How Does Heat Transfer to Food in an Oven?

The first method is conduction, which works through contact. The surface of food touching the baking sheet receives heat directly from the heated metal; this is the fastest of the three methods. This is why the bottom of börek (a Turkish savory pastry) browns before the top and why the side of a potato resting on the baking sheet gets darker. The sheet material determines how quickly heat transfers: metal conducts heat quickly, while glass and ceramic heat slowly but retain heat for a long time. That is why the bottom of dough placed on a preheated sheet cooks much faster. For the same reason, food cooked on a wire rack heats from the bottom through air alone, not by conduction.

The second method is convection, which works through air. Hot air in the oven circulates around the food, releases its heat onto the surface, cools and rises, and is replaced by new hot air. The faster this circulation is, the faster the surface heats. In fan mode, the fan drives this circulation, noticeably speeding heat transfer; that is why the temperature is set 10–20°C / 18–36°F lower than specified in the recipe in fan mode. Placing baking sheets too close to each other or to the oven walls blocks this circulation.

The third method is radiation, and it does not require contact. A glowing heating element emits radiation directly, much like the warmth you feel in front of a stove, and this radiation heats the surface in its path. This is why the top of food browns within minutes in broil mode; there, almost only radiation is at work. Radiation increases rapidly with proximity to the source, so the surface of a sheet on the top rack can burn while the inside remains raw. The oven’s interior walls also emit radiation as they heat up.

All three processes work at once, but their contributions vary depending on the situation. In conventional mode, radiation and conduction are prominent; in fan mode, convection dominates. Food covered with foil is shielded from radiation, but continues to receive heat by conduction from below. Paper covering the food does the same thing. Moisture on the food’s surface also changes this balance; a wet surface cannot rise above 100°C / 212°F until its water evaporates, and it does not brown during that time.

Knowing these three methods makes most oven problems solvable. In a dish with a burned bottom, there is too much conduction: the sheet is dark or too close to the lower rack; the solution is to raise the rack or use a light-colored sheet. In a dish with a dark top, there is too much radiation: lower the rack or cover the top with foil. In a dish that stays pale everywhere, there is not enough convection: the sheet is overcrowded, so space out the pieces or switch to fan mode. Knowing how much each of the three methods contributes lets you correct the result without changing the recipe’s temperature.