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?
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.
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.