Retrogradation

Retrogradation is what happens when cooked starch cools and its molecules slowly line up again into firmer, more ordered bundles. It is the main reason bread goes stale, cold rice turns hard and grainy, and leftover potatoes and pasta firm up. It is the partial reverse of gelatinization, when starch first swells in hot water.

Why it works

When starch cooks, its grains swell and its long molecules spread out and trap water. As the food cools, the molecules begin to bond to each other again and squeeze out some of that water. The food feels firm, dry and crumbly even though it has not lost much moisture. This happens fastest at fridge temperatures, a little above freezing, and much more slowly at room temperature or in the freezer.

How to use it

• Bread: store it at room temperature in a bag or bread box for 1 to 3 days, or slice and freeze it. Do not keep it in the fridge, where it stales fastest. Gentle heat reverses staling for a while, so toast slices or warm a loaf in the oven at 180°C (350°F) for 5 to 10 minutes.
• Fried rice: day-old, chilled rice has firmed and dried, so the grains stay separate in a stir-fry instead of turning mushy.
• Potato salad and pasta salad: firmed starch holds its shape when dressed.

Common mistakes

• Keeping bread in the fridge.
• Expecting reheated stale bread to stay soft. It firms again quickly as it cools, so eat it warm.
• Leaving cooked rice out to cool slowly. Rice can carry spores of bacteria that survive cooking. Cool it within 1 hour, refrigerate it, and use it within 24 hours, as the UK Food Standards Agency advises. See cooling food safely.
• Reheating rice more than once. Reheat it once, until steaming hot all the way through.

In recipes

Recipes do not usually name it, but "use day-old rice", "use stale bread" for bread pudding, panzanella or breadcrumbs, and "chill the potatoes before dressing" all rely on retrogradation.