Cooking science

Ice crystals in frozen food

Water expands when it freezes and it freezes into crystals with sharp edges. Inside food, those crystals puncture cell walls — and the size of the crystals, which depends on how fast the food froze, decides how much damage is done.

Big crystals or small ones

When water freezes slowly, a few crystals form and grow large. When it freezes quickly, many crystals form and each stays small.

Large crystals do far more damage. They puncture cell walls, and when the food thaws the cells cannot hold their contents. Liquid runs out, the structure collapses and the texture goes soft and watery.

Small crystals do much less. This is the entire reason commercial blast freezing exists and why a domestic freezer, which freezes slowly, damages food more than an industrial one.

Which foods survive freezing

Freeze well

Bread, pastry, raw meat, fish, stock, soups, stews, butter, hard cheese, most fruit for cooking, blanched vegetables.

Freeze poorly

Salad leaves, cucumber, whole tomatoes, potatoes in most forms, soft cheese, egg-based sauces, anything meant to be crisp.

Why cell-heavy foods suffer most

Lettuce and cucumber are nearly all water in cells. Rupture them and there is nothing left but liquid.

Why bread survives

Its structure is already set and largely dry. There is little cellular water to damage.

Why stews improve

Nothing in them has an intact cell structure to lose, and the flavours marry.

Why some damage is useful

Freezing tenderises squid, breaks down berries so they release juice, and softens quince and damsons for preserving.

Freezing better at home

• Freeze in small portions. A thin flat package freezes much faster than a deep block.
• Turn the freezer down before putting a lot in, if it has a fast-freeze setting.
• Do not overload it at once. A freezer full of warm food freezes everything slowly.
• Open-freeze on a tray first for berries, chips and anything you want to stay separate.
• Exclude air. Freezer burn is dehydration at the surface, not crystal damage, and air is what causes it.
• Blanch vegetables first. It deactivates enzymes that would otherwise degrade colour and flavour in the freezer.
• Thaw slowly, in the fridge. Thawing and refreezing grows crystals dramatically. How to thaw safely is a Food Standards Agency matter.

What to do with food that has already suffered

1. Accept the texture is not coming back. No technique reverses ruptured cells.
2. Use thawed soft fruit for jam, compote, smoothies or a sauce, where collapse is irrelevant or helpful.
3. Use thawed vegetables in soups, stews and purées rather than trying to roast or stir-fry them.
4. Squeeze thawed spinach or greens hard and use them in a filling or a gratin.
5. Drain thawed mince thoroughly and expect it to release more liquid in the pan. Brown it in batches.
6. For anything watery, drive the liquid off deliberately in a hot dry pan before adding anything else.

Common questions