Two Kinds of Starch Molecules

The parent plant lays down starch molecules in microscopic, solid granules that fill the cells of the seed storage tissue. All starch consists of chains of individual molecules of the sugar called glucose. But there are two different kinds of starch molecules in starch granules, and they behave very differently. Amylose molecules are made from around 1,000 glucose sugars, and are mainly one extended chain, with just a few long branches. Amylopectin molecules are made from 5,000 to 20,000 sugars and have hundreds of short branches. Amylose is thus a relatively small, simple molecule that can easily settle into compact, orderly, tightly bonded clusters, while amylopectin is a large, bushy, bulky molecule that doesn’t cluster easily or tightly. Both amylose and amylopectin are packed together in the raw starch granule, in proportions that depend on the kind and variety of seed. Legume starch granules are 30% or more amylose, and wheat, barley, maize, and long-grain rice granules are around 20%. Short-grain rice granules contain about 15% amylose, while “sticky” rice starch granules are almost pure amylopectin.