Cooking science

Osmotic moisture movement

Salt and sugar pull water across the membranes in food. That single effect explains salting aubergines, brining a chicken, curing salmon, macerating strawberries and why a salted salad goes limp.

What is happening

Cells in food are surrounded by membranes that let water through more readily than dissolved substances. When you put salt or sugar on the outside, the concentration outside the cell is higher than inside, and water moves outwards to even it up.

That is why salted vegetables weep, why sugared fruit produces syrup, and why a piece of fish left in a salt cure firms and darkens. The food is losing water, and with it a certain amount of dilute flavour.

Run it the other way and the same principle puts water in. A brine is salty enough to season but dilute enough that, over hours, salt penetrates the meat and the meat absorbs water along with it. The proteins are also altered so they hold that water better during cooking, which is the real reason brined meat stays juicy.

Uses in the kitchen

Salting aubergine or courgette

Draws out water so they fry rather than steam, and collapses the sponge structure so they absorb less oil.

Salting cabbage or cucumber

Removes water so a slaw or pickle stays crisp rather than diluting its dressing.

Brining poultry and pork

Adds water and seasons throughout. Most effective on lean meat that dries easily.

Dry-brining meat

Salt on the surface draws moisture out, dissolves in it, and is reabsorbed as a concentrated brine. Better browning than a wet brine, because the surface ends up drier.

Curing fish

Salt and sugar draw enough water out to firm the flesh and change its texture entirely.

Macerating fruit

Sugar draws out juice, which becomes a syrup and softens the fruit.

Timing matters more than quantity

• Salt meat well ahead or immediately before. Salted 40 minutes before cooking, meat is at its wettest on the surface and browns badly. Either salt at least a few hours ahead, or salt as it hits the pan.
• Salt salad leaves at the table. Dressing a salad early is osmosis working against you.
• Salt vegetables for frying half an hour ahead, then pat them thoroughly dry.
• Do not salt mushrooms early if you want them browned. They release a great deal of water and will stew.
• Sugar on fruit works within minutes and goes on working. Fruit macerated overnight is soft and swimming.

What it does not do

Osmosis is often invoked to explain things it does not cause. Salting a steak does not "seal in" juices, and there is no evidence that searing does either. Salt drawn into meat does not make it dry; over-cooking does, and brined meat survives over-cooking better than unbrined.

It is also worth being clear that drawing water out of food is not a food-safety measure in itself. Curing and preserving depend on much more than moisture loss — on concentration, on acidity, on temperature and on time — and any preservation intended to keep food safe should follow guidance from a food-safety authority rather than a general principle like this one.

Common questions