Why does bread go stale, and why does the fridge make it worse?

Stale bread is not simply dry bread. The main process is retrogradation, where starch molecules that gelatinized during baking gradually recrystallize into an ordered structure as the bread cools and sits, squeezing out water and making the crumb firm and crumbly. Moisture also migrates from the interior toward the crust, which is why the crumb hardens while the crust softens, the opposite of fresh bread. Because it is a physical restructuring rather than moisture loss, staling can be partly reversed by heating: warming bread in an oven re-gelatinizes some of the starch and restores softness temporarily, though it will stale faster afterward.

The counterintuitive practical rule follows directly. Refrigeration accelerates staling because starch retrogradation proceeds fastest at temperatures just above freezing, roughly in the range a refrigerator maintains, so bread goes stale several times faster in the fridge than on the counter. Freezing, by contrast, is well below that range and essentially halts the process, which makes the freezer the correct storage for bread you will not eat within a couple of days, ideally sliced first so you can take what you need. At room temperature, keep bread in a paper bag or bread box for crusty loaves to preserve crust texture, or a plastic bag for soft sandwich bread, accepting that plastic softens the crust. The one thing refrigeration does help with is mold in hot humid conditions, which is a genuine tradeoff.