When an American chef makes rice pilaf, she begins with raw long-grain rice. Once cooked, the individual grains remain pleasantly separate, soft but not sticky.

When a Japanese chef prepares sushi, he reaches for cooked short-grain rice. He can easily shape the sticky rice into mounds on which to lay pieces of fish, or he can pack it neatly into sushi rolls.

Switch the two kinds of rice and you’d get pilaf that clumps together and sushi rolls that fall apart.

Before you cook rice, it helps to understand its specific characteristics. The most important factor is the composition of the rice’s starch.

What is starch?

Starch is made through photosynthesis. Using energy from the sun, plants combine carbon dioxide and water into simple glucose, or sugar molecules. These molecules either link into great long chains to make the kind of starch called amylose, or they link into many short, branched chains to make amylopectin. Both kinds of starch are packed tightly together in granules.

All plants contain both types of starch but with different ratios of amylose and amylopectin. Long-grain rice has more amylose; shortgrain has more amylopectin.

Heat makes rice exude starch, some kinds more than others.When you heat starch in a liquid, the molecules of both the starch and the liquid move faster, and water seeps into the granules. As the temperature rises, more water gets in and the granules swell. Somewhere near the boiling point of water, some of the swollen granules pop, and starch rushes out into the liquid. (When you’re making a sauce or a gravy, this is when it thickens.)

In short-grain rice, the starch granules (which are mostly amylopectin) swell and pop at around 160° to 170°F. High-amylose longgrain rice doesn’t finish swelling until about 200°F, meaning that in the same cooking time, it gives off less starch than short-grain does, which means that long-grain rice stays separate. Medium-grain rice is between the two in its starch characteristics.

Rice for risotto has special attributes.Arborio rice, used to make the Italian rice dish called risotto, is a medium-grain rice (though it’s often called short-grain rice because its plump shape after cooking makes it look short). Risotto is cooked by adding a little liquid at a time, which makes its surface granules exude a lot of starch while its center remains firm.

Starch troubles in long-grain rice

As cooked long-grain rice cools, those long amylose molecules move more slowly and bond tightly to each other. Long-grain rice, because it has more amylose than short-grain, can become rock-hard.

Not only does cooked long-grain rice harden, but once cooled, much of its starch becomes insoluble and it won’t soften even if you add liquid. If you reheat the rice, the bonds of amylose will break, and the rice will become soft again. But in dishes where the rice is cooked and chilled but not reheated—as in rice salads—amylose crystallization can be disastrous. You end up with pebble-like rice that isn’t softened even by a vinaigrette.

为了避免这个问题,把米饭和潮湿redients (such as vinaigrette in the case of rice salad) while the rice is still hot from cooking. The additional liquid soaks in to keep the amylose molecules separate so they can’t bond and harden. Alternatively, you can use medium-grain rice, which has less amylose and will not harden as it cools.

Long-grain rice can also cause pudding problems.While you can use cooled and hardened rice in rice pudding, you’ll want to eat it while it’s still warm. Once cooled, the amylose will make the rice hard once again.

Long-grain rice can contribute to another problem in rice pudding—the settling of the rice. If it’s not cooked long enough, or if it has been cooked and cooled, it doesn’t exude starch to aid in thickening. If the custard doesn’t thicken fast enough to suspend the rice, the rice falls to the bottom of the dish.

For this reason, many rice pudding recipes call for shortor medium-grain rice, which, because it exudes starch at a lower temperature, is a safer bet. Many long-grain rice pudding recipes either initially overcook the rice so that it’s very starchy, or they call for enough eggs and a high cooking temperature to help set the pudding quickly.

Choosing the right rice for the dish.

All rice is grouped into three categories: long-grain (the length is at least three times the width), medium-grain (the length is about two times the width), and short-grain (the length is less than two times the width). Longgrain and short-grain rices are not interchangeable; medium-grain rice is similar to short-grain.

Type of rice	Long-grain	Medium-grain	Short-grain
Description	High in amylose. Cooks into fluffy, separate grains.	Lower in amylose than long-grain, but not as low as short-grain. Can be used in place of short-grain.	High in amylopectin and low in amylose. Cooks soft and sticky.
Popular types	Carolina, basmati, jasmine	Arborio, black japonica	Sushi, Spanish, pearl
Good for	Rice pilafs, rice salads (tossed with dressing while warm), white rice side dishes	Rice salads, rice puddings, risottos	Sushi, paellas, stir-fries, rice puddings
Problems	Cooled rice becomes rock-hard and stays hard unless reheated and served warm.	Can clump as it cools.	Can become mushy if cooked with too much liquid.