Why does bread become stale even if it doesn’t go mouldy?

When bread becomes stale, it loses its soft, fresh texture and becomes dry and hard. As it happens, this isn’t just because it dries out. Staling is mainly caused by a process called starch retrogradation.

When bread is baked, the heat causes starch molecules in the flour to absorb water and swell, forming a soft, gel-like mass. As the bread cools after baking, these starch molecules slowly begin to crystallise again over time. This recrystallisation pushes water out of the starch structure and into other parts of the bread, making the crumb firm and less pleasant to chew.

The surprising part is that staling can happen even if the moisture content doesn’t drop significantly. In fact, bread stored in the refrigerator often goes stale faster, because the lower temperature speeds up starch crystallisation. This is why bakers recommend storing bread at room temperature or freezing it. Freezing keeps molecules from moving too much and thus stops the retrogradation process until the bread is thawed.

While staling changes texture, mould growth is a separate process caused by fungal spores that feed on the bread’s nutrients, usually when stored in warm, humid conditions.

Bread can thus taste ‘old’ long before mould appears simply because its starch molecules have reorganised themselves at a subatomic level. Understanding this can help you keep bread fresh for longer: by freezing leftover slices and toasting them when needed.