How does food become contaminated?

How does food become contaminated?

Posted on Sep 03, 2015

How does food become contaminated? Let me count the ways…



It’s actually pretty easy for food to become contaminated, so it’s not really that surprising that at least 5 million of us become ill each year as a result of eating or drinking contaminated food. However, more worrying perhaps for those of us who are involved in producing, processing, preparing, supplying or serving food, is the sharp increase in compensation claims being submitted through personal injury lawyers from those made ill from food poisoning.


So how does food become contaminated?

Contamination is when something unpleasant or harmful ends up in food, and this can happen in one of three ways:


1. Physical contamination

This is when items such as hair, glass, plasters, dirt, insects or other foreign bodies are present in food. Physical contamination is usually visible, and can happen at any stage of the production or preparation process.

So where do physical contaminants come from?

From buildings or equipment

This includes bits of plaster or brick, flakes of paint, broken glass or tiles, screws and nails, and pieces of wire or cable. Potentially, any of these could be dangerous and might result in broken or chipped teeth, cuts to the lips, mouth or throat, choking or worse, if swallowed.

From food packaging

This includes bits of wood, glass, cardboard or polythene, as well as elastic bands, staples, string or ring pulls.

From pests and insects

Including, rats, mice, flies, maggots, rodent hairs, tails or droppings, wasps, slugs and caterpillars. Not only are they contaminants in their own right, but harmful bacteria from their bodies can also be spread through this means.

From food handlers

Including fingernails, plasters, pieces of jewellery or clothing, chewing gum, sweet wrappers, pen tops and, of course, hair (we lose about 75 of them a day; even more if our hair is greasy).


Want to see the top five grossest things found in food?


Top 5 grossest things found in food



2. Chemical contamination

This is when food contains pesticides or cleaning/other chemicals. Products like bleach are too strong to be used in food preparation areas and have the potential to contaminate. Chemical contamination can also happen when certain foods come into contact with harmful metals, through metallic containers or when soil or water is contaminated by pollution.

Sometimes this may only taint food, but other chemicals such as bleach are very harmful and can cause severe vomiting, organ damage, or even death.


How do pesticides contaminate food?

  • through pesticides and fungicides that are used on a farm
  • through additives to animal feeds that may be present in animal and poultry meat
  • through plants and animals exposed to contaminated water or soil, that later becomes food.


How do cleaning chemicals contaminate food?

Food can become contaminated if cleaning chemicals aren’t used properly or if the wrong chemicals are used. Not all cleaning chemicals are safe to use in food areas – eg, bleach is too strong. Contamination happens when food comes into contact with equipment, work surfaces or food containers that have been treated with these unsuitable chemicals.


How does metal contaminate food?

Chemicals from metals can also contaminate food. Some metals such as cadmium (sometimes used in kitchen equipment such as fridges and cookers) and zinc (which is used for galvanising), are harmful and must not come into contact with food. Tin cans can also be a hazard as they are made of iron, coated with tin plate. If unlined, acidic foods such as fruit (or tomatoes) can react with the tin, and be absorbed by the food. This food is harmful and must not be eaten. Once canned foods are open, the contents need to be transferred to another container. Food left in open tin cans starts to react and will end up being tainted and tasting metallic.


3. Biological contamination

This is when food becomes contaminated by bacteria or a virus, parasites or mould. In food safety terms, there are two types of harmful bacteria: pathogenic (causes food poisoning) and spoilage (causes food to rot, decompose and perish). Basically, germs can infect food on the farm, in the kitchen, and just about anywhere in between, if adequate safety precautions aren’t taken, so let’s look at how raw food becomes contaminated before we bacteria-laden humans


Raw meat and Dairy

Many germs that cause illness in humans live in the intestines of healthy animals raised for food – including E Coli for example. Typically, this bacteria remains in the discarded portions of the animal, though occasionally meat can become contaminated with this bacteria if it comes into contact with just tiny amounts of the animal’s intestinal contents.

Equally worrying, following a year-long survey of supermarket chickens, a report published at the beginning of 2015, revealed that almost 73% of them are infected with campylobacter – the largest cause of food poisoning in the UK. Not surprisingly, eggs, raw milk and cheese are often contaminated too.


Fresh produce and fish

Eating fruits and vegetables are any essential part of a healthy diet, but they too can be contaminated before they even hit the shelves. There have been many cases when produce - from spinach to raspberries – became contaminated with bacteria, viruses or parasites that are usually found in the intestines of animals.


So how does this happen?

There are many ways that produce can become infected. It is important to remember that fruits and vegetables are grown in soil around farm animals, wild animals, birds, and fowl. Animal waste may be deposited in water that then irrigates the crop, or in the field itself. 

Water may be used to wash the produce might not be safe to consume, or improper sanitation procedures used in the field may also lead to contamination. Sadly, organic fruit and vegetables are just as likely to be contaminated with germs as standard produce. Fish too can become contaminated by bacteria from the ocean water they live in.

Finally, a word on parasites. Parasites are organisms that lives in another organism – the host – and often harm it. 

Numerous parasites, including roundworms, flukes and tapeworms, can be transmitted through food, particularly:

  • undercooked fish, crabs and mollusks, wild salmon and sea trout
  • undercooked meat
  • raw aquatic plants such as watercress
  • raw vegetables contaminated by human or animal waste.

Contamination can, and does occur at any stage of the food journey – whether that be during preparation, storage, cooking, hot holding or reheating – though some stages are more perilous than others. Similarly, there are a number of foods that are more susceptible to becoming contaminated, and these are referred to as high-risk foods.

The problem with pathogenic bacteria is that you can’t see, taste, feel or smell it, which makes it really difficult to detect, and even worse, it grows – at alarming rates, and therein lies the biggest problem.

In order to grow, bacteria require four things: food, moisture, time and warmth. 

Although room temperature (approximately 21°C) is ideal, these bacteria can grow rapidly, multiplying every 10 to 20 minutes, at anywhere between 5°C and 63°C. This temperature range is called ‘the danger zone’ – you may be able to remember it by thinking it’s between the age of starting school (5) and pension age (63 ish).

danger zone



Putting that into context, just one single cell of bacteria at breakfast time left in the danger zone has grown to become one million by teatime.  This means that food left for any length of time in the ‘danger zone’ can quickly become dangerous to eat.  If you’re not careful, bacteria can multiply on food whenever you:

  • defrost it
  • prepare it or handle it
  • cook it 
  • cool it
  • keep it ‘hot’ 
  • reheat it.


The most common ways that food is contaminated during the preparation process are:

storing or displaying ready-to-eat foods at room temperature for longer than two hours

Removing food from the fridge to prepare it – whether it’s a Master Chef meal you’re making or a beef sandwich, exposes food to the danger zone, and so the food becomes warmer and reaches room temperature, bacteria will start to multiply rapidly.


Remember that barbecue when your chicken drumstick was burnt on the outside but pink and raw inside? Whilst most bacteria is killed when subjected to temperatures of at least 70°C for at least two minutes, food only becomes safe if every part of it is subjected to 70°C, all the way through. If not, bacteria can survive, and once your burgers are served they’ll start to party.

insufficient re-heating

Not reheating food for long enough at a high enough temperature is also a common cause of food poisoning. Toxins produced by certain bacteria aren’t necessarily killed by reheating either. Microwaves can be dodgy too, as they don’t tend to cook things evenly and often leave ‘pockets’ of food that may not have reached a sufficient temperature.

inadequate hot holding

Keeping food hot once it’s cooked for a number of hours during service time can cause problems, as hot food counters don’t always keep food safe at temperatures over 63°C. Just as in cooling, spores can germinate and any bacteria not killed during cooking can also reactivate and multiply rapidly. 

cooling or defrosting at room temperature for longer than two hours

Removing high-risk food from the oven and allowing it to cool naturally over several hours is risky. Any bacteria not killed in the cooking process, and those that survived by forming spores (protective coats, like a shell), will soon reactivate and start to get busy, and by the time it’s cool enough to refrigerate, or serve, you’ve got a problem. 

If the item is to be cooled (and stored for later consumption), time is critical.

poor standards of personal hygiene and infected food handlers

Humans are great vehicles for contamination. In fact we’re walking food safety hazards. We’re really good at spreading bacteria from our bodies around, and because we’re always touching and prodding and picking at stuff we’re also masters of cross contamination too.

Here are just a few of the ways we do it: 

  • dipping our fingers in to taste (especially when we haven’t washed our hands after licking our fingers to separate food bags or wrap)
  • drying our hands on our overalls or on cleaning cloths
  • coughing, sneezing, spluttering and spitting
  • munching or smoking in food areas


Cross contamination

Think of it as speed dating for germs. 

Cross contamination happens when harmful bacteria from one food or kitchen surface, equipment, or hands is transferred to another food. It is especially dangerous if spread on to ready-to-eat food that won’t undergo any further treatment (such as cooking) that would usually kill the bacteria and make the food safe.

The correct use of chopping boards


food hygiene training


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