Freezing

What freezing actually does

Two things, and neither is sterilisation.

It removes available water by turning it into ice. Frozen water is not available to microbes any more than bound water is, which is why freezing belongs in the same family as salting and drying rather than in the same family as cooking.

It slows everything down. Enzyme activity, oxidation and chemical change do not stop entirely at domestic freezer temperatures; they become very slow. That is why frozen food has a quality life rather than an indefinite one: it is not going off, it is gradually getting worse.

Crucially, the microbes are still there and still viable. They resume when the food thaws, from wherever they had got to. Freezing preserves the microbiological state of the food at the moment it went in — which means freezing something that was already on the turn preserves that, too.

Why texture suffers, and what to do about it

Water expands when it freezes, and it forms crystals. Those crystals puncture cell walls. When the food thaws, the punctured cells cannot hold their water any more and it runs out — which is why thawed strawberries collapse, thawed lettuce is inedible, and thawed meat sits in a pool of liquid.

The size of the crystals is what governs the damage, and crystal size depends on how fast the food froze. Fast freezing produces many tiny crystals that do relatively little harm. Slow freezing produces fewer, larger crystals that do a great deal.

That gives four practical rules. Freeze in small or thin portions, which freeze faster than a large block. Do not overload the freezer at once, which raises its temperature and slows everything down. Keep the freezer cold and stable — repeated partial thawing lets small crystals merge into large ones, which is most of what makes long-frozen food disappointing. And choose what to freeze: high-water, delicate foods suffer most, and dense or already-cooked ones suffer least.

What freezes well, and what does not

Excellent

Bread and baked goods, raw meat and fish, stock and soup, stews and braises, butter, grated cheese, herbs in oil, pastry, berries for cooking, blanched peas and beans.

Good with care

Cooked rice and pasta if cooled fast and frozen promptly. Cakes without icing. Milk, which separates but recombines when shaken.

Poor

Salad leaves, cucumber, whole tomatoes, raw potato, soft fresh cheeses, boiled eggs, custards, anything relying on crispness.

Splits or separates

Cream sauces, mayonnaise, yoghurt, single cream, egg-thickened sauces. Emulsions break when the water crystallises out.

Fine but changed

Cooked potato in a stew goes grainy. Herbs go limp but keep flavour. Berries collapse and become fruit for cooking rather than for eating raw.

Freezing and thawing well

• Cool food before it goes in. Putting hot food in a freezer raises the temperature of everything around it and slows every item's freezing.
• Freeze flat and thin. A bag of stew laid flat freezes faster, thaws faster and stacks better than a tub.
• Exclude air. Freezer burn is sublimation — ice going straight to vapour from the surface — and it dries and toughens whatever is exposed. Wrap tightly.
• Label with what and when. Everything looks the same after a month, and this is a genuinely universal failure.
• Blanch vegetables first. Freezing slows enzymes; it does not stop them. Enzymes in unblanched vegetables carry on degrading colour and flavour over months. A brief blanch deactivates them.
• Open-freeze small items — berries, chips, dumplings — spread on a tray before bagging, so they do not freeze into a single mass.
• Thaw in the fridge rather than on the worktop, so the surface does not sit warm while the middle is still frozen.
• Use the liquid that comes out. In meat and in fruit it carries flavour, and pouring it away is a real loss.

Where the safety questions actually are

Freezing raises several genuine food safety questions and they are not the ones people usually worry about.

The questions worth asking are about thawing, because that is when food re-enters a temperature range where microbes grow, and about refreezing, and about how long food may safely be kept frozen and after thawing. Those are all matters of specific times and temperatures.

FoodHQ does not publish them. FoodHQ does not publish salt percentages, acidity figures, processing times, curing quantities or keeping windows. Those are safety numbers set by a food safety authority rather than by a cookery writer. In the UK the Food Standards Agency is the authority — take the figures from them or from a body of equivalent standing, and use this page to understand what those figures are doing. What this page can tell you is the mechanism: freezing pauses, thawing restarts, and the moment of restarting is where the care belongs.

Common questions