Word of the Week: Retrogradation

The secret life of starch.

Published Mar. 23, 2017.

Working at the intersection of food and science, we come across a LOT of scientific jargon and terminology, from chemical processes (see: nixtamalization), to physical phenomena, to species names (see: Aspergillus oryzae). We’ll be sharing many of these fun words with you in this ongoing series, Word of the Week. Get ready to impress at your next cocktail party.

Ever eaten cold leftover rice? Then you, like so many of us, have the imperishable sense memory of retrogradation, the way the grains rattle as you scoop them and grind grittily between your teeth.

Starch—the delicious form in which many plants store energy—is composed of microscopic round granules, each of which contains two kinds of starch molecules: amylose and amylopectin, with somewhat different properties. Amylose is a long, chain-shaped molecule, and amylopectin is a bushy, branching molecule. The characteristics of different starches—the difference between waxy potatoes and floury potatoes, for example—are often due to their differing ratios of amylose to amylopectin, as well as the size of their starch granules.

When starchy foods—rice, pasta, bread dough—are cooked in the presence of water, all those individual granules of starch absorb water and swell up. The amylose and amylopectin molecules in the granules, formerly clinging together, relax a bit and come apart, allowing water to seep in among them. This causes the food to become softer and more digestible, as the starch and water form a gel. This process is called gelation. (It’s often called gelatinization instead, which makes it sound like there’s gelatin involved. There’s not, just starch and water.) Amylopectin gelates at a lower temperature than amylose, and swells up more, because the molecule’s shape allows it to take in a lot more water.

So, say you’ve made a delicious, fluffy, steamy batch of long-grain rice. Long-grain varieties of rice tend to cook up separate and fluffy, because their starch contains a higher percentage of amylose, which means the granules tend to stay more intact as they swell. When shorter-grain rice cooks, its bushy amylopectin swells up a lot, starting at a lower temperature—earlier in the cooking—and ruptures the starch granules. The loose starch leaks out of the grains and forms a gluey gel, which gives sushi rice its cohesion and risotto its luxurious smoothness.

But you made a lot of rice, and you have some left over. It’ll be great tomorrow, right? You put it in the fridge.

As the rice cools and the hours pass, the amylose molecules that were gelated by the heat start to creep back together, squeezing out some of the water that held them apart, and forming microscopic crystals throughout the grain. The water that was keeping the molecules separate in the gel is bound up inside the amylose crystals.

In the morning, the grains of rice are hard and feel dry. They haven’t actually lost water: It’s just sequestered away in the amylose crystals. The starch has retrograded. (One reason many cooks prefer day-old rice for making fried rice is because, with the moisture locked inside, the surfaces of the grains are nice and dry so they sear rather than steam in the hot wok.)

The same thing happens with all starchy foods. Foods containing less amylose and more amylopectin, like baked potatoes or cold pasta, don’t get as crunchy as our rice, just firmer. Short-grain rice, with its higher ratio of amylopectin to amylose, is still pleasantly chewy the morning after. That’s because when amylopectin retrogrades, which it does more slowly than amylose, it makes softer, less tightly bound crystals.

Retrogradation isn’t just an annoyance; it’s also a useful property of starch, and not just for fried rice. For instance, the injunction to WAIT after you take a fresh-baked, incredible-smelling loaf of bread out of the oven before you cut into it is not just to torment you: The important thing that happens during those agonizing minutes is that the amylose in the bread retrogrades as it cools, turning from a gummy and somewhat formless hot mess into a sliceable, chewable structure.

Over the next few days, of course, the amylopectin in the bread retrogrades, which is part of what makes it turn stale.

Starch in its retrograded form is much less digestible, because the starch crystals can’t be readily broken down by our stomach enzymes, which one researcher has taken advantage of to devise a lower-calorie way of cooking rice. Sudhair James’s method involves cooling cooked rice mixed with oil, which locks away some of the starch in the form of bound-together complexes of retrograded amylose and fat. Those can’t be readily digested, and thus don’t contribute calories to the meal.

Those starch and fat complexes, which lock the starch in its retrograde form, are an exception to a very good rule: Reheating starchy foods, like rice or bread, will reliably cause the starches to re-gelate and become delectably tender once more.

Graphics by Sophie Greenspan.


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