Cooking science

The surface-to-core gradient

Cut a thick steak in half and there are three zones, not two: a dark crust, a band of grey overcooked meat beneath it, and a pink centre. That grey band is the temperature gradient made visible, and how wide it is tells you exactly how the steak was cooked.

The gradient is unavoidable; its width is not

While food is cooking, the surface is hot and the centre is cool, with everything in between at some intermediate temperature. Every part of that gradient is cooking at its own rate.

The meat just under the crust spends the whole cook at a high temperature and ends up well past where you wanted it — that is the grey band. The centre spends the cook catching up and finishes where you aimed.

The gradient cannot be eliminated while heat comes from outside. What can be changed is how steep it is: the fiercer the heat, the steeper the gradient and the wider the overcooked band.

How each technique changes the band

Very hot pan, thin steak

A steep gradient across a short distance. The band can be most of the steak.

Very hot pan, thick steak

A steep gradient, but a wide steak, so a narrower band relative to the whole.

Reverse sear

Cook low until the centre is nearly there, then sear briefly. The gradient is shallow for most of the cook and the band is very narrow.

Sous vide then sear

The whole piece is brought to one temperature, so there is effectively no gradient until the brief sear. The narrowest band of all.

Low oven roast

A shallow gradient. Evenly cooked, and it needs a separate step to brown.

Resting

Narrows the band after the fact. Heat keeps moving inwards and the gradient flattens.

What to do about it

• Buy thicker cuts. The band is roughly a fixed depth, so it is a smaller proportion of a thick steak.
• Reverse sear anything thick. Low oven or low pan until nearly done, then a hard sear for a minute a side.
• Rest properly. The gradient continues to even out for as long as the meat rests.
• Do not cook from fridge-cold if the piece is thick. It starts the gradient wider.
• Accept it on thin cuts. A minute steak will be cooked through and that is what it is for.
• Use a probe in the centre, not near the surface, and take carryover into account.

Reverse searing, which is the practical answer

The technique that most directly narrows the band, and it needs no special equipment.

1. Salt the meat and leave it uncovered in the fridge for a few hours if you can.
2. Cook it in a low oven, or in a barely warm pan, until the centre is close to where you want it. This takes far longer than searing first and it needs very little attention.
3. Rest it briefly and pat the surface completely dry. Surface moisture is what stops the sear working.
4. Get a heavy pan extremely hot and sear for around a minute a side, no more. The inside is already cooked; this is browning only.
5. Rest again briefly and slice. The band under the crust should be a few millimetres rather than a centimetre.
6. The same logic works in reverse: sear first, then finish low. Both separate browning from cooking through.

Common questions