Ice crystals and texture

What the crystals do

Cells are bags of water held in walls or membranes. As the food cools, water begins to freeze — and it does not freeze uniformly. Crystals nucleate at particular points and then grow, drawing more water to themselves as they do.

A growing crystal is a hard, expanding, sharp-edged object inside a soft structure. It pierces membranes and ruptures cell walls. When the food later thaws, those cells cannot hold their contents, and the water that was inside them runs out — the "drip loss" in the bottom of a packet of thawed mince, and the puddle under a thawed strawberry.

The food has not lost anything chemically. It has lost the structure that was holding its water in place, and that structure does not come back.

Why speed is the whole variable

Two things happen when water freezes: crystals nucleate, and crystals grow. How the balance falls between them decides everything.

Cool food quickly through the freezing range and a great many nucleation sites form at once. The available water is shared between all of them, so you get an enormous number of very small crystals, each doing very little damage.

Cool it slowly and few sites nucleate. Those few keep growing, drawing water from a wide area, and you end up with a small number of large crystals that tear through a great deal of structure.

This is why commercial blast freezing produces noticeably better frozen peas than a domestic freezer produces frozen anything, and why every piece of domestic advice about freezing is really advice about freezing faster: thin portions, spread out, not too much at once, in a freezer that is genuinely cold.

Recrystallisation — the slow damage

The second mechanism, and the reason long-frozen food gets worse even though nothing appears to be happening.

• Ice is not static in a freezer. Small crystals are thermodynamically less stable than large ones, and over time water migrates from small crystals to large ones. The number falls and the average size rises.
• Temperature fluctuation drives it. Every time the freezer warms slightly — a door opened, a defrost cycle, a warm item added — a little ice melts, and it refreezes onto the larger crystals rather than back where it came from.
• This is what "quality life" means. Food frozen well and then stored through months of fluctuation ends up with the large-crystal damage that fast freezing avoided.
• It is why ice cream goes grainy. Exactly the same mechanism, and it is why an ice cream that has partly thawed and been refrozen is never the same again.
• A full, cold, rarely-opened freezer is a better one. Thermal mass keeps the temperature stable, and stability is what limits it.

Why some foods survive and others do not

It comes down to how much water there is, how it is held, and whether the structure mattered in the first place.

Destroyed — lettuce, cucumber, whole tomato, strawberry to eat raw

Very high water content held in large turgid cells, and the crispness was the whole point. Nothing survives.

Barely affected — bread, pastry, butter, stock

Either low in free water, or the structure is set by fat, starch or gluten rather than by turgid cells.

Improved or unaffected — stews, braises, soups

The cells were already broken down by cooking. There is no structure left to damage.

Split — cream sauces, mayonnaise, yoghurt

Emulsions. Freezing crystallises the water out, which forces the fat and water phases apart, and they do not recombine.

Grainy — cooked potato, custard

Starch and protein networks that separate from their water when it crystallises out.

Fine if cooked afterwards — most fruit and vegetables

The texture is lost either way once you cook them, so freezing costs nothing you were going to keep.

Common questions