Why Does Bread Go Stale? It Isn’t Just Drying Out

A fresh loaf has a crisp crust and a soft, springy crumb. A couple of days later the crust is leathery and the inside is hard and crumbly. Most people assume the bread has simply dried out. Surprisingly, that is only a small part of the story. The real change happens at the level of the molecules in the starch.

Bread is mostly starch

Flour is about 70 to 75% starch. Starch is made of two kinds of molecules built from long chains of glucose:

• amylose, mostly long straight chains;
• amylopectin, larger, highly branched molecules.

In raw flour, starch is packed into hard, partly crystalline granules.

What baking does to starch

When dough goes into a hot oven, the starch granules absorb water from the dough and swell. Above about 60 °C their orderly structure breaks down in a process called gelatinisation. The starch becomes a soft, disordered gel, and that is what gives fresh bread its tender, moist crumb.

Staling: starch slowly recrystallises

As soon as the bread starts to cool, the starch molecules begin to settle back into order. This is called retrogradation.

• Amylose lines up quickly, within the first hours. This sets the structure of the loaf and is the reason freshly baked bread is easier to slice once it has cooled.
• Amylopectin recrystallises much more slowly, over days. As it forms crystals, it traps water inside them. The crumb becomes firmer, more crumbly and feels drier, even if the total amount of water has hardly changed.

At the same time, moisture moves from the moist crumb towards the drier crust. The crust loses its crispness and turns soft and chewy, while the inside feels harder.

The experiment that proved it

In 1852 the French chemist Jean-Baptiste Boussingault sealed fresh bread in airtight containers so that no water could escape. The bread still went stale. When he reheated it, it became soft again. If staling were just drying, that would have been impossible. Modern studies using X-rays and other techniques have since shown the starch crystals forming and melting.

Why the fridge is the worst place for bread

Retrogradation runs fastest at temperatures just above freezing. At typical fridge temperatures of around 4 °C, bread goes stale several times faster than at room temperature.

In the freezer, at about −18 °C, the molecules can hardly move and staling almost stops. That is why freezing is by far the best way to keep bread for more than a couple of days.

Why toast tastes fresh again

Heating stale bread above about 60 °C melts the amylopectin crystals and releases the trapped water, so the crumb softens again. This is why a stale roll warmed in the oven or a slice of toast can taste almost fresh.

The effect is temporary. As the bread cools down, retrogradation starts again, often faster than before, so reheated bread should be eaten straight away.

How bakers slow down staling

Bakers have several tricks to keep bread soft for longer:

• More water and fat in the dough slow down recrystallisation, which is why brioche and soft sandwich bread stay fresh longer than a lean baguette.
• Enzymes such as certain amylases trim the starch chains so they crystallise less easily.
• Emulsifiers bind to starch and get in the way of crystal formation.
• Sourdough fermentation and whole-grain flours also tend to slow staling.

A baguette, with its thin crumb and little fat, is famously best within hours of baking.

In short

• Bread goes stale mainly because starch recrystallises (retrogradation), not just because it dries out.
• Amylose sets within hours; amylopectin crystallises over days and firms the crumb.
• Moisture moves from the crumb to the crust, making the crust leathery.
• Staling is fastest in the fridge and almost stops in the freezer.
• Reheating melts the crystals and refreshes bread temporarily.