Understanding Heat in the Kitchen
Conduction, convection and radiation — and why each produces a different result
Heat is the central tool of cooking, and yet most people use it without understanding the three fundamentally different ways it transfers into food. The difference between a crisp crust and a stewed interior is usually a question of which mechanism is doing the work.
There is one instrument in cooking more important than any pan, knife or thermometer: heat itself. Almost every technique in the kitchen is a controlled application of heat — and yet most people cook without a clear picture of how heat actually travels from a heat source into food. This matters because the three mechanisms of heat transfer each produce different results, and knowing which one you are relying on explains why things work the way they do.
The three mechanisms
### Conduction
Conduction is the transfer of heat through direct physical contact — from a hot pan surface to the food resting on it, from the outer layers of food inward to the centre.
Cast iron and stainless steel conduct heat well. Thin-gauge pans conduct it unevenly, creating hot spots. This is why a heavy-based pan produces more even browning than a thin one — the mass of the pan acts as a heat reservoir that distributes energy across its surface as the food draws it away.
The important implication of conduction: it moves from the outside of food inward. The outside of a steak is always hotter than the centre. Getting the inside to the right temperature while not burning the outside is a conduction management problem — which is why thick steaks work better than thin ones, and why reverse-searing (bringing the steak to temperature slowly in the oven before searing) works so well.
### Convection
Convection is heat transferred through the movement of a fluid — either liquid or gas. When you roast vegetables in an oven, the hot air circulates around them, transferring heat from every direction simultaneously. When you boil pasta, the turbulent water carries heat to the pasta surface from all sides.
A fan oven (convection oven) works by forcing air to circulate, increasing the rate of convective heat transfer. This is why fan ovens cook faster at the same nominal temperature, and why they produce more even browning — the forced airflow prevents cold spots from forming.
Convection in liquid is very efficient: water conducts heat roughly 25 times more effectively than air at the same temperature. This is why lowering food into 80°C water produces faster, more consistent cooking than putting it in an 80°C oven — and the basis of sous vide cooking.
### Radiation
Radiation is heat transferred as electromagnetic waves, without any medium — like the heat you feel from a fire without touching it. In cooking, the most common source of radiant heat is the grill (broiler), which heats the surface of food directly through infrared radiation.
Radiant heat is exceptionally fast and extremely directional — it only reaches surfaces with line-of-sight to the heat source. This is why food must be turned under a grill, and why the top surface browns while the bottom stays soft. It is also why you can char the outside of a marshmallow almost instantly with a blowtorch while the interior stays cold.
The practical consequences
**Why a preheated pan matters for searing:** A cold pan relies on conduction from the pan surface, but if the pan's thermal mass is low (or it is not fully preheated), the food draws heat from the pan faster than the burner replaces it, and the temperature drops. The surface temperature falls below the 140°C threshold for browning, and the food starts to stew in its own moisture. A preheated pan with high thermal mass maintains its temperature even when cold food contacts it.
**Why ovens are inconsistent:** Ovens rely on convection from hot air, but air is a poor conductor of heat and varies in temperature throughout the cavity. Fan ovens reduce this by forcing circulation. The floor of the oven is usually hottest (direct radiant heat from the element beneath), which is why bread benefits from being baked on a preheated stone — conductive heat from below + convective heat from above.
**Why frying produces faster browning than baking at the same temperature:** Oil conducts heat roughly four times more effectively than air. 180°C oil will brown food dramatically faster than 180°C oven air, because more heat is reaching the food surface per unit time.
**Why sous vide is so precise:** Water's high thermal conductivity means the food reaches and stabilises at the water temperature reliably, without overshoot. The food cannot exceed the temperature of the water, which eliminates the grey zone of overcooked exterior.
Heat is not a single thing. How it moves, how quickly, and from which direction, determines everything about what your food becomes.