How does freezing preserve food?
When frozen, many changes in food which lead to it spoiling are prevented or slowed down considerably. For example, bacteria and moulds cannot grow in the cold of the freezer. When food is frozen, much of the water is 'locked up' as ice which means that many other chemical changes in food are also slowed down.
How long can we keep frozen food for?
This depends upon the type of food and the storage temperature. Most foods obey the rule the colder the better. Domestic freezers have a star rating indicating the temperature they are designed to operate at.
On the packaging of a frozen food, there is guidance stating how long the food can be stored using freezers of a given star rating.
As a guide here are some suggested maximum storage times at -18°C for a variety of foods:
|Product||Practical Storage Life (in months)|
|Corn on the cob||12|
|Raw meat and meat products|
|Beef joints, Steaks||12|
|Lamb joints, chops||10|
|Pork joints, chops||6|
|Fish and Shellfish|
|Oily fish (herring, salmon, mackerel, etc)||4|
|White fish, cod, haddock, etc||8|
|Flat fish, sole, plaice, etc||10|
|Prawns, lobster, crab||6|
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Does that mean that frozen food stored for longer is dangerous?
No. Bacteria - some of which cause food poisoning - cannot grow at freezer temperatures, so the storage life reflects the length of time that the product is likely to remain in good condition. The products are usually labelled 'Best Before' rather than 'Use By'. Eventually, even well-frozen food will deteriorate and become unpalatable while still remaining safe to eat.
Is thawing before cooking necessary?
Always follow the manufacturers' cooking guidelines. The purpose of thawing foods thoroughly before cooking is to ensure that during cooking the food is heated sufficiently to kill harmful bacteria. If the food is large (e.g. a whole chicken or beef joint) and not defrosted prior to cooking, it will remain uncooked in the centre while being burnt or overcooked on the surface. Many ready-prepared foods can be safely cooked directly from the frozen state because the manufacturers have designed the cooking method to ensure that the food is properly cooked.
Can thawed foods be frozen again?
Refreezing thawed foods is not advisable from either a safety or quality point of view. Retail packed frozen foods must be labelled 'Do not refreeze after thawing'. The main reason is to ensure that some consumers don't use inappropriate thawing methods, e.g. thawing at room temperature for too long a time, or letting the thawed food get too warm before refreezing is started.
What is the storage life of thawed foods?
Thawed foods should be treated as carefully as chilled foods, i.e. kept in the refrigerator. Care should be taken to ensure that juices which are released during and after thawing do not drip onto refrigerator surfaces. Store thawed meats covered on a tray at the bottom of a fridge for no more than 24 hours unless manufacturers advise to the contrary.
Is it better to freeze fresh food at home or buy commercially-frozen products?
Commercially-frozen products will retain their eating and nutritional qualities better than home frozen products. There are several reasons for this.
Commercial freezers blow cold air over foods so they freeze food more rapidly than is possible in a domestic freezer. Generally, this results in less damage to the structure of the food, leading to reduced amounts of water being lost on thawing.
Vitamin content is often higher because the moment a vegetable is picked, or a food is cooked, changes start to take place. Many nutrients, such as vitamins begin to break down and are lost over time; freezing slows these changes down dramatically. Therefore the shorter the length of time before freezing, the greater the retention of vitamins. For example, garden peas lose between 10-20% of their vitamin C content within a day of picking. Peas are commercially frozen within 2 or 3 hours of harvesting.
In short, domestic freezers are designed to store frozen food, rather than freeze fresh produce.
What do producers do to ensure your frozen foods are of high quality?
Even before they are frozen, many producers take care to select vegetables that are of similar quality to those which are sold fresh. They also select varieties that freeze well. There are many strains of most common vegetables, they do not all freeze equally well. After selection and washing, vegetables are blanched in boiling water or over steam for a short time to disable natural enzymes that could otherwise cause the product to deteriorate during frozen storage.
This whole process of harvesting, washing and blanching is complete within a few hours. The vegetables are then frozen very rapidly in a freezing tunnel before being packaged. The package is not just there to contain the product. It also helps protect the food from dehydration during storage.
Long-term storage of seasonal products is done in cold stores operating at -25°C and colder to ensure that the product retains its quality. Care is taken in the distribution chain to ensure that changes are kept to a minimum, the process being rather like a relay race with the baton of responsibility being passed on from producer to distributor to retailer. The temperature history is monitored and recorded to ensure that the baton has not been dropped.
What can consumers do to keep frozen food as fresh as possible?
There are simple steps that consumers can take to ensure that frozen food is of as high a quality and nutritious value as possible.
Always buy frozen foods at the end of a shopping run and pack them together in insulated bags or boxes (available from most retailers).
Put in the freezer as soon as possible. Don't wait...Refrigerate
Purchase a fridge thermometer. These are widely available from retailers. Check the temperature of your freezer. Is it operating at -18°C or below? If it is not, turn the refrigeration unit up if possible.
For more guidance, please see our tips for freezing fish.