Microwaves Do Not Cook Food From the Inside Out

It is a familiar little drama. A square of yesterday's lasagne goes into the microwave, the door clicks shut, the plate turns slowly in the warm yellow light, and two minutes later out it comes, edges bubbling, cheese sizzling at the corners. You take a forkful from the middle, and it is still cold.

If microwaves really cooked food from the inside out, as many people believe, that should be impossible. The cold middle is the clue to what is really going on.

The myth everyone repeats

The idea is easy to understand. A microwave has no glowing element and no flame, and heat seems to appear out of nowhere inside the dish. So it is tempting to picture the oven reaching straight to the centre and working outwards. The US Food and Drug Administration says plainly that this is not so: although the heat is produced directly in the food, microwave ovens do not cook food from the inside out.

What the evidence says

Inside the oven, microwaves bounce off the metal walls and pass into the food, where they make its water molecules vibrate. According to the FDA, that vibration causes friction between the molecules, and the friction produces heat. The catch is that the microwaves do not travel far. Nebraska Extension, at the University of Nebraska-Lincoln, gives a rule of thumb that microwave energy reaches 1 to 1.5 inches into food, roughly 2.5 to 4 centimetres, and that thicker pieces take longer to heat through.

• 1–1.5 in how deep microwave energy reaches into food, as a rule of thumb
• 3 min the shortest standing time recommended after microwaving

So in a thick dish, the outer layers are heated by the microwaves themselves, while the inside, as the FDA puts it, is cooked mainly by heat conducted inwards from the hot outer layers. That is the slow part of the job. Heat creeping through a deep slab of pasta and sauce takes time, which is why the edges race ahead while the centre lags behind.

What people often get wrong

The myth survives because it is half right. A microwave really does make heat inside the food, rather than on a hot surface the way a frying pan or a grill does, and the FDA says exactly that. The mistake is about where it starts. The heat begins in the outer layer, not at the centre, and it is the middle of a thick dish that is always the last to be cooked.

The practical advice follows straight from the physics. Nebraska Extension notes that microwave cooking can be uneven, which is why many ovens have a turntable, and suggests stirring, rotating or turning food over halfway through to get rid of cold spots. It also recommends letting food stand for at least three minutes after the oven stops, which completes the cooking, and then checking the inside with a food thermometer.

So the next time the lasagne comes out with a cold heart, do not blame the oven for working backwards. It did exactly what it always does. Spread the food out, give it a stir, and let it rest for a few quiet minutes on the counter while the warmth finds its own way to the middle.