The determining factor for cooking time is not the cut’s weight but the distance heat must travel from its surface to its center. When thickness increases to 2 times its original size, that distance also increases to 2 times its original length, but because heat moves more slowly as it penetrates, cooking time increases considerably beyond 2 times. At the same weight, a thin, spread-out piece cooks much faster than a thick, compact piece.
Heat moves slowly through food, from the surface toward the center. The surface quickly approaches the oven temperature; the layer just beneath it receives heat from the surface, the layer below receives heat from it, and the same process continues toward the center. As each layer heats up, less heat passes on to the next, so the progress is not at a constant rate. The closer heat gets to the center, the longer it takes to travel the same distance. A piece of food is cooked when its center, not its exterior, reaches the desired temperature.
The clearest practical result is the difference between weight and thickness. A flat 2-kilogram lamb shoulder cooks much faster than a round leg of the same weight. Flat patties made from 1 kilogram of ground meat cook in a few minutes, while a thick roll made from the same amount of ground meat cooks in half an hour. That is why recipe rules such as “so many minutes per kilogram” work only as rough estimates for pieces with roughly similar shapes; the rule breaks down when the shape changes.
Thickness has more than a proportional effect. When thickness doubles, cooking time increases not 2 times but roughly three to four times. A 2-centimeter-thick fish fillet cooks in a few minutes, while a 4-centimeter piece takes much longer. For the same reason, oven temperature is lowered for thick pieces: high heat browns the outside quickly but does not speed up the heat reaching the center, leaving a piece burned outside and raw inside. The right approach for a thick piece is to lower the temperature and extend the cooking time.
In practice, this information makes three decisions easier. First, thick pieces are cut open into 2 sides (butterflied) or brought to an even thickness; that is why schnitzel is pounded thinner. Second, pieces of different thicknesses are not cooked together on the same tray; thin pieces dry out while thick ones remain raw. Third, measure the internal temperature of thick pieces instead of relying on time. A thermometer removes the guesswork in this situation, and thresholds such as 74°C / 165°F for poultry can be followed precisely.
Starting cold also lengthens the cooking time. A thick piece taken straight from the refrigerator starts heating at a much colder temperature in its center. A frozen piece uses heat first to thaw. That is why cooking time for thick, frozen pieces can be as long as 2 times the usual cooking time. Frequently opening the oven door extends cooking time for thick pieces even further. If you are cooking thick pieces, you can also change the method: cook them for a long time at a low temperature, then brown them briefly at high heat to reduce the difference between the center and the surface. Resting the thick piece after it comes out of the oven also balances this difference somewhat.
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