Vol. VII · Starches and Sugars Plate 3 of 11retrogradation, staling, bread, cold rice, fridge, freezer, reheating, resistant starchMS

Retrogradation.

A loaf goes stale in the fridge faster than on the counter. The cause is not drying but starch quietly re-crystallising, fastest just above freezing.

After starch has gelatinised, its chains are free in the water. As the food cools and stands, they slowly find each other again and line up into crystalline bundles, pushing water out: retrogradation. Amylose (V.3) does it within hours, amylopectin over days. The results are familiar: bread that is firm and crumbly though it has lost little water, cold rice that is hard, a starch-thickened sauce that sets and weeps in the fridge.

Two surprises follow. Retrogradation is fastest at refrigerator temperatures, slower at room temperature and almost stopped in the freezer — so the fridge is the worst place for bread, and the freezer the best. And it is reversible: warming above about 60 °C melts the crystals again, which is why toast, reheated rice and steamed old buns taste fresh for a while. The workshop stores cooked starch at three temperatures.

Below: the fraction of starch re-crystallised over two days at the fridge, the counter and the freezer, and at your temperature (a teaching curve with the published pattern: fastest just above freezing).

What you will learn
Workshop
Teaching model of retrogradation: an Avrami-type curve whose rate peaks near 4 °C and falls at higher and much lower temperatures, following the pattern in bread-staling studies.

Not drying — crystallising

After a day, starch in the fridge has re-crystallised 98% in this model, on the counter 65%, in the freezer 2%: the fridge stales bread about 1.5 times faster than the counter. So keep bread you will eat in a day or two at room temperature, wrapped; freeze the rest, sliced, and toast it from frozen. Rice for fried rice is the exception that proves the rule: day-old refrigerated rice is wanted precisely because its grains have firmed and separate in the wok (XXI.4).

Reheating melts the crystals: bread warmed in a hot oven for a few minutes or toasted is soft again inside, steamed buns and rice recover in the steamer, and a set sauce loosens when warmed. The effect lasts until the food cools and the chains find each other again. Retrograded starch also digests more slowly — "resistant starch" — which is why cooled and reheated potatoes and rice raise blood sugar a little less than fresh ones.

At the stove

Put one slice of the same bread in the fridge, one in a bag on the counter and one in the freezer. After a day, compare them cold, then toast all three. The fridge slice is the stalest cold, and all three recover in the toaster.

In 1852 the French chemist Jean-Baptiste Boussingault showed that stale bread had lost almost no water and that heating it in a sealed tube made it fresh again — the first evidence that staling is a change in the starch, not drying.

Terms

Retrogradation
Gelatinised starch re-crystallising on cooling and standing.
Staling
The firming and crumbling of bread, mainly from retrogradation.
Resistant starch
Starch that resists digestion, including retrograded starch.
Refreshing
Reheating stale starch foods to melt the crystals.
Staling is starch re-crystallising, fastest just above freezing. Keep bread on the counter or in the freezer, never the fridge, and reheat to refresh.

Sources

  1. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
  2. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.