General Knowledge
Why does bread go stale even in a sealed bag?
- The starch recrystallises - it is not just drying
- Air gets in
- Bacteria grow
The answer is The starch recrystallises - it is not just drying. Starch retrogradation, which is why warming bread revives it briefly.
Because going stale is not drying out. It is starch retrogradation: during baking the starch granules absorb water and swell into a soft gel, and as the loaf cools those molecules slowly rearrange back into an ordered crystalline structure, squeezing the water out and turning firm. A sealed bag stops evaporation and does nothing about that.
The proof is that heat reverses it. Warm a stale loaf and it goes soft again for a while, because the crystals melt - which is exactly why toast works and why refreshed bread stales again faster afterwards.
The temperature detail is counterintuitive and useful: retrogradation is fastest just above freezing, so bread goes stale several times faster in a fridge than on the counter. The freezer stops it almost completely, because the water is locked up as ice. Bread kept out will dry and mould instead, which is a different problem, and the reason the fridge feels like the right idea.