Freezer burn is water leaving as vapour without ever melting

Ice evaporates, and a freezer is a drying environment

Solids with a vapour pressure lose molecules to the air above them without passing through a liquid stage, and ice is one of them. That process is sublimation, and it is why washing hung outside in a hard frost eventually dries, and why old ice cubes shrink and go cloudy in a tray nobody touches. A freezer, for all that it is full of frozen water, is a remarkably dry place as far as any exposed surface is concerned.

This is a different failure from the coarsening that happens when a tub warms and refreezes. Recrystallisation moves water around inside the product and leaves the total amount unchanged. Sublimation removes it altogether, and the water that leaves doesn’t come back. That is why freezer burn cannot be reversed by any storage improvement, and why the affected part of a dessert is permanently changed rather than temporarily out of condition.

The driving force is a difference in vapour pressure

Water moves from where its vapour pressure is higher to where it is lower, and vapour pressure rises with temperature. The compressor cycling on and off keeps parts of a freezer slightly warmer than others, and the coldest surface in the cabinet — usually the evaporator coil or the back wall — is where the water ends up. The frost that builds up there is, quite literally, moisture that has left the food.

Every temperature fluctuation drives another round of this. A door opened, a warm item added, a defrost cycle running: each warms the product surface slightly, raises its vapour pressure, and sends more water towards the cold spot. A freezer that is opened often loses more moisture from its contents than one that is left alone, independent of the average temperature it holds.

What is lost is texture and aroma rather than safety

Freezer-burned food isn’t unsafe. Nothing has grown in it, because nothing can at those temperatures, and the change is entirely physical. What has happened is that the surface has dried into a porous, cavity-riddled layer, and those cavities then fill with air, which is why the affected patch looks pale and feels rough. The colour change is partly optical and partly oxidation, since a dry porous surface offers a great deal of area for oxygen to work on.

In a frozen dessert the aroma loss is often the more noticeable problem. Volatile compounds leave along with the water vapour, so a burned tub reads as flat and slightly stale even where the texture is acceptable. Cutting away the affected layer is a reasonable response, and it is the one professional kitchens use, since the interior is generally unaffected.

Packaging is the only defence that actually works

Because the mechanism is mass transfer from a surface, the fix is to stop the surface being exposed. A material with low permeability to water vapour, in close contact with the food and with as little trapped air as possible, prevents the whole thing. Air trapped inside the package is the weak point: the water simply moves from the food to the coldest part of the package interior, which is why an ice cream tub with a large headspace grows a layer of frost on the underside of its lid.

Pressing a sheet of film directly onto the surface of ice cream before the lid goes on is the domestic version of this, and it works well. Filling a container rather than half-filling it works for the same reason. Colder storage helps too, though less than people expect, since a colder cabinet lowers vapour pressure but also tends to run a more aggressive defrost cycle. Barrier first, temperature second.

The same process run deliberately produces freeze-dried fruit

Sublimation is not only a nuisance. Push the pressure down far enough and ice will sublimate briskly at temperatures well below freezing, which is the basis of freeze-drying: freeze the material solid, hold it under vacuum, and let the water leave as vapour without the structure ever collapsing into a liquid phase. Because nothing melts, the cell structure and the shape survive, and the result is a light, porous solid.

This is why freeze-dried raspberries in a bar of chocolate look like raspberries and behave like meringue, and why they are so intensely flavoured — the water has gone and everything else has stayed. It is also why they must be kept sealed, since a freeze-dried fruit is powerfully hygroscopic and will take water back out of the air as readily as it gave it up. The same physics, running in the same direction, is a defect in a freezer and a technique in a factory.