E13 Thermal radiation

Topic

Thermal radiation transfers heat via infrared electromagnetic waves; unlike heat conduction and heat convection—which represent distinct modes of heat transfer—it requires no material medium for propagation. In the kitchen, thermal radiation operates inside an oven, on a grate, under a broiler, or within a toaster, becoming the dominant heat transfer mechanism precisely when the food is neither in direct contact with a hot surface nor surrounded by a circulating fluid. Because the food is removed from direct contact or circulating fluids, thermal radiation consequently supersedes conduction and convection as the primary mechanism for transferring heat to the food.

According to the Stefan-Boltzmann Law, any body with an absolute temperature above zero emits thermal radiation proportional to the fourth power of that absolute temperature. The surface emissivity—which is close to 0.9 for most moist foods—quantifies the amount of thermal radiation actually emitted by the surface compared to that of an ideal black body at the same absolute temperature; for instance, a golden-brown or fatty surface on the food may exhibit slightly lower emissivity than a moist surface. A higher absolute temperature in the food results in significantly greater thermal radiation emission from its surface—again, according to the Stefan-Boltzmann Law—because the radiation depends on the fourth power of the absolute temperature rather than on the absolute temperature itself. Consequently, the net flow of thermal radiation between the heating element and the food surface scales with the difference between the fourth power of the heating element's absolute temperature and the fourth power of the food surface's absolute temperature; thus, a small rise in the heating element's absolute temperature generates a much larger increase in this net radiation flux than a simple proportional relationship would predict—it is precisely this fourth-power dependence on absolute temperature that explains the blast of heat felt when approaching a hot grill. A further increase in the heating element's absolute temperature causes the net thermal radiation flux toward the food surface to rise disproportionately, following that same fourth-power relationship.

For this reason, the model—based on the thermal radiation and fourth-power-dependent net flux described earlier—aligns with Wien's Law, indicating that a heating element at typical cooking temperatures emits radiation primarily in the near- and mid-infrared ranges. This radiation is readily absorbed by water and the food surface, thereby promoting the Maillard reaction and surface caramelization without necessarily cooking the food's interior yet. In practice, an equivalent radiant coefficient allows this thermal radiation to be treated as an additional input alongside the convective coefficient established by another part of the model, with both contributions summed in the total heat balance for the food. Consequently, a higher combined value for the equivalent radiant and convective coefficients results in a higher total heat balance for the food.

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