Starch is a complex carbohydrate which is insoluble in water. Starch (in particular cornstarch) is used in cooking for thickening sauces. In industry, it is used in the manufacture of adhesives, paper, and textiles.
In biochemistry, starch is a mix of two polymeric carbohydrates (polysaccharides) called amylose and amylopectin, in which the monomers are glucose units joined to one another head-to-tail forming alpha-1,4 linkages. The overall structure of amylopectine is not, however, simply a linear polysaccharide chain, since occasionally, two glucose units are joined to one in an alpha-1,6 linkage, forming a branch point. Structurally, the starch forms clusters of linear polymers, where the alpha-1,4 linked chains form linear columns of glucose units branching at the alpha-1,6 links. The relative content of amylose and amylopectin varies between species. The primary function of starch is to act as an energy storage molecule for the organism. In plants starch for storage is produced in specialized cellular organs, amyloplasts.
Starch is often found in the fruit, seeds, or tubers of plants. The four major resources for starch production and consumption in the USA are corn, potatoes, rice, and wheat. As an additive for food processing you will find; arrowroot, guar gum, locust bean, and tapioca commonly used as well. Commonly used starches around the world are: arracacha, buckwheat, cassava, konnyaku, kudzu, oca, and sago.
When a starch is pre-cooked it can then be used to thicken chilled foods. This is referred to on packaging as modified food starch. Agar, carrageenan, gelatins and pectins are used in very much the same way.
Clothing starch or laundry starch is a liquid that is prepared by mixing a vegetable starch in water (earlier preparations also had to be boiled), and is used in the laundering of clothes. During the 19th century and early 20th century, it was stylish to stiffen the collars and sleeves of men's shirts and the ruffles of girls' petticoats by applying starch to them as the clean clothes were being ironed.
Aside from the smooth, crisp edges it gave to clothing, it served a practical purpose as well. Dirt and sweat from a person's neck and wrists would stick to the starch rather than fibers of the clothing, and would easily wash away along with the starch. Then, after each laundering, the starch would be reapplied.
Animal starch is the common name of glycogen. It is not the same as ordinary starch.