I did a quick search, both here and on Google, but couldn’t find the answer. Anyone know why bread tends to get soggy when heated in the microwave?
I don’t know for certain but microwaves excite the water molecules to produce heat, so the water in bread probably evaporates quickly, and the vapor will be reabsorbed by the bread but condensed on the surface. If you heat it very long, the water’s gone for good and you will have very dry, tough bread. I never put bread in the microwave more than a few seconds.
Next time you make toast, take the toast immediately out of the toaster and put in on a plate. Wait a couple of minutes and check the underside of the toast. It will be soggy, for similar reasons.
http://126.96.36.199/curricular/nbba/lab2.htm (emphasis added)
Water of hydration is not normally perceptible as wetness, however when the microwave begins to drive it off, before evaporation, it accumulates unbound in the pores etc. of the bread. That excess is perceived as sogginess.