In order to understand why things go stale, we must first understand what bread actually is.
Bread is essentially a network of flour proteins (called gluten) and starches. Suspended throughout this network of large molecules is CO2 that is produced by the fermentation of yeast in the dough. These bubbles give bread its fluffy texture.
Starch is a polymeric carbohydrate, which basically means "a really big, complex molecule made up of carbon and hydrogen (and oxygen in this case)" It's made up of two different kinds of molecule, amylose and amylopectin, which are themselves made of glucose. Plant starch is about 20-30% amylose and 70-80% amylopectin.
When heated up in the oven for instance, starch molecules weaken and allow water molecules to enter or get in between the chains of sugar molecules and join with them. This swells the starch molecule. Mmm. Warm and squishy bread.
Where does the water come from? Either the protein in the bread itself, or water added to the dough.
Once you take it out of the oven, the process reverses itself. Starch molecules begin to crystallize in a process known as retrogradation. The water returns to the proteins in the wheat, or if you microwave it again or leave it exposed to the open air you're liable to have some of it evaporate away. If you just keep letting this happen you'll end up with crutons, or very, very crunchy chips. This is how things go stale.
Basically, if you lower temperatures even more than room temperature, this process gets even faster. The water molecules detach themselves even faster in the fridge than at room temperature, and starch molecules revert to their original shapes and become brittle, crystallized things again. _Putting your bread in the fridge will make it go stale 6 times faster than at room temperature._
Fantastically, you can reverse the staling process by adding some moisture and putting it in the oven and setting it to "warm". similarly to how it was made in the first place.
And yeah. This applies to basically anything carb-y. Chips, crutons, bread, muffins, the list goes on. Anything with a starch in it that gets stale is effected by this process.
It has a lot less to do with evaporation and a lot more to do with temperature. A wet slice of bread is not a fluffy slice of bread. Not the same thing at ALL.
If you want to try de-staling some croutons by running them under the faucet you will be terribly disappointed.
Stale bread weighs the same as "fresh" bread. Moisture is not lost to the staling process. This is an important distinction to thinking that stale bread is "dry".
@j605 evil, evil people
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