Rice grains contain amylose, which is a component of starch (a carbohydrate.) How soft or firm a rice cultivar is when cooked is influenced by its amylose content.
Glutinous rice varieties are distinguished by their glossy, waxy grains that when cooked do not expand in volume, are sticky, moist, and remain firm. This is due to their low amylose content. High amylose varieties are nonglutinous. Their grains expand considerably in volume to become extremely flaky. When cooked, their texture is dry and less tender, with a tendency to harden when cooled. Rice varieties with an intermediate amylose content are tender and moist upon cooking and do not harden when they cool.
Rice varieties are ranked according to the following amylose content scale:
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In the U.S. rice is classified by its kernel length into market classes. Typically, most long grain cultivars are intermediate in their amylose content. Specialty cultivars have a high amylose content. Cultivars considered short grain or medium grain have a low amylose content.