You will learn more about salting, smoking, refrigeration and other food preservation methods in Study Session 10.
The salt draws out water from the meat so the aW is reduced. This makes the conditions too dry for harmful microorganisms to multiply in the salted meat.
No matter whether food is fresh or processed, the rate of its deterioration or spoilage is influenced by the environment to which it is exposed. The exposure of food to oxygen, light, warmth or even small amounts of moisture can often trigger a series of damaging chemical and/or microbial reactions. Changing the environment can help to delay spoilage. For example, storing foods at low temperatures reduces spoilage because both microbial and enzymic decay is faster at higher temperatures.
Packaging is a means of safeguarding food when it is raw, or after it has been processed or prepared. It helps to protect food against harmful contaminants in the environment or conditions that promote food spoilage including light, oxygen and moisture. The type of packaging is a key factor in ensuring that the food is protected. Packaging of foods in cans, jars, cartons, plastics or paper also serves to ensure food safety if it is intact, because it provides protection against the entry of microorganisms, dust, dirt, insects, chemicals and foreign material.
In Study Session 8, you have learned that:
Now that you have completed this study session, you can assess how well you have achieved its Learning Outcomes by answering these questions. Write your answers in your Study Diary and discuss them with your Tutor at the next Study Support Meeting. You can check your answers with the Notes on the Self-Assessment Questions at the end of this Module.
Match each term with the correct definition.
Using the following two lists, match each numbered item with the correct letter.
Organisms that can be transmitted to susceptible hosts and cause disease
The entry and development or multiplication of infectious agents in the body of humans or other animals
Disease caused by pathogenic organisms or toxins transmitted to humans by food
Lives and reproduces only in the presence of oxygen
Lives and reproduces only in the absence of oxygen
Transfer of harmful microorganisms or their microscopic stages (eggs, larvae) from one source to another
You are asked by a school head teacher to explain to some students about food contamination by microorganisms. Write a plan of what you will tell them, including explaining why microorganisms are dangerous and under what conditions they grow and multiply.
Your plan should include the following points:
Suppose you went to a village for a house visit and found complaints of foodborne illness among the villagers. What possible ways of food contamination do you suspect and how will you teach the villagers about them?
You might suspect that the food handlers who prepare and serve food have been infected and have transmitted the infection through food to healthy people. Alternatively the food handlers’ personal hygiene is so poor that transfer of pathogens to food items occurs often. The food might have become contaminated with chemicals like pesticides during transport, storage and preparation, or there may have been accidental or deliberate chemical contamination of food items.
Hence you should tell them how they can prevent their foods from any contamination by avoiding cross-contamination, using correct food storage practice, frequent handwashing by food handlers, thorough cooking, separating raw food from cooked food, and avoiding coughing and sneezing over food.
Study Figure 8.9 and explain why the two peppers look different.
The two peppers look different because one has been spoiled by enzyme reactions, which have also allowed moulds to grow on the fruit. Enzymes are important to the normal functioning of living cells, but after the pepper is harvested and stored autolytic enzymes begin to spoil it.