Starch is a white, powdery substance found in the living cells of green plants. It can be found in the seeds of corn, wheat, rice, and beans and in the stems, roots, and tubers (underground stems) of the potato, arrowroot, or cassava (tapioca) plants. Starch is a carbohydrate, one of the most important foods. Starchy foods are an important source of energy for human beings and animals. When starch is digested in the body, energy is directly obtained from it.
During photosynthesis (the food-making process in green plants), the energy of sunlight changes water and carbon dioxide into glucose and oxygen. Plant cells can quickly convert glucose into starch. Tiny starch granules (grains) are formed in most green leaves during the day. At night, the starch is converted back to sugars, which then move to the root, stem, seeds, fruit, and other parts of the plant. The sugar may be used for growth, or stored again as starch.
Use in foods.
Starch or flour that contains starch is often used in cooking to thicken mixtures. The mixtures usually become pasty or jellylike. Uncooked starch does not dissolve in water. But when rice, macaroni, and other starchy foods are cooked, the starch granules swell and absorb water. This property of starch is called gelatinization. Cooked, gelatinized starch is easily broken down in the body by digestive enzymes (chemicals). However, uncooked starch is too insoluble to be digested easily.
During the cooking of some foods, the starch may change into other substances. For example, slightly scorched starch becomes dextrin, a sticky carbohydrate used as glue on envelopes. During breadmaking, a small amount of starch becomes the sugar maltose. Maltose is fermented by yeast and changed into carbon dioxide and alcohol. The carbon dioxide forms bubbles in the bread dough and makes it rise.
Chemists use iodine to test for the presence of starch in food. When a small amount of iodine is added to a starch solution, it becomes blue-black. Under the microscope, starch appears as tiny granules. Cornstarch granules are rounded, irregular polygons (many-sided figures) about 10 to 20 microns in diameter. Potato starch granules are oval and may be more than 100 microns in diameter. Rice starch has tiny granules about 3 to 5 microns in diameter. With experience, a person can identify a starch by how it looks under a microscope.
Industrial uses.
Starch has many uses in industry. It is used to size (stiffen) weaving yarn and to finish cloth. Starch gives strength and a smooth, glossy finish to high-quality paper. Starches are also used in making pasteboard, corrugated board, plywood, and wallboard. A starch called amylopectin is produced from waxy maize (a kind of corn). Amylopectin produces clear and fluid pastes.
To manufacture cornstarch, corn is soaked in warm water and sulfur dioxide for two days. The softened kernels are torn apart and the germ (part of the inside) is removed. The kernel fragments are then ground and screened (sifted) down to starch and gluten (proteins). The starch is then filtered, washed, dried, and packaged. Similar processes are used to manufacture starch from waxy maize and sorghum.
To make potato starch, the potatoes are washed and ground, and the starch is separated from potato fibers by screening. After further separation, the starch is washed and dried. Arrowroot and tapioca starch may be produced by similar methods.
Wheat starch can be manufactured by kneading (mixing) wheat flour into a dough. The starch is washed out of the sticky mass by a stream of water.
Rice starch is made by soaking the grain in an alkaline chemical, which dissolves the gluten but not the starch. The starch is then separated and washed.
