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?
Short-grain rice exudes more starch, making it sticky. Sushi cheft prefer short-grain rice: its stickier texture also makes it easier to eat with chopsticks.
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.
Creamy with a "bite" is the hallmark of great risotto. Cooking arborio rice in a little liquid makes the starch granules on the outside of the rice grains pop, while the center granules only swell. The popped granules exude starch, making the dish creamy, while the unpopped granules keep the centers of the grains firm.
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.