In this article
Freezing is an effective preservation method that we have used to preserve our food for centuries. It does this by using cold temperatures to prevent the growth of microorganisms, such as yeasts, moulds and harmful bacteria, that can cause food spoilage and foodborne illnesses.
If food is not frozen or defrosted safely, it can encourage harmful bacteria to grow, which increases the risk of foodborne illness. In fact, according to many local authorities, storing and defrosting foods incorrectly are some of the main causes of food poisoning.
According to the Food Standards Agency (FSA), approximately 2.4 million cases of foodborne illness occur every year in the UK, which is up from the 2009 estimate of around one million. Globally, an estimated 600 million (almost 1 in 10 people in the world) fall ill after eating contaminated food, and 420,000 die every year (World Health Organization). The number of reported cases is thought to be significantly lower than the actual number, as not everyone with symptoms goes to their doctor when they are ill.
These statistics highlight that everyone should take the risk of food poisoning seriously. Whether you are preparing food at home or in your workplace, you can reduce the risk by following some simple food safety practices, such as safe freezing and defrosting.
This article will cover the importance of freezing food properly and defrosting it safely. It will also look at the different defrosting methods available.
The importance of defrosting food safely
There is often a misperception that when food is frozen, it is completely safe, as any germs are destroyed by freezing temperatures. Unfortunately, this is not the case. Freezing does not kill harmful bacteria in food. The bacteria remain alive but become dormant (inactive) at freezer temperatures of -18°C and below. The cold stops the bacteria from growing and producing toxins; they essentially go into hibernation.
When you take food out of the freezer or leave frozen food out for too long before putting it away, the thawing process begins, and it starts to defrost. Once this happens, the harmful bacteria in the food ‘awaken’, and they become active. As they become active, they will start to grow if the food temperature moves into the danger zone. According to the Food Standards Agency, the danger zone relates to a temperature range where most harmful bacteria will grow.
It isn’t just the temperature that influences the growth of harmful bacteria. Other factors include:
As food starts to defrost, it becomes wetter, and the moisture content increases. If the other conditions are optimal for bacterial growth and the food is left in warmer temperatures for long periods, the bacteria can quickly multiply. This will increase the risk of food poisoning.
Certain foods carry a higher risk of food poisoning, as they provide the ideal conditions for harmful bacteria to grow. For example, meat and poultry are considered to be high risk as they:
- Contain harmful bacteria due to their nature.
- Are usually moist.
- Have a high protein content.
- Require cooking before consumption.
One of the most common causes of food poisoning is not defrosting frozen food properly before cooking, particularly meat and poultry. If it is not thoroughly thawed and is still frozen or partially frozen, it can result in:
- Ice in the centre of the food.
- Longer cooking times.
- The centre of the food not being cooked when the outside appears to be done.
- Food poisoning as harmful bacteria in the centre will not be destroyed by hot enough temperatures.
To reduce the risk of food poisoning, food must be defrosted fully, and there are several methods you can use.
Safe defrosting methods
Many foods can be cooked straight from frozen without needing to be defrosted first, e.g. oven chips, vegetables and some ready meals. However, this isn’t the case for some foods, and they will need to be defrosted fully and safely before cooking. Some examples of these types of frozen foods are:
- Poultry, e.g. chicken, turkey, duck and goose.
- Meat, e.g. pork, ham, beef and lamb, and processed meat products, such as mince, sausages and burgers.
- Fish, e.g. cod, salmon and haddock fillets, and seafood.
- Some foods cooked before freezing, e.g. lasagne and chilli con carne.
The amount of time it will take for food to defrost safely will depend on the type of food, and the quantity, e.g. large joints of meat will take longer to thaw than small pieces.
Defrosting food in the refrigerator
This is the safest method of defrosting food, as it thaws at a low temperature (5°C or below) and keeps it from entering the danger zone. Defrosting food in a refrigerator prevents the growth of harmful bacteria and reduces the risk of food poisoning.
Take extra care when defrosting poultry, meat and fish due to the higher risk of food poisoning. To safely defrost these foods in a refrigerator, you should:
- Check the instructions on the packaging for the defrosting times if purchased frozen. Large whole birds and meat joints can take days to defrost, so it is important to plan and account for thawing times.
- Take it out of the packaging, remove any giblets (if present) and put it into a sufficiently sized clean dish to capture any thawing liquid. Cover the dish.
- Where possible, defrost raw poultry, meat and fish in a separate refrigerator to ready-to-eat and cooked foods.
- Put raw poultry, meat and fish below any other foods (on the bottom shelf) when being defrosted if you do not have a separate refrigerator. This will prevent the thawing liquid from dripping on foods and contaminating them with harmful bacteria.
- Poor away the thawing liquid to prevent overflowing. However, take extreme care not to contaminate other areas in the kitchen.
- Ensure the food has fully defrosted before cooking, i.e. no ice crystals present, and the texture is soft and flexible.
- Cook the food at the correct temperature and time once fully defrosted.
- Always cook defrosted poultry, meat and fish within 24 hours and keep it in the refrigerator if you are not cooking it straight away. This will keep it out of the danger zone.
- Once poultry, meat and fish have been fully defrosted, they should not be refrozen unless thoroughly cooked.
Using cold water to defrost food
If you have limited time and cannot wait for food to defrost overnight in the refrigerator, you can either submerge it in cold water or put it under running cold water to speed up the thawing process.
To safely defrost foods in cold water, you should:
- Place in a sealed bag, an airtight container or its original packaging.
- Fill a clean bowl with cold water and completely submerge the bag/container. Change the water every 30 minutes to ensure it remains cold. Never use warm or hot water, as this can encourage harmful bacteria to grow.
- Leave the food under running cold water until it has fully defrosted as an alternative to the submerged method. Never defrost raw poultry and meat directly under cold running water, as this can result in harmful bacteria splashed around the kitchen and contamination on surfaces. Always put it in a sealed container first.
- Always clean and disinfect the sink before and after defrosting food.
Using the microwave to defrost food
An even quicker way to defrost food is using the defrost setting on a microwave. You can use this method for all types of food, even poultry and meat. However, take extreme care when using a microwave to defrost high-risk foods. It is also important to remember that heat in a microwave is not distributed evenly, so you will need to monitor the food closely during defrosting.
To safely defrost food in a microwave, you should:
- Only use this method for defrosting small items that you will cook immediately after.
- Remove any packaging that is not microwave-safe, i.e. plastic wrappings.
- Place small amounts of food on a microwaveable plate (or in a container) and cover it.
- Ensure the microwave is on the correct defrost setting. If it isn’t, the food could start to cook, which could affect food safety. This can also happen if you leave it in the microwave for too long.
- Ensure the food is defrosting evenly, e.g. if it is a liquid, stir it regularly, or if it is solid, turn it.
- Check that it has fully defrosted, i.e. no ice crystals present.
- Cook the food immediately after defrosting it in the microwave.
You can use a combination of methods to speed up the defrosting process. It is important to note that some methods may affect the quality of the food, e.g. defrosting fish in the microwave can make it dry.
Defrosting food safely
Freeze food safely first
Before you even think about defrosting food, it is important to freeze it properly in the first instance. You should put frozen food into a freezer as soon as possible after purchase or delivery.
If it isn’t put away quickly, it can begin to defrost, and harmful bacteria can start to grow. Frozen food should not be allowed to defrost unless it is to be cooked or used immediately after.
- Always put frozen foods in the freezer as soon as possible to prevent thawing.
- Ensure all food stored in the freezer is properly wrapped, as it may suffer from freezer burn.
- Ensure that foods with use-by dates are not frozen after these dates.
- Divide foods into smaller portions and put them in containers or freezer bags before freezing.
- Label any non-pre-packed foods with the name and any allergens.
- Label any frozen food with the date frozen.
- Use frozen food within 3-6 months, but this will depend on the type of food.
Freezing cooked food
If you freeze any cooked food, you must ensure that it is chilled as quickly as possible to prevent it from entering the danger zone. If you don’t have a blast chiller, you can use the following methods after cooking to reduce the chilling time:
- Divide food into smaller portions and cut joints of meat in half.
- Cover pans or trays of food and place them in a cooler area or stand them in cold/iced water.
- Stir food regularly to release the heat.
- Cover and move hot food to the coolest and most ventilated place.
- Spread food, such as rice, out on a tray.
- Use a cool setting on the oven if this function is available.
Under no circumstances should hot food be put straight into the refrigerator or freezer. This can increase the temperature inside, which can:
- Make other foods warm, which can encourage bacterial growth and increase the risk of food poisoning.
- Defrost the freezer.
- Damage the appliance.
Always chill foods properly and as quickly as possible before putting them in the freezer.
Ensure the temperature of the freezer is right
The freezer temperature must also be correct for food to be frozen properly, i.e. it should be kept at or below -18°C. Check the temperatures at least once a week to ensure it is working and freezing food properly.
Defrosting dos and don’ts
- Always check the packaging for instructions on whether foods need to be defrosted fully before cooking.
- Plan ahead to ensure you leave enough time to defrost foods, as it may take a long time (several days) for larger items such as joints of meat to thaw in the refrigerator.
- Only defrost the necessary amount of food and keep the rest of it frozen. You can break up large pieces into smaller pieces, e.g. prawns.
- Where possible, use the safest method of defrosting, which is in the refrigerator overnight.
- If using other defrosting methods, such as cold water or the microwave, monitor the process closely
- When defrosting raw foods, keep them away from other foods to prevent cross-contamination.
- Once food has fully defrosted, use it immediately and definitely within one day.
- Don’t leave high-risk foods to defrost at room temperature.
- Don’t defrost food in hot/warm water.
- Don’t refreeze defrosted foods unless they have been thoroughly cooked and chilled.
- Don’t place defrosting raw foods (e.g. meat, poultry and fish) above other foods in the refrigerator.
- Don’t cook food if it has not defrosted fully, particularly high-risk foods.
Always check that food has defrosted thoroughly before cooking. The best way to do this is to check the food for any ice crystals, which indicates that it has not fully thawed out. If there are any ice crystals, continue defrosting and check again later. If you are unsure, never take the risk. You can always provide an alternative meal if you cannot guarantee safe defrosting.
Not defrosting food properly, particularly high-risk foods, is a common cause of food poisoning. Therefore, to minimise this risk, you should always ensure that food that requires freezing is frozen properly first and then defrosted safely using one of the above methods.
Wherever possible, always defrost food in the refrigerator overnight, which will keep it out of the danger zone and prevent harmful bacterial growth.