Vol. II · Heat Plate 10 of 11microwave, dielectric heating, penetration depth, uneven heating, standing time, defrosting, cold spotsHMS

The Microwave.

A microwave oven heats food from within its outer layers, not from the outside in — but not evenly through, either.

A microwave oven fills its cavity with radio waves at 2.45 GHz that make water molecules in food twist back and forth; the friction heats the food directly, without hot air or a hot pan. That is why a mug of soup heats while the mug stays cooler. But the waves are absorbed as they go: most of their power is spent in the outer centimetre or two of a water-rich food. A thick piece is heated in its outer layers and must wait for conduction to warm its centre.

Ice absorbs microwaves far less than liquid water, so in a frozen block the first patches to thaw absorb more, heat faster and can cook while the rest is still frozen. And the standing waves in the cavity leave hot and cold spots, which the turntable only partly evens out. The workshop heats a slab of mash and lets it stand.

Below: the temperature across a slab of mashed potato after microwaving, and after standing, with the coolest point marked.

What you will learn
Workshop
A slab of mashed potato (properties of potato) heated by microwaves absorbed with the chosen penetration depth, then left to stand in 22 °C air (thermal bench with internal heating). Red: after heating. Blue: after standing.

Hot outside, cold heart

A 40 mm slab heated for three minutes runs from 13 °C at its coolest to 62 °C at its hottest: the outer layers, where the microwaves are absorbed, race ahead, and the centre lags. Two minutes of standing let conduction carry heat inward and lift the coolest point to 19 °C. A 15 mm slab heated the same way is far more even (spread 6 °C), because almost all of it lies within the penetration depth. Thin layers, a ring shape with a hole in the middle, and stirring halfway all help for the same reason.

Frozen food is harder still. From −18 °C, four minutes leave a coolest point of −6 °C while the hottest reaches 33 °C: thawed regions absorb more and run away, frozen ones barely warm. That is why defrost settings pulse the power, giving conduction time between bursts. For safety, reheated leftovers should reach 74 °C throughout (the U.S. Food Code's reheating temperature, XII.11) — measured at the coolest point, after standing.

At the stove

Reheat a bowl of stew or rice for the time you usually use, then check it with a probe in three places: near the edge, halfway, and in the middle. Stir, stand for a minute, and check again. Arrange food in a ring and cover it loosely next time; the cool centre will be smaller.

The microwave oven came from radar: in 1945 the engineer Percy Spencer noticed that a magnetron he was testing had melted a bar of chocolate in his pocket. The first commercial microwave, in 1947, stood almost six feet tall.

Terms

Dielectric heating
Heating by an oscillating electric field that makes polar molecules such as water rotate.
Penetration depth
The depth over which most of the microwave power is absorbed; a centimetre or two in moist food.
Standing time
Time after microwaving for conduction to even out the temperature.
Runaway heating
Thawed patches absorbing more power, heating faster, while frozen parts stay cold.
Microwaves heat the outer centimetre or two; conduction does the rest. Use thin layers, stir, and let food stand — then check the coolest point.

Sources

  1. U.S. Food and Drug Administration, Food Code 2022 (sections 3-401.11, 3-403.11, 3-501.14, 3-501.16, 3-402.11 and the definition of time/temperature control for safety food).
  2. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.