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It's fairly well known that stale bread will go soft in the microwave. It makes hard bread edible (albeit a little bit chewy sometimes.)

It's obvious that moisture is being released from somewhere in the bread, and this is what causes the softness.

But the bread doesn't blacken as it would if the carbohydrates were decomposing to carbon, so where (chemically) does this moisture come from?

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  • $\begingroup$ Poked around a bit and I think the answer is the gluten content. Know when you cut into a cooked steak how juice will ooze out? That is because cooking denatures the proteins in the steak. Microwaving does the same thing to bread. It denatures (removes water) from the gluten and the rest of the bread absorbs the moisture. $\endgroup$ – MaxW Nov 8 '15 at 23:27
  • $\begingroup$ I'd say the residual water acts a plasticiser. When quickly heating the bread, it goes above Tg, becoming soft. If that's true, it should become stiff again quickly when cooling. Does it? $\endgroup$ – Karl Jun 29 '16 at 0:03
  • $\begingroup$ Another possibility: fresh bread contains water, we can see that once water evaporates it gets hard. Does some water remains? Probably, but this might not be the only reason why it softens when heating it up. When you heat the 'dry' bread up it seems reasonable that some component in the mixture fuses and because of it, it softens. Probably water helps too...chemistry.stackexchange.com/questions/96459/… seems relevant $\endgroup$ – user43021 Aug 23 '19 at 15:17
  • $\begingroup$ @MaxW 5 years later, but the same happens to gluten free breads as well for whatever it's worth $\endgroup$ – David Mulder Apr 7 '20 at 14:01
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I believe this has to do with its moisture content. While certainly dry, it still has some moisture, and what little moisture is left gets excited by the microwaves, heats up (the heat helps it soften too) and steams the bread essentially. I think that MaxW's explanation of glutens for the question 'Where does the moisture come from?' is a likely explanation.

Note that if the bread is sufficiently dry enough, this little microwave trick wont help. It is usually advised that you add a few drops of water to the bread before heating up in the microwave in this way. This is to ensure the bread doesn't actually become harder in the microwave, as it boils the water and will drive it off if microwaved in excess.

Also, note that bread WILL become blackened, charred and start smoking if left in the microwave long enough. I did this by typing 30 minutes instead of 30 seconds. It started smoking around 5 minutes.

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