Microbiological hazards, mainly bacteria, are the most common cause of food poisoning. They are far too small to see, so you cannot tell by looking, smelling or tasting whether food is contaminated.

In this module, you will learn how bacteria grow, which foods are most at risk, the main causes of food poisoning, and how to control bacteria with temperature, time and good hygiene.

Microbiological contamination is caused by tiny living organisms, including:

• Bacteria.
• Viruses.
• Moulds and yeasts.
• Parasites.

They come from many sources, such as raw food, soil, water, air, pests, waste and people. Many microorganisms are harmless, or even useful, but some cause illness or make food spoil.

High standards of food safety are what prevent outbreaks of food poisoning, and they rest on six essentials. First, food must be protected from contamination risks at every stage, from delivery to sale. Second, bacteria that are already present must be prevented from multiplying to levels that could cause illness or spoilage, mainly by controlling temperature and time. Third, food must be properly cooked or processed to eliminate harmful bacteria.

Fourth, food that is unfit or contaminated must be disposed of safely, so that it cannot find its way back onto the shelves. Fifth, use by dates must be checked regularly and stock rotated effectively. And sixth, thorough cleaning and disinfection must keep the environment safe for storing, preparing and displaying food. Each of these practices is explained in detail in this course. Together they form a chain, and a weakness in any one link can undo the others.

Investigations usually find that an outbreak had more than one cause. Common causes include:

• Hot or cold food held at the wrong temperature.
• Food cooled too slowly before refrigeration.
• Food undercooked, or not reheated thoroughly.
• Frozen poultry not fully thawed before cooking.
• Cross contamination from raw to ready-to-eat food.
• Food from unreliable suppliers.
• Food handlers who are ill, or have poor hygiene.

Food poisoning can start within hours of eating contaminated food, or sometimes weeks later. It usually lasts from one to seven days. Symptoms include diarrhoea, stomach pain, vomiting, nausea, fever, and dehydration.

Anyone can get food poisoning, but it is most dangerous for:

• Babies and young children.
• Older people.
• People who are ill, or recovering from illness.
• Pregnant women and their unborn babies.

Bacteria are found everywhere: in soil, water and air, on animals, and on people. Most are harmless, and some are used to make foods such as cheese and yoghurt. Two groups matter in food safety:

• Spoilage bacteria, which make food look, smell or taste bad.
• Pathogenic bacteria, which cause illness. Food containing them usually looks, smells and tastes normal.

Only laboratory testing can show that pathogenic bacteria are present, so we must prevent them getting into food and stop them multiplying.

Bacteria need four things to multiply: food, moisture, warmth and time. Remove any one of these, and growth slows or stops.

In the right conditions, bacteria multiply by splitting in two, as often as every 10 to 20 minutes. So 200 bacteria can become 400 in ten minutes, 800 in twenty minutes, and 1,600 in thirty minutes. A small number can reach a dangerous level in just a few hours.

The temperature danger zone is between 5 °C and 63 °C. This is the range in which bacteria multiply most quickly.

The ideal temperature for bacteria to grow is 37 °C, which is human body temperature.

To stop bacteria multiplying, keep cold food at 5 °C or below, and hot food at 63 °C or above. Keep high-risk food out of the danger zone as much as possible.

• Chill high-risk food at 5 °C or below.
• Cook food to a core temperature of 75 °C, or an equivalent, such as 70 °C for two minutes.
• Hot-hold food at 63 °C or above.
• Cool cooked food as quickly as possible, and put it in the fridge within two hours.
• Store frozen food at -18 °C or below.

Cold does not kill most bacteria; it only slows or stops their growth. Thorough cooking is what destroys them.

Some bacteria protect themselves by forming spores, which can survive cooking, drying and disinfection. When conditions improve, the spores release bacteria that start to multiply again.

Some bacteria also produce toxins, or poisons, that ordinary cooking will not destroy. Examples include Bacillus cereus in cooked rice, Staphylococcus aureus from people’s hands, and Clostridium botulinum.

This is why quick cooling, correct hot-holding and cold storage are so important.

Spores are one of the reasons food safety cannot rely on cooking alone. Some pathogenic bacteria can form spores, a tough protective state that lets them survive harsh conditions such as high temperatures, drying and disinfection. Some spores can even withstand boiling for up to five hours before they are destroyed. While the bacteria are inside the spore, they cannot multiply and so cannot cause illness.

The danger comes later. When conditions outside the spore become favourable, for example when cooked food is left to cool slowly at room temperature, the spore releases the bacteria and they begin to multiply. To stop this happening, keep hot food at 63 °C or above, cool food quickly, and store cold food at 5 °C or below. Most spores stay inactive at these temperatures. Remember too that some bacteria, such as Staphylococcus aureus and Bacillus cereus, produce toxins that normal cooking does not destroy.

• Campylobacter is a very common cause of food poisoning. It is mainly linked to raw and undercooked poultry, and spreads easily by cross contamination. It can also be found in red meat, unpasteurised milk and untreated water. Never wash raw chicken: splashing spreads the bacteria.
• Salmonella lives in the gut of animals and people. It is found in raw meat, poultry, eggs, unpasteurised milk and untreated water, and can reach fruit and vegetables through contaminated soil or water. Symptoms usually start 12 to 72 hours after eating.

It helps to know how these two common bacteria behave. Campylobacter symptoms typically appear within two to five days of exposure, and include stomach pain, fever, diarrhoea, nausea or vomiting. It spreads mainly through cross contamination from raw chicken, or from undercooked poultry dishes such as chicken pâté, and most cases are linked to contaminated poultry. It can also be found in red meat, unpasteurised milk and untreated water.

Washing raw chicken splashes the bacteria onto hands, surfaces, clothing and utensils, which is why it must never be done. Salmonella lives in the intestines of animals and humans and is found in faeces. During production, it can pass onto food products, and fruit and vegetables can be contaminated through contact with faeces, for example when manure is used to enrich soil. Its symptoms include diarrhoea, vomiting, stomach pain, fever and headache, usually starting 12 to 72 hours after exposure.

• E. coli O157 can cause severe stomach pain and bloody diarrhoea, and very serious illness in children. Even a small number of bacteria can cause illness. It spreads through undercooked meat such as burgers, unwashed vegetables, unpasteurised milk, and contact with infected people or animals.
• Listeria causes listeriosis. It is rare, but can be fatal for vulnerable people, and is especially dangerous in pregnancy. Unlike most bacteria, it can grow slowly in the fridge. It is linked to chilled ready-to-eat foods, such as deli meats, pâté, smoked fish, soft cheeses and pre-packed sandwiches.

E. coli is commonly found in the intestines of animals and humans, and most strains are harmless. The strain E. coli O157 is different. Exposure to even a small amount can lead to illness, with severe stomach pain and bloody diarrhoea. Symptoms usually appear within three to four days and can last for up to two weeks. It often spreads through raw or undercooked meat such as burgers, through contaminated foods like unwashed vegetables or unpasteurised milk, and through contact with an infected person.

Listeria is found widely in nature and can contaminate many foods, especially ready-to-eat items that are not cooked again, such as deli meats, pâté, smoked fish, soft cheeses, unpasteurised dairy products, and pre-packed sandwiches and salads. The illness it causes, listeriosis, often begins with mild symptoms such as aches, pains, nausea and diarrhoea, but for people with weakened immune systems and for pregnant women it can be very serious.

• Bacillus cereus forms spores and toxins. It is often linked to cooked rice and other starchy foods that are cooled slowly or kept warm. One type of illness causes vomiting within a few hours, and another causes diarrhoea and cramps after about 6 to 15 hours.
• Clostridium perfringens also forms spores. It is found on raw meat and poultry, and multiplies quickly in large meat dishes, stews and gravies that are kept warm, or cooled slowly. Symptoms are stomach cramps and diarrhoea, usually within a day.

Bacillus cereus is a toxin-producing bacterium that can multiply quickly at room temperature, and it causes two different kinds of illness. In the first, the bacteria release toxins in the small intestine after the food is eaten, causing diarrhoea, cramps and sometimes nausea, usually six to fifteen hours later. Meat, milk, vegetables and fish can all be involved. In the second, the toxins are produced in the food before it is eaten, causing vomiting and nausea within thirty minutes to six hours.

Starchy foods, especially rice, are the most common culprits. Clostridium perfringens is a spore-forming bacterium found in many environments, including the intestines of people and animals. It thrives in low-oxygen conditions, such as the centre of a large pot of stew or gravy. Outbreaks typically happen when large quantities of food are kept warm or at room temperature for long periods, and symptoms of cramps and diarrhoea usually appear within a day.

Viruses can cause foodborne illness, but they cannot multiply in food. They need living cells to multiply.

• Norovirus causes vomiting and diarrhoea, and spreads very easily from infected people, contaminated surfaces and food handled by someone with the virus. Thorough hand washing with soap and water is the best protection.
• Hepatitis E is linked to undercooked pork and pork products, and to poor hygiene. Cook pork thoroughly, and keep personal hygiene high.

High-risk foods are ready-to-eat foods that support the growth of bacteria, and will not be cooked again. Examples include:

• Cooked meat and poultry, pâté, pies, stews, gravy and sauces.
• Milk, cream, soft cheese and custard.
• Shellfish, cooked rice, and products made with raw egg, such as mayonnaise.

Low-risk foods are dry, or high in sugar, salt or acid, such as cereals, dried pasta, jam and canned food. Once water is added, or a can is opened, they become high-risk.

Cross contamination happens when bacteria move from raw food, or from a contaminated source, onto ready-to-eat food. It can happen:

• Directly, when raw food touches ready-to-eat food, or drips onto it.
• Indirectly, through hands, cloths, knives, boards, slicers, scales or packaging.

Keep raw and ready-to-eat food apart at every stage, use separate, colour-coded equipment, and wash your hands between tasks.

Spoiled food is unfit to eat. Unlike harmful bacteria, spoilage usually shows. Look out for:

• Sour smells, slime, or unusual textures and flavours.
• Mould, which spreads through food even if the surface is removed.
• Discolouration, gas, bulging cans, and leaking packs.

Damaged packaging can let in bacteria, and may be a sign of pests. Remove damaged items from sale at once, label them, and keep them apart from other stock until they are returned or thrown away.

Spoilage happens when conditions such as light, oxygen, heat, humidity and temperature allow bacteria, moulds and yeasts to grow, until the product becomes unfit to eat. Unlike pathogenic contamination, spoilage usually causes changes you can see, taste or smell. Spoiled food is wasted food, so preventing it saves money as well as protecting customers. Spoilage speeds up when food is handled carelessly, stored at the wrong temperature, packed in unsuitable or damaged packaging, or kept in poor hygiene conditions.

One example of illness linked to spoilage is scombrotoxic poisoning. It comes from oily fish such as tuna, mackerel, sardines, anchovies and herring that have not been stored properly, allowing high levels of histamine to build up. Normal cooking temperatures do not remove it. Symptoms appear within about thirty minutes and resemble an allergic reaction, with rashes, nausea, dizziness and a burning sensation in the mouth. Keeping fish chilled at all times, from delivery to sale, helps prevent it.

Preservation, such as canning, pasteurising, freezing, drying, pickling, smoking and vacuum packing, slows or stops bacteria. But vacuum-packed, smoked and pasteurised foods usually still need refrigeration, and opened preserved food must be treated like fresh food.

Some hazards come from natural toxins. Oily fish such as mackerel or tuna that is not kept chilled can build up histamine, which cooking does not destroy. Throw away mouldy, bruised or damaged produce, and green or sprouting potatoes.

Food preservation extends shelf life and limits the growth of spoilage and pathogenic bacteria. Common methods include high temperatures, as in pasteurisation, UHT treatment, cooking, canning and bottling, which destroy bacteria; low temperatures, such as freezing, which slow their growth; removing moisture by dehydration; adding preservatives such as benzoates, nitrites and sulphites; acid fermentation, as in pickling; and smoking, especially for meat and fish. Vacuum packing and modified atmosphere packaging limit moisture and oxygen to slow spoilage, but vacuum-packed food must still be refrigerated, because some bacteria thrive without oxygen.

Once a can or jar is opened, its contents must be treated like fresh food. As a food handler, you also have responsibilities of your own. You must make sure spoiled or unfit preserved food is not used or sold, know where each preserved product should be stored, store it correctly, and package it properly if it is produced in your shop.

Natural toxins are compounds produced by living organisms. They are harmless to the organism that makes them, but can harm people who eat them. Toxins arise in several ways. Plants may produce them as a defence against predators or pests. Moulds and other microorganisms may produce them when they infest food. And fish or shellfish may take in toxin-producing organisms, such as certain algae, which are safe for the fish but harmful to humans.

Several natural toxins are known risks, including aquatic biotoxins in shellfish, lectins in some raw beans, mycotoxins from moulds, and the toxins in poisonous mushrooms. To reduce the risk, never assume that food is safe simply because it is natural. Discard bruised, damaged or discoloured produce, especially if it is mouldy, and throw away anything that smells or tastes wrong. Only sell or use mushrooms and wild plants that have been reliably identified as safe.