How does bread go stale?

Thanks for that excellent question, reader Cindy! In fact the word “stale” is akin to “aged”, but in a good way. “Ripened” is more like what it means. We moderns, addicted as we are to perfectly fresh bread, would scarcely think of applying a world like that to a past-its-prime loaf. But the ancients (and not-so-ancients) did, mostly because they had no alternative.

But what is staling exactly? Most people think of it as the “drying out” of bread, but that’s not the half of it. If it were, fresh bread kept in a tight sheath of plastic wrap would never go stale. I think we’ve all experimented with double and triple layers of Reynold’s Wrap long enough to know what a fool’s paradise that is. So what is it with bread that it starts to harden the moment it’s removed from the oven? It all has to do with the behavior of starch molecules.

Starch is made up of two base components, both of them long-chain sugars, also known as carbohydrates: amylose and amylopectin. Both are made up of many many units of glucose, and that makes them similar. Yet those units of glucose are configured differently, and that causes them to behave differently. Amylose is built like a narrow bundle of reeds, with all of its glucose units (up to 1,000 or so) arranged in straight, parallel chains. Amylopectin, in the other hand, looks more like a shrub, with its glucose units (up to 20,000 of them) going off every which way.

Hundreds or thousands of both make up a typical starch “granule” (or single grain of flour) with the long straight amylose in nice orderly layers (starch crystals) and the amylopectin in big bushy heaps. Add water and heat to that scenario (dough making and baking) and things start to change. The bonds that keep the carbohydrate molecules bunched together weaken, and water molecules start getting in between them. The starch granule swells and even sheds some of its starches (gelates), which is how the “structure” of bread is created.

The process continues until the finished bread is taken out of the oven, at which point the process starts to reverse itself. The carbohydrate molecules start to reorder themselves. It doesn’t happen quickly, but it does happen inexorably. The carbs, especially the amyloses, become re-attracted to one another and begin stacking themselves back up again in neat piles, making hard crystals once again. The water molecules are forced out from between them, and shortly evaporate.

So you see, bread goes stale not just because it’s dryer, but because its structure is also harder. If you own a microwave you’ve no doubt noticed that you can re-gelatinize starch to some extent with a little fast heat. But with much of the water already gone, the effect is fleeting, barely enough time to butter that scone and stuff it into your mouth!