Why does bread go stale?
Fresh bread is one of life's simple pleasures: a crisp crust on the outside and a soft, springy inside, which bakers call the crumb. Yet after only a day or two on the kitchen counter, the same loaf can become firm, crumbly and far less pleasant to eat. Most people would say that the bread has simply dried out. Surprisingly, that explanation is only partly correct.
The main reason bread goes stale is a change in starch, the substance that makes up most of the flour. When dough is baked, heat and water cause tiny grains of starch to swell and lose their neat internal pattern, and this gives fresh bread its soft texture. As the loaf cools, however, the starch slowly begins to line up again into a more ordered structure. As this happens, the starch holds water less tightly, and the crumb feels hard and dry, even though much of the water is still inside the loaf.
This explains why bread that is tightly wrapped in plastic still goes stale. Wrapping stops the loaf from losing water to the air, but it cannot stop the starch from slowly changing.
The crust changes too, but in the opposite direction. Over time, moisture moves outwards from the crumb into the crust, so the crust loses its crunch and becomes soft and leathery.
Temperature also matters. Perhaps surprisingly, the starch changes happen fastest at the cool temperatures found inside a fridge. A fridge helps to stop mould, but a loaf kept there usually goes stale sooner than one kept in a bread bin at room temperature. Freezing, on the other hand, almost stops the process, which is why sliced bread can be kept in the freezer for weeks and toasted straight from frozen.
Stale bread can also be partly rescued. Heating it in a hot oven for a few minutes loosens the starch again, and the crumb becomes soft for a while. The effect does not last, though, so the bread should be eaten immediately. Loaves made with some fat or sugar generally stay soft for longer than simple ones made only from flour, water, yeast and salt. And bread that is past its best need not be wasted: it makes excellent toast, and it can be turned into breadcrumbs for cooking.