Cooking science

Starch retrogradation

Retrogradation is gelatinised starch slowly putting itself back in order as it cools and stands. It is why day-old bread is firm, why cold rice is hard, and why a loaf in the fridge goes stale faster than one on the counter.

The slow reordering

Gelatinisation destroys the ordered structure inside a starch granule and releases long polymer molecules into the surrounding water. Those molecules are not stable in that free state: over hours and days they gradually re-associate with each other into ordered, crystalline regions again.

As they do, they squeeze out the water they were holding and form a firmer, more rigid network. The bread feels drier and harder even though very little water has actually left it — which is the counter-intuitive part, and the reason wrapping bread does not stop staling.

It happens fastest at cool temperatures above freezing, which is why the fridge is the worst possible place for bread. It happens very slowly below freezing, which is why the freezer is the best.

What it explains

Stale bread

Firm, dry crumb after a day. Mostly retrogradation, not moisture loss.

Why the fridge stales bread faster

The temperature range just above freezing is where retrogradation runs fastest.

Why the freezer works

Below freezing the polymers cannot move enough to reorder. Frozen bread is close to fresh when thawed.

Why toasting revives bread

Heat reverses some of the reordering. It is temporary — the bread firms again quickly once cool.

Cold rice and potatoes

The same process. Chilled rice is firm and separate, which is exactly why it makes better fried rice than fresh.

Why fat and sugar slow it

Both interfere with the polymers re-associating, which is why enriched breads and cakes stay soft far longer than a lean loaf.

What to do about it

• Freeze bread, sliced. Toast from frozen. This is by a distance the best method and almost nobody does it.
• Never refrigerate bread. It is the fastest way to stale it.
• Keep a cut loaf cut-side down on a board, or in a paper bag, at room temperature for a day or two.
• Refresh a stale loaf by spraying it lightly with water and putting it in a hot oven for a few minutes. It genuinely works, and it works once.
• Use enriched doughs where keeping matters. Brioche and milk bread stay soft for days.
• Use stale bread deliberately — breadcrumbs, croutons, panzanella, bread sauce, French toast, ribollita. It is better for all of them than fresh.

Two starches, two speeds

Starch is a mixture of two polymers and they retrograde at very different rates, which explains a good deal of what a cook observes.

The straight-chain fraction reorders within hours of cooling. That is why bread firms noticeably on the day it is baked, and why a loaf is never quite as soft in the evening as it was at lunchtime.

The branched fraction reorders over days, slowly and steadily. That is the slow decline of a loaf across a week, and it is also the part that heat can most readily reverse — which is why a stale loaf revived in the oven genuinely comes back.

Rice and potatoes follow the same pattern. Chilled rice firms in hours and then changes little, which is why day-old rice is so much better for frying than fresh, and why potato salad made with warm potatoes has a different texture from one made with cold.

Common questions