ELI5: Why does putting a mug of water in the microwave keep bread from getting soggy when heated?
The water acts as a ‘dummy load’ when you have it in the microwave with bread. So it reduces the amount of energy going to the bread. Think of it in terms of cranking down an energy knob. The higher the energy the faster it heats up which means it gets soggy. If you have less energy and cook it slowly and evenly it won’t get soggy.
It’s assumed the bread is frozen. Why is the bread frozen? It stays fresher longer. Good for when you’re living by yourself and don’t go through food as fast. As a poor graduate student who knows the struggle lol.
Can I lower the power for the same effect? Yes, basically it will do the same thing. Lowering the amount of energy reduces the amount of 'work’ used to heat the bread. You’ll have a lower temperature and this will allow for the heat to dissipate evenly throughout the bread.
Wait a minute, what happens if I heat room temperature bread? If you heat it the moisture from the bread will be released and moisten the bread. If you do this with water that water will be 'added’ to the atmosphere in the microwave and may condense to make the bread wet on the outside. Of course if you heat it long enough it will dry out or burn.
Why does bread get soggy? At room temperature for example the bread is saturated with moisture, kind of like a damp sponge that has been sitting out for a couple hours. If you heat it up it softens the pores of the material and you get that 'soggyness’. When heating frozen bread at high temperatures it can be concentrated in certain areas and the heat may not be able to spread out. If you lower that temperature the heat can spread out and let the vapor escape.
I worked in a bioenergy lab for a year with some people who studied pyrolysis with domestic and industrial microwaves. We had a metal 'mixer’ to mix up the wood pellets or other biomass. We used water as a dummy load so that the microwaves would not be reflected back and damage the magnetron.