How Bacteria Grow in Food and How to Control Them

Bacteria can multiply very quickly when the right conditions are present. By understanding what bacteria need to survive and grow, food handlers can take practical steps to reduce the risk of food poisoning and keep food safe.

What Conditions Do Bacteria Need to Grow?

The main factors affecting bacterial growth include:

• Temperature
• Time
• Moisture
• Food and nutrients
• Acidity
• Oxygen, depending on the type of bacteria

Controlling these conditions can slow bacterial growth or prevent bacteria from multiplying.

How Temperature Affects Bacterial Growth

Temperature is one of the most important factors affecting the growth of bacteria in food. Many harmful bacteria grow particularly well at temperatures close to normal body temperature, which is approximately 37°C.

However, bacteria can multiply across a much wider range of temperatures.

The Food Temperature Danger Zone

In food safety, the temperature danger zone is generally considered to be between 5°C and 63°C. Within this range, many harmful bacteria can multiply quickly, increasing the risk of food poisoning.

Potentially hazardous food should be kept outside the temperature danger zone whenever possible.

Keeping Chilled Food Safe

Refrigeration is an important method of controlling bacterial growth. Chilled food should generally be kept at 5°C or below, although the precise storage requirements may depend on the food and the food business’s procedures.

In England, Wales and Northern Ireland, food that requires temperature control must normally be kept at 8°C or below. Setting refrigerators at 5°C or below provides a useful safety margin.

Refrigeration does not usually kill bacteria. Instead, it slows their growth significantly. Fridge temperatures must therefore be checked, recorded where required and monitored regularly.

Safe Refrigerated Storage

To keep refrigerated food safe:

• Check fridge temperatures regularly
• Avoid overfilling refrigerators
• Allow cold air to circulate around food
• Keep refrigerator doors closed whenever possible
• Store raw and ready-to-eat foods separately
• Follow use-by dates and stock-rotation procedures

How Freezing Controls Bacterial Growth

Freezing is another effective way to control bacterial growth. Food should generally be stored at -18°C or below.

At freezing temperatures, bacteria will normally stop multiplying. However, freezing does not necessarily kill harmful bacteria. Some bacteria can survive and begin multiplying again when the food defrosts.

Food must therefore be defrosted safely and used within the appropriate time. Always follow the manufacturer’s instructions and the food business’s procedures.

How Cooking Destroys Harmful Bacteria

Cooking food thoroughly is essential because heat can destroy many harmful bacteria. In general, the higher the temperature, the less time is required to reduce bacteria to a safe level.

As a commonly used food safety control, food should reach a core temperature of at least 75°C for 30 seconds, or an equivalent time and temperature combination.

Alternative equivalent combinations include:

• 70°C for 2 minutes
• 65°C for 10 minutes
• 60°C for 45 minutes

The correct cooking standard will depend on the food, the cooking process and the food business’s documented food safety procedures.

Checking Core Food Temperatures

When checking food with a temperature probe:

• Insert the probe into the thickest or coldest part of the food
• Avoid touching bones, trays or cooking containers
• Clean and disinfect the probe before and after use
• Take more than one reading when necessary
• Record the result if required by workplace procedures

Bacterial Spores and Toxins

Some bacteria can produce spores or toxins, creating additional food safety risks.

Bacterial spores can survive conditions that would normally kill active bacteria. They may become active and begin multiplying when suitable conditions return.

Certain bacterial toxins may also remain in food after cooking. This means that reheating contaminated food may not always make it safe.

Correct storage, hygiene, temperature control and food handling are therefore just as important as thorough cooking.

Using Preservation to Control Bacteria

Food can be preserved by changing the conditions that bacteria need to grow. Common preservation methods include:

• Adding salt
• Adding sugar
• Using vinegar or other food-safe acids
• Pickling
• Brining
• Drying
• Chilling
• Freezing

These methods can make food less suitable for bacterial growth. However, the preservation method must be suitable for the food and carried out correctly.

Brining Food

Brining is a traditional preservation method in which food is placed in a saltwater solution. The concentration of salt and the length of storage must be carefully controlled to ensure that the process is effective and safe.

Why Moisture Encourages Bacterial Growth

Bacteria need available moisture to survive and multiply. Removing moisture by drying food can greatly reduce bacterial growth.

Dried foods such as uncooked rice, dried pasta and powdered products are generally less likely to support bacterial growth while they remain dry.

However, some bacteria and bacterial spores can remain dormant in dry food. They may become active when water or other moisture is added. Rehydrated or cooked foods must therefore be handled and stored safely.

What Are High-Risk Foods?

High-risk foods are usually ready-to-eat foods that provide suitable conditions for harmful bacteria to grow. They often contain high levels of moisture and protein and may not receive any further cooking before being eaten.

Examples of high-risk foods include:

• Cooked meat and poultry
• Pâté
• Gravy, soups and stews
• Milk and cream
• Custard
• Soft cheeses
• Eggs and egg products
• Cooked rice
• Cooked seafood and shellfish
• Prawns, crab and oysters
• Prepared sandwiches and filled rolls

High-risk foods must be stored, handled, prepared and cooked correctly to reduce the risk of food poisoning.

Food Safety Risks from Raw Food

Raw meat, poultry, eggs, seafood and unwashed produce can carry harmful bacteria, viruses or parasites. Although some raw foods will be cooked before being eaten, they can still contaminate hands, equipment, surfaces and ready-to-eat food.

For example, harmful bacteria from raw meat or poultry can transfer to cooked food if the same chopping board, knife or utensils are used without proper cleaning and disinfection.

How to Prevent Cross-Contamination

To reduce the risk of cross-contamination:

• Keep raw food separate from cooked and ready-to-eat food
• Store raw meat and poultry below ready-to-eat food
• Use separate equipment and utensils where appropriate
• Wash hands thoroughly after handling raw food
• Clean and disinfect food-contact surfaces correctly
• Use suitable food storage containers
• Follow the food business’s colour-coded equipment system

Food Safety Risks from Ready-to-Eat Food

Ready-to-eat foods include foods that will not be cooked or reheated before they are eaten. Examples include:

• Prepared fruit
• Salads
• Raw vegetables
• Sandwiches
• Cooked meats
• Cheese
• Prepared desserts

These foods can present a particular risk because there may be no further cooking stage to destroy harmful microorganisms.

Fresh produce can sometimes carry bacteria, viruses or parasites. Where appropriate, fruit, salad and vegetables should be washed using clean drinking-quality water and handled in a way that prevents further contamination.

Using Stock Rotation to Keep Food Safe

Effective stock rotation helps ensure that older food is used before newer stock. A commonly used system is first in, first out, known as FIFO.

Food handlers should:

• Check use-by and best-before dates
• Place newer stock behind older stock
• Label opened or prepared food correctly
• Follow the food business’s storage-time limits
• Dispose of food that is no longer safe

Controlling Bacterial Growth in Food

Controlling bacterial growth requires good food hygiene, effective temperature control, correct storage and the prevention of cross-contamination.

By understanding how bacteria survive and multiply, food handlers can take practical steps to keep food safe, prevent food poisoning and protect public health.