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Why does heating bread (cinnamon buns) in a microwave give it a rubbery texture, when a regular oven doesn't? What are the chemical or structural changes?

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up vote 10 down vote accepted

It is simply that the microwave heats primarily the water molecules, causing the bread to steam. A normal oven heats all of the molecules of the bread, and by the time the water is heavily steaming you will have pulled it out of the oven.

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I thought that it was also related to the way the bread is heated. In a conventional oven it is heated by conduction, allowing the water content to evaporate as heat flows in. In a MO heat transfer is too quick and uniform. – Dr. belisarius Nov 17 '10 at 0:57
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Primarily water, but also fats and sugars and other polar molecules. – endolith Nov 17 '10 at 1:18
    
@belisarius: I'm sure you're right about conventional ovens. I think the microwave thing isn't just about how fast it is, though--microwaves penetrate the loaf, heating water on the inside as well as the outside. So steam is generated well inside the loaf at the same time as outside, causing the effect we all know. – bikeboy389 Nov 17 '10 at 14:27
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@Bikeboy389 That was I tried to imply with the word "uniform". Anyway be careful with heating water on the inside as well as the outside. as bread is not "transparent" for microwaves (surely MW penetrate deeper than infrared). You almost sure have seen that when defrosting food too quickly and got a nasty cooked oustside with a frozen inside. – Dr. belisarius Nov 17 '10 at 14:41
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But I still don't understand why heating the water selectively would have this effect on the structure of the bread. – endolith Nov 17 '10 at 15:47

I am not entirely sure about this, but my theory is based on the way microwaves interact with water. Microwaves are resonant with the rotational frequency of water's dipole, and works by using frequencies that are not quite resonant so that instead of causing rotations some of the energy is lost to friction which increases the vibrational frequencies of the water, which is a synonym for saying that it adds heat, but it also increases the rotational frequency of the water. When the entire water molecule is forced to rotate, the many of the critical hydrogen bonds that give bread its structure are likely broken resulting in a collapsing of the bread and more different hydrogen bonds form that are more stable and thus more rubbery.

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