Last updated on 8 June 2012
Lorna Rowswell, from the hygiene and microbiology team at the Food Standards Agency, explains how freezing keeps food safe.
Freezing is good because it kills off germs doesn’t it?
Most types of bacteria survive freezing, but they do become inactive while the product is frozen.
No, this is a common misunderstanding. Most types of bacteria survive freezing, but they do become inactive while the product is frozen. This is because of the low temperature and lack of available water. Frozen food will, therefore, keep indefinitely, although the taste or texture of the food can deteriorate over time due to ice crystal damage.
When the food starts defrosting, its core temperature rises and water becomes available, providing the ideal conditions for bacteria to grow. And, as a rule of thumb, the warmer the temperature the more active bacteria are, which is why we recommend defrosting food slowly and safely, preferably overnight in the refrigerator. This way there is less risk of harmful bacteria growing in the food.
Once defrosted, food will spoil in the same way as if it were fresh, so handle defrosted foods in the same way as you would raw.
If freezing keeps food safe, why is it bad to refreeze thawed food?
Refrozen food has a higher risk of causing food poisoning because, as explained, when food is thawed, bacteria can multiply rapidly – particularly if at room temperature. So if the product is refrozen these bacteria will survive, and when defrosted again, it is more likely that the bacteria would reach harmful levels and cause food poisoning.
However, if you cook defrosted food before refreezing (for example turning defrosted chicken into a chicken curry), any harmful bugs will be killed off, making the food safe before freezing. Any defrosted food can be stored in the fridge for up to two days before it needs to be cooked or thrown away.