Starch

Starch is a polymer of only glucose units. Starch is the main food reserve in plants and consequently the major carbohydrate of the human diet. Common food starches are derived from seeds (e.g. wheat, maize, rice, and barley) and roots (e.g. potato and cassava).

Two kinds of covalent linkages occur between the glucose molecules in starch, giving rise to two kinds of glucose chains. Amylose is composed of straight chains, with carbon 1 of one glucose linked to carbon 4 of the next glucose. This is called alfa-1,4 linkage*. Amylopectine is made up of straight chains like amylose but has occasional branches where the carbon 6 of a glucose is linked to the carbon 1 of another glucose ( alfa -1,6 linkage**).

*see glossary: linkage

**see glossary: glucosidic bound

It was long thought that starch is completely digestible. However some starch (in raw potato's and unripe bananas for example) occurs in a partially crystalline form in granules. This crystalline starch is resistant to digestion by pancreatic enzymes. The crystalline structure is lost when starch is heated in water, thus permitting digestion to take place. Recrystallization takes place to a variable extent after cooking and may result in higher resistant starch values than originally present in the raw food. As a result of food processing (cooking, cooling, freezing), resistant starch values in a food may be elevated up to 20%.