Science Questions

How does a microwave burn food?

Mon, 5th Sep 2016

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Question

Jason Deutsch asked:

How does a microwave burn/char things without infrared radiation.

Answer

Kat Arney put this to Adam Townsend...Old Microwave

Adam - I was unconvinced so this morning, I did a little experiment so I brought a little present into the studio. Andrew is going to take this one and describe this present to the audience.

Andrew - I've got a secure Tupperware tub and inside it is a small piece of whole grain bread. On it, it has a very definite charred mark in the middle like youíve held it against a cigarette lighter.

Adam - Can you smell that here?

Kat - I just got a whiff. Thatís definitelyÖ

Andrew - Itís also very solid and hard.

Adam - It is incredibly solid, incredibly hard. That smell though is the same smell that my flat now completely smells of. So donít try this at home. So I put a piece of bread in the microwave for just 2 minutes and after 2 minutes, the middle just started smoking. And so, you get this nice black burn mark.

Kat - What has happened? A microwave does work by making the water in the food vibrate. So how can it do something that seems like a dry fire?

Adam - The first thing that happens here is that all the water gets evaporated off so you get loads of steam. Microwaves are set the frequency that water is very receptive to which means that water heat up. But all of the stuff, in bread in this case, is still getting slightly excited by the microwave energy. So itís just that water is normally more receptive. So you take away the water. What's left while it still tries to absorb some stuff, some of the starch in the bread for example, and so, it also heats up, and ends up stinking up your kitchen.

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Burning and charring of food is actually a chemical reaction caused by heat in which the material breaks down to form other compounds. There are chemists here who will be able to describe the actual process but I suspect that the browning, charring of food is a combination charcoal and caramelisation plus a whole load of other chemicals.
Wood for example will begin to carbonise at 280C using up oxygen, above this temp the process becomes exothermic- generating it's own heat.

Think about food burning in the bottom of a pan, you don't need direct flame or infrared. Colin2B, Wed, 13th Apr 2016




I thought that is a Physics question Colin rather than chemistry?

I am uncertain of the answer, my observation of the process tell's me that firstly the food  becomes dehydrated, the matter is then not losing heat by losing water and gases, so the matter gains more  heat by the entropy gaining more energy which is a much greater rate than the rate of emittance of energy, creating ''burnt'', what ever burnt actually is I have no idea, 

What is burnt? 

Is it like ''dead'' atoms?

Thebox, Wed, 13th Apr 2016

Most food contains water, which is a polar molecule - the oxygen side is slightly more negative, and the hydrogen side is slightly more positive.

Microwaves have a local electric field which spins the water molecules around, absorbing microwave energy and turning it into molecular vibrations, which we feel as heat. It is this heat which causes the delicious chemical reactions.

The absorption spectrum of water has a broad peak around 1-3cm wavelength.
See the blue curve at: https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water#Microwaves_and_radio_waves

A wavelength of 12cm (2.45GHz) is usually used in microwave ovens. This is longer than the peak absorption, but is used because it penetrates the food more deeply, heating it more uniformly, and killing any bacteria more thoroughly. evan_au, Wed, 13th Apr 2016

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