Chemically, starch is composed of two different molecules, amylose and amylopectin. In amylose, the glucose molecules are linked in a "linear" fashion; however, the tetrahedral chemistry of carbon (and the bond angles that result from this chemistry) gives amylose an overall spiral shape. Amylopectin, on the other hand, has a linear arrangement of glucose molecules that includes, at regular intervals, a different kind of linkage between two adjacent glucoses. This different linkage results in the formation of a branched structure and an overall treelike shape for this molecule. Plant starch is typically 20 to 30 percent amylose and 70 to 80 percent amylopectin. The classic test for the presence of starch is reaction with iodine. If starch molecules are present in a substance, the addition of iodine yields a deep blue color, which results from I2 being trapped inside the spiral structures of amylose molecules.
Starch molecules are broken down by enzymes known as amylases. The digestibility of a specific starch is influenced by its physical form. In plants starch is present in microscopic granules, which impair the enzymatic digestion of starch molecules obtained from plants. Cooking starch-containing items results in the hydration of starch molecules and the swelling of starch granules, increasing the rate and enhancing the enzymatic breakdown of starch. Amylases also convert starch to glucose.
