Cooking science

Heat penetration

Heat reaches the surface of food quickly and travels inwards slowly. Everything about timing, tin choice and why a hotter oven often makes things worse follows from that one fact.

Food is a poor conductor

Metal conducts heat well; food does not. Once the surface is hot, the heat has to work inwards through the food itself, and that is slow — much slower than most people expect.

So there is always a gradient: the surface is at or near the oven temperature, and the centre lags well behind. In a thin piece that gap is small and closes quickly. In a thick one it is large and takes a long time.

The practical consequence is that raising the oven temperature widens the gap rather than closing it. The surface gets hotter and browns faster; the centre warms at roughly the same rate. That is precisely how you get a cake dark outside and raw in the middle, or a chop burnt and cold.

Thickness, not weight

The single most useful correction to how people estimate cooking time.

Double the thickness

Roughly four times the time. The relationship is closer to squared than to linear.

Double the weight, same thickness

Roughly the same time. A wider tray of the same-depth food is not slower.

Two small joints against one large

Two small ones cook far faster than one of the same total weight, and more evenly.

A deep cake tin

Needs a lower temperature and a much longer time than the same batter in two shallow tins.

A rolled or stuffed joint

Thicker than it looks. Time it by diameter, not by weight.

Bone

Conducts differently from meat and slows the area around it. Meat near the bone is usually last to cook.

How to work with it rather than against it

• Lower and longer for anything thick. The gap between surface and centre narrows as the oven temperature falls.
• Two-stage cooking. Sear hard then finish in a moderate oven, or cook low then blast to brown. Both separate browning from cooking through.
• Cover what is browning too fast. Foil over the top of a cake or a joint slows the surface without slowing the centre.
• Even thickness. Butterfly, spatchcock or pound to a uniform depth and everything finishes together.
• Let it come towards room temperature for a thick cut. Starting fridge-cold widens the gradient before you begin.
• Rest it. The gradient evens out after the heat comes off, which is what resting is for.
• Use a probe for anything large and expensive. Everything else is estimation.

Estimating a cook you have not done before

A workable method when the recipe does not match what is in front of you.

1. Find the shortest distance from the outside to the centre. For a joint that is usually half the smallest cross-section, not the length.
2. Compare it with something similar you have cooked and timed before.
3. Scale by roughly the square of the ratio of those distances, not by weight.
4. Drop the oven temperature if the new piece is much thicker, or the outside will be finished long before the middle.
5. Check early, not late. A piece taken out underdone can go back; one taken out overdone cannot.
6. Write down what actually worked. Your oven and your tins are the variables no recipe knows about.

Common questions