Starch, fiber, and glycogen are the polysaccharides most commonly found in foods.
The glucose derived from photosynthesis in plants is stored as starch. As a plant matures, it not only provides energy for its own needs, but also stores energy for future use in starch granules. Microscopic starch granules are found in various foods such as rice, tapioca, wheat, and potato. A cubic inch of food may contain as many as a million starch molecules. Amylose and amylopectin are the two major forms of starch found in these granules. The glucose molecules in both of these starch molecules are joined together with a bond (alpha-1, 4) that is capable of being digested by human enzymes. Amylose is a straight-chain structure of repeating glucose molecules, while’ amylopectin is highly branched (every 15 to 30 glucose units). The majority of starchy foods in their natural state usually contain a mixture of about 75 percent amylopectin and 25 percent amylose.
The concentrations of amylose and amylopectin in a solution determine the starch’s ability to hold water. The higher the amylose content, the more likely the starch will gel (form a solid structure) when mixed with water and heated. Cornstarch is high in amylose, while potato starch and tapioca are high in amylopectin, so cornstarch will form the gels needed in custards, gravies, and other foods better than tapioca starch. Heat, enzymes, and acid are used to break starches down into smaller, sweeter segments called dextrins. The sweeter taste of toasted bread, compared to its untoasted counterpart, comes from the dextrins formed in the toaster.