Cooking science

Crisping

Crispness is a mechanical property, not a flavour. A crisp layer is rigid and brittle enough to fracture suddenly under a bite — and what makes it brittle is the absence of water.

Why dryness makes crunch

A dried layer of starch, protein or batter is a rigid, glassy solid. Bite it and it cannot deform — it cracks, and the crack propagates suddenly, which is what produces both the sensation and the sound of crispness.

Water is a plasticiser. It gets between the molecules and lets them slide past each other, so the same layer with a small amount of moisture in it bends instead of breaking. The transition is surprisingly sharp: a few per cent of water is the difference between a crisp crust and a leathery one.

That is why crispness is so fragile. Anything that adds water back — steam from the food beneath, humidity in the air, a sauce, a lid, a pile of other food — softens it within minutes.

Getting food crisp

• Get the water out first. Dry the surface, parboil and steam-dry potatoes, dry-brine skin uncovered.
• Use enough heat. Crisping needs the surface well above 100°C, which means it needs to be dry first.
• Use a coating that dries hard. Cornflour, semolina, rice flour and panko all crisp better than plain flour alone.
• Do not crowd. Steam released by neighbours is the enemy.
• Use fat. It conducts heat into the surface far better than air and helps it dehydrate evenly.
• Finish hot and briefly. A double fry, a final blast in a hot oven, or a minute under a grill.

Keeping it crisp

Harder than making it, and mostly about not trapping steam.

• Drain on a rack, not on kitchen paper. Paper traps steam under the food and the base softens.
• Salt immediately but sauce at the last possible moment.
• Serve straight away. Crispness has a short life and no technique extends it much.
• Do not cover or stack. A lid on a bowl of chips is the fastest way to ruin them.
• Reheat in an air fryer or a hot oven, never a microwave, which adds steam and softens further.
• Keep the sauce separate where you can — a dressed crisp thing is a soft thing within a minute or two.

What crisps, and what it takes

Different materials reach brittleness by different routes, which is why the techniques differ.

Starch

Gelatinises, then dehydrates into a rigid glassy layer. Chips, roast potatoes, batter, bread crust.

Protein

Coagulates and then dries. Chicken skin, pork crackling, fish skin, cheese crisps.

Sugar

Melts and then sets hard on cooling. Caramel, brittle, a bruleed top.

Fat

Renders out and leaves a dry solid behind. Crackling and bacon are both this plus protein.

Water content

Below roughly five per cent for most of these. It is a small number and a sharp threshold.

Why fat helps

It conducts heat into the surface far better than air and dehydrates it evenly, which is why fried things crisp more reliably than baked ones.

What a crust needs, and what destroys it

• Dryness. Crispness is a dry, rigid, porous structure. Any water in it and it is not crisp. Everything else follows from this.
• Heat high enough to drive water off faster than it arrives from inside the food.
• Somewhere for the steam to go. A rack, not a plate; a single layer, not a pile; uncovered, not under foil.
• Fat helps, because it conducts heat to the surface efficiently and because a fried surface dehydrates faster than a baked one.
• Starch or a coating gives something to crisp that the food itself may not provide — flour, cornflour, breadcrumbs, semolina.
• What destroys it: covering it, stacking it, saucing it early, refrigerating it, or letting it sit next to something wet. See moisture migration.

Common questions