How a microwave oven heats food, and why the middle stays cold
Do microwaves cook food from the inside out? Why the middle stays cold
Many people believe that a microwave oven cooks food from the inside out. Then why is the middle of a thick dish still cold when the edges bubble? The answer is how far the waves get into your food.
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1
The timer beeps. The edges of Rono's lasagne bubble, but a bite from the middle is cold. Miva, a friend who came for dinner, is surprised. She always heard that microwaves cook from the inside out.
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2
It is the other way around. The water in the food takes the energy of the waves so well that they fade on the way in. They only reach the first few centimetres, so only that outer layer gets its molecules turning.
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3
In a thin slice, the few centimetres from the top and from the bottom meet in the middle, and the whole slice gets hot. A thick slice has a middle that the waves never reach.
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So the middle must get its heat from its neighbours. Fast molecules bump into slower ones, a little deeper each time. This slow spread is called conduction, and through food it takes minutes.
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In the end, a microwave cooks from the outside in, like a normal oven. The difference: a normal oven heats only the surface, and a microwave heats an outer layer a few centimetres deep.
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That is why food packets say "stir" and "let stand". Stirring puts cold food next to hot food, and standing gives conduction time to finish the job.
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7
Then Miva takes something out of her bag. It is a block of soup straight from her freezer, hard as a stone.
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Quick check
Two plates of rice.
Show the answer
The flat ring. Every part of it is a few centimetres or less from the surface, so the waves reach all of it. The middle of the tall pile must wait for conduction.
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Go further
Where the heat goes next.