Bread question.. any baker chemist geniuses out there?

Why does toasting make bread “harder” while microwaving makes bread softer?

I am actually wanting to know if there is a chemistry-ish reason behind this.
My “guess explanation” follows, but I’d sure like to know if others (esp someone educated in this such as a baker or chemist) agree.

I understand that toasting actually removes moisture from the bread by convective heat on the surface evaporating the water… that’s a no-brainer.

But… in the microwave:
I understand that a microwave excites water/oil. molecules in the bread and therefore causes it to heat up… But then I guess my question is… I understand why toasting would make bread “harder”… but why do excited water molecules make bread softer? It is not like the bread was frozen water to begin with.

Exciting the liquid water molecules could do nothing (alone) to make it softer (water is either boiling or not; it doesn’t get significantly “softer” or more malleable with heat). Does heat that is cast off by the excited water in the bread then loosen bonds between other molecules in starch/wheat/flour/sugars etc, or is it due to fats and oils found in bread (which are also excited by a microwave) that were in a solid state and become excited into liquid state that make the bread softer?

My guess is that bread is a “glass” (vitrified thing) due to the sugars and starches in it; and as when ironing fabric, bread becomes more malleable with heat. (in a microwave, the heated water/oil transfers heat to the glassy starch/sugars and loosens them). Toasting just removes all water from the outer layer and therefore due to friction increased by the missing water, the toast is “harder”, but the inner layer is still as soft and gooey as if it were microwaved because it has heat transferred into it via convection.

I’d sure like an expert to confirm this thought process, and let me know if microwave softness is due to oil/fat in the bread melting or starch becoming softened (or both). Also, I am not sure if fat is also a “glassy” substance or not, but my guess is that it is not, and there is likely not enough fat in bread to make a difference. but my old zumdhal chem books are deep in my basement, and I figured someone here would know better.

A microwave makes the bread softer because it heats the liquid in the bread to the boiling point – i.e., it steams the bread. You’d get the same result if you sprinkled the bread with a little water, wrapped it in foil, and heated it in a regular oven.

Also, your premise isn’t quite waterproof. Microwave a bread roll in the microwave a little too long and you have a rock that can do some damage.

Also, if you break a sufficiently thick piece of bread in half, straight out of the toaster, you will also notice the inner bread is “softer” and steamed. You are essentially steaming bread with the microwave method from the inside-out and caramelizing and steaming the bread from the outside- in with a toaster.

Rather than glass, I think clay is to ceramics/pottery as dough (paste) is to bread, I believe it’s a better analogous association. Both processes are essentially just baking/drying and evaporating the water out of a solutions with different structural properties. You are talking about twice cooking an essentially baked paste that hasn’t had all of the water evaporated from it- a second firing, so to speak. The water that bread contains, and which makes it palatable and moist enough that you don’t break a tooth, and also provides an environment suitable for the growth of mold, can be heated to further remove water… the more you evaporate off through steaming the harder it gets. Or maybe dough and bread is more like a chemical resin in behavior?

Isn’t a water, yeast, ground wheat, and salt solution essentially a starchy polymer?

The Melting Point, Boiling Point, and Glass Transition qualifiers for polymers

Well, I think 3 things:

  1. I can agree with a “starchy polymer”.
  2. I can kinda agree with “ceramic”, but as devilsknew noted, bread is not like fully fired clay. I’d say bread is more like a pyrometric cone, a partially fired ceramic that gets softer when heated and harder when cooled, and when eaten warm, it is like removing the cone from the kiln ‘uncooled’.
  3. leaving bread in microwave too long is like a fully fired ceramic in that all the water is removed; at some point, the water-removal overcomes the melting of the “starchy polymer” and you get your rock.
    So yeah, I’ll go with “ceramic” and consider this one “figured out” unless others speak up and say that we are way off base here. Thanks devilsknew.