Food Safety and the Different Types of Food Contamination

There are many scenarios that might cause food contamination, but most are one of three categories; biological, chemical or physical.
March 16, 2016

Food contamination happens when something gets into food that shouldn’t be there. 

While there are many scenarios that might cause food contamination, most fall under one of three categories; biological, chemical or physical contamination. 

Biological Contamination

Biological contamination is when bacteria or toxins contaminate food and is a common cause of food poisoning and food spoilage.

Food poisoning can happen when harmful bacteria, also called pathogens, spread to food, and are consumed. Bacteria are small microorganisms that split and multiply very quickly. In conditions ideal for bacterial growth, one single-cell bacteria can split so many times that in just seven hours, it has multiplied into two million.

Some bacteria such as salmonella, staphylococcus and listeria are extremely toxic by themselves. And, sometimes it’s not the bacteria that are toxic to humans, but the process of the bacteria multiplying and producing waste. However, not all bacteria are harmful to humans; many are quite beneficial, such as those found in yoghurt.

As a Food Handler, it’s your job to control the spread of harmful bacteria by maintaining food safety. Bacteria can be found everywhere and are impossible to see with the naked eye. Some of the most common places for bacteria to grow are:

  • The human body
  • Dust
  • Raw meat
  • Pets and pests
  • The air
  • Kitchen cloths
  • Food Handler's clothing

How Bacteria Survive

Food: Bacteria need a constant source of food to survive, especially protein. High protein foods such as meat are particularly vulnerable to biological contamination from bacteria, which means they’re considered high-risk foods.

Water: Water is essential to bacterial growth and without it, most bacteria will die. Which is why drying foods as a way of preservation are so effective and have been performed for thousands of years.

Oxygen: Most bacteria require air to survive, these are called aerobic bacteria. Although some bacteria - called anaerobic bacteria - can survive without oxygen. Which is why it’s still possible to get food poisoning from canned food items.

PH Levels: PH refers to food acidity and is measured on a scale of 1 (acidic) to 14 (alkaline). Most fruits generally have a PH level of between 1 - 5.9, so are considered acidic. While many alkaline foods such as vegetables have a PH level at the other end of the scale. Bacteria thrive in neutral foods that are neither acidic or alkaline and generally have a PH level of between 6 - 8.9. Foods such as meat and seafood are prime examples of neutral foods.

Time and temperature: Bacteria need both time and the right temperature to multiply to dangerous levels. A temperature of between 5ºC and 60ºC -  also referred to as the ‘danger zone’ - allows for maximum bacterial growth, so it’s important not to keep food at this temperature for too long.

High-risk foods are those that have ideal conditions for bacterial growth. This means they’re usually:

  • Neutral in acidity
  • High in starch or protein
  • Moist

Foods such as seafood, cooked rice or pasta, and dairy are all considered high-risk because they provide the perfect environment for bacteria to grow. This is why it’s essential to practice proper food handling when dealing with these foods.  

Low-risk foods are those that don’t have particularly good bacterial growth conditions. These foods are:

  • High in acidity
  • High in salt or sugar
  • Dried
  • Canned or vacuum packed

Some examples of low-risk foods would be pickles, uncooked rice or pasta and jams. Although these foods are not common sources of biological contamination, the appropriate care must still be taken when handling them.

Chemical Contamination

Chemical contamination occurs when food comes into contact with chemicals and can lead to chemical food poisoning.

Some common sources of chemical contamination can include:

Kitchen cleaning agents: Proper storing of kitchen cleaning chemicals is essential. Never keep food stored in the same place as your cleaning chemicals, and always use cleaning products designed especially for kitchen use.

Unwashed fruits and vegetables: The pesticides and fungicides often used on fruits and vegetables to help them grow free from diseases are harmful if consumed. Which is why it’s vital to properly wash all fruits and vegetables before eating them.

Food containers made from non-safe plastics: Single-use items - such as plastic containers - are not designed to be reused again and again. Always store food in containers that are specially designed to safely be reused.

Pest control products: Items like fly spray and rat poison are extremely hazardous if consumed. Always store these products away from food items.

Chemicals used in equipment maintenance: Some kitchen machines and equipment with moving parts - such as slicers and mixers - can need regular oiling. Always use food-safe oil to help make sure this doesn’t contaminate the food you use them to prepare.

Physical Contamination

Physical contamination happens when actual objects contaminate foods. Sometimes when a food is physically contaminated, it can also be biologically contaminated. This is because the physical contamination might harbour dangerous bacteria, for example, a fingernail.

Common sources of physical contamination are:

Hair: Always wear hair neatly tied back and use a hair net if possible.

Glass or metal: This can occur when kitchen items are not maintained. Cracked or broken crockery and utensils should be thrown away, as well as any food that might have come into contact with it.

Pests: Pests - such as mice, rats and cockroaches - leave droppings that can contaminate food. Also, pests themselves - such as flies and insects - can also make their way into food.

Jewellery: Always keep jewellery to a minimum when preparing and handling food.

Dirt: Because dirt is so small, it’s easy not to notice that it’s contaminating your food. It usually gets into the food from unwashed food and vegetables.

Fingernails: Always keep nails short and clean to prevent contamination. Also, avoid wearing fake nails as these can fall off and may contaminate food.


Cross-contamination is the accidental transfer of contaminants from one surface or substance to another, usually as a result of improper handling procedures. In a food setting, the term refers to the transfer of contaminants from a surface, object or person to food. Cross-contamination usually refers to biological contamination but can also be physical or chemical.

This can happen in many different ways and some of the more common sources of cross-contamination include:

Clothing: Dirty clothes can transport bacteria from one place to another. If possible, clothing should be replaced when moving from one work area to another. You should also thoroughly wash your face and hands. This is especially important when working with allergens or high-risk foods.

Utensils: Separate utensils should always be used to prepare different types of foods. For example, never use the same chopping board or knife to prepare raw meat and ready-to-eat foods.

Personal hygiene: Coughing, sneezing or even touching your face and hair before handling food can also result in cross-contamination. Washing hands regularly when handling food is essential.

Pests: Flies, cockroaches, mice and rats carry harmful bacteria, which they can transport from one place to another. Pest control is vitally important in the workplace when it comes to preventing cross-contamination.

Raw food storage: One of the most common types of cross-contamination is when raw food comes into contact with cooked or ready-to-eat food. If this happens, it’s a good idea to assume the raw food has been contaminated. Raw food should always be covered and stored below cooked foods to prevent this type of contamination.

Waste control: Garbage should be stored and sealed correctly to prevent cross-contamination. It should always be stored away from other items in the kitchen to ensure it never comes into contact with food preparation. Regular cleaning and sanitising of waste bins should also be carried out to prevent the risk of pest infestation.

Preventing Food Contamination

The best way to prevent food contamination from happening is through food safety training and education. Food Handlers need to be trained in important food safety concepts and practical knowledge, such as:

  • safe cooking temperatures
  • proper storage and preparation of high-risk foods (also called potentially hazardous foods)
  • effective cleaning and sanitising techniques
  • the importance of personal hygiene and their legal responsibilities with regards to food safety

AIFS provides online food safety training for all levels of responsibility in a food business or organisation, including our online Food Handler course and Food Safety Supervisor course. Find out more information about our nationally recognised food safety courses


This public health information was produced and distributed by the Australian Institute of Food Safety Foundation.