Cooking science

Turgor loss

A crisp vegetable is crisp because each of its cells is inflated with water pressing outward against its wall. Lose that pressure and the vegetable goes limp — and whether that is reversible depends entirely on whether the cell walls are still intact.

Water pressure is the structure

Plant cells have rigid walls and a membrane inside them. When the cell is full of water it presses outward against the wall and the whole tissue is stiff. That pressure is turgor, and it is what makes celery snap and lettuce crunch.

Lose water and the pressure falls. The cell shrinks away from its wall, the tissue softens, and the vegetable goes limp — which is what happens to a lettuce left out, a cucumber in a salted salad, or a courgette on a hot tray.

The important distinction is whether the wall and membrane are still intact. If they are, the cell can take water back up and the vegetable revives. If heat or freezing has destroyed them, it cannot.

What removes turgor, and whether it is reversible

Simple drying out

Reversible. A wilted lettuce or limp celery in cold water rehydrates within an hour.

Salt

Reversible up to a point. Salt draws water out by osmosis; rinsing and soaking recovers some of it.

Heat

Not reversible. Cooking destroys the membrane, so the cell cannot hold water again. This is why cooked vegetables never go crisp.

Freezing

Not reversible. Ice crystals puncture the walls.

Age

Progressive and largely irreversible as the tissue breaks down.

Bruising and cutting

Local and permanent where the cells were ruptured.

Reviving what can be revived

1. Trim a fresh cut across the base of anything with a stem — celery, spring onions, herbs, asparagus.
2. Submerge in very cold water, or stand stems in it like flowers.
3. Leave for thirty minutes to an hour. A fridge is better than a warm room.
4. Ice in the water speeds it and firms leaves noticeably.
5. Dry thoroughly afterwards, particularly for salad leaves, or the dressing will slide off.
6. For root vegetables that have gone bendy, the same soak works and takes longer.
7. If it does not revive within an hour, it will not. Cook it instead.

Using turgor loss deliberately

• Salting aubergine or courgette before frying, to collapse the cells so they absorb less oil.
• Salting cabbage for sauerkraut, where drawing the water out creates the brine the whole ferment depends on.
• Salting cucumber for a salad, so it does not water down the dressing.
• Salting shredded cabbage for a slaw, then rinsing and drying, which keeps it crunchy rather than weeping.
• Wilting spinach or chard deliberately, which is turgor loss used as a cooking step.
• Salting tomatoes before a tart, so the pastry does not go soggy.

Reading a limp vegetable

Before deciding whether to revive, cook or discard something, work out which kind of limpness it is.

1. Look at the cut ends and the base. Slimy, brown or smelling sour is decay, not water loss, and no amount of soaking helps.
2. Feel the weight. A vegetable that has lost water is noticeably lighter than it was, and one that has begun to rot is not.
3. Bend it. Rubbery and flexible is water loss. Soft and collapsing under its own weight is structural breakdown.
4. Look for wrinkles. Wrinkled skin is dehydration and is very often reversible.
5. If it is dehydration, cut a sliver off the base and stand it in cold water for an hour. Carrots, celery, radishes, herbs and leaves all come back.
6. If it is decay, it goes in the bin. Cutting the bad part off works for a firm vegetable with a local blemish and not for a soft one.
7. If it is neither and it has simply aged, cook it. Age costs texture and crispness, and almost nothing that matters in a soup or a braise.

Common questions