The Science of Heat Transfer in the Kitchen

Heat control is one of a cook’s most important skills. Many cooking mistakes can be traced to misunderstandings about heat: how it is produced, how it varies over time and space, and how its flow can be controlled.

What is heat? Heat is energy in transit, transferred because of a temperature difference. Temperature, by contrast, is a state variable that describes the thermal condition of a system. Because heat is inherently a process of energy transfer, it is often clearer to think in terms of heat transfer rather than heat alone.

Heat is transferred in three principal ways: conduction, convection, and radiation. Conduction is the transfer of thermal energy through direct molecular interactions within a material or between materials in contact. In cooking, it occurs through solids and through fluids that are not undergoing significant bulk motion.

Convection involves the motion of a fluid—either a liquid or a gas—which transports thermal energy from one region to another. In engineering, the term often refers specifically to heat transfer between a surface and a moving fluid, such as between an egg and the circulating hot water around it. The fluid motion may be forced by a pump, fan, or stirring, or it may arise naturally from density differences caused by heating.

Radiation transfers energy through electromagnetic waves and does not require a material medium. This is how energy travels from the sun to Earth, but radiation also occurs over short distances in the kitchen—for example, between a glowing heating element and a slice of bread.

We will first study conduction and convection by cooking a soft-boiled egg, then examine radiation by making toast.