When your food goes from frozen to fossilised. Should you risk it?
You may have noticed that ice cubes kept in the freezer for a long time have a tendency to shrink as water is lost. This is because in dry freezer air, ice can transform directly into water vapour without first becoming a liquid, in a process known as sublimation.
Ice sublimates in an effort to equilibrate with the vapour pressure of air in the freezer. The water lost from food items during sublimation is redeposited on the food’s surface, and on refrigeration cools as ice crystals.
The moisture loss means that fruit and vegetables can become shrivelled and dry, while meats can develop a leathery texture and dark spots. The dehydration creates pockets in the food which are then open to the air, accelerating oxidation. In fatty foods, this oxidation can cause unpleasant flavours. Food with freezer burn is unpalatable, but safe to eat.
All foods are susceptible to freezer burn, but those with higher moisture levels will develop it more rapidly. To reduce the likelihood of freezer burn, you can vacuum seal food to remove the air that drives the sublimation process. Or if you don’t have a vacuum sealer, wrap the products as tightly as possible and stack them to limit the exposed surface area.
Finally, make sure you regularly rotate your freezer stock so that items are not left languishing for long periods. Some people suggest filling containers with water and leaving them open in the freezer to help maintain humidity and reduce sublimation. Self-defrosting freezers can cause more burn. They regularly melt the ice layer on refrigeration coils, but this keeps the vapour pressure low, driving sublimation.
Freezer burn is similar to the freeze-drying process that food companies use to create long-life foods for astronauts and mountaineers, as well as instant coffee and some dried fruits. The industrial process involves freezing foods, dropping the pressure using a vacuum, then removing the ice.
Read more:
- Why do ice cubes crack when you put them in water?
- How did people make ice before freezers?
- Where is the coldest part of the fridge, the bottom or the top?
Asked by: Aiden Chambers, London
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