Culinary function

Transfer heat

Every cooking method is a way of moving energy into food, and there are only four routes. Knowing which one a method uses explains most of what it can and cannot do — including why a 100°C pan of oil cooks faster than a 200°C oven.

The four routes

Conduction

Direct contact. A pan to a steak, a hot tray to a potato, and the movement of heat from the surface of food towards its centre. Efficient in solids and slow through food itself.

Convection

A moving fluid carrying heat — air in an oven, water in a pan, oil in a fryer. How most of the heat in most methods actually arrives.

Radiation

Energy travelling as waves with no medium at all. A grill element, glowing charcoal, the walls of a hot oven. It heats the surface only.

Volumetric

Energy deposited through a depth rather than at a surface. A microwave, and nothing else in a domestic kitchen.

Most methods use several

An oven is convection plus radiation plus conduction from the tin. A grill is radiation plus a little convection.

Why the medium beats the temperature

Water at 100°C cooks a potato faster than air at 200°C, and oil at 180°C cooks a chip faster than an oven at 200°C. The temperatures suggest the opposite, and the medium is why.

Air is a poor conductor and holds little energy per unit volume, so it transfers heat into food slowly. Water and oil are both far better on both counts. There is also a still, insulating layer of fluid that clings to any surface — the boundary layer — and it is much thicker and more insulating in air than in liquid.

That boundary layer is why a fan makes such a difference. Moving air strips it away and lets heat reach the surface directly, which is why a fan oven cooks faster at a lower setting and why an air fryer browns better than an oven at the same temperature.

It is also why 80°C in a sauna is comfortable and 80°C water is not.

What each route is good for

• Conduction — searing, griddling, anything where you want a hard crust on one face. Also the limiting factor in how fast the centre of anything cooks.
• Convection in air — roasting and baking. Gentle, even, and it allows a dry surface, so it browns.
• Convection in water — boiling, simmering, poaching. Fast and capped at 100°C, so it cannot brown.
• Convection in oil — frying. Fast, hot, and it browns.
• Radiation — grilling and broiling. Intense at the surface and shallow, which is why thick food chars outside and stays raw inside.
• Volumetric — microwaving. Fast, and it cannot brown.

Getting heat where you want it

• Match the pan to the burner. A pan wider than the ring heats in a circle and browns only in the middle.
• Use thermal mass for searing. Cast iron and carbon steel hold heat, so the pan barely drops when cold food arrives. This matters far more than the dial setting.
• Use the fan for anything you want dry and browned, and conventional heat for delicate bakes that a fast-moving surface would dry out.
• Move food between zones on a grill or a barbecue rather than trying to find one setting that does both jobs.
• Add liquid and the ceiling drops to 100°C. That is the moment a pan stops browning and starts braising, and it is a decision worth making deliberately.
• Preheat the vessel, not just the oven — a tin, a stone or a heavy pan supplies conducted heat that air alone cannot.

Common questions