Starch is a natural component of many plants and acts as a thickener when cooked with water. It is one of the three forms of carbohydrate along with simple sugars and fiber. Like others forms of carbohydrate, starch contains the elements carbon, hydrogen and oxygen. Food sources of starch include cereals and grains, starchy vegetables and some beans. Potatoes, corn, rice, tapioca and wheat are all high in starch. Some fruits contain small amounts of starch but are generally much higher in sugar than starch.
A starch molecule is a long chain of glucose units. A single glucose unit contains six carbon atoms, twelve hydrogen atoms and six oxygen atoms. Starch is made up of two types of chains. Amylose refers to a straight chain of glucose units, while amylopectin refers to a branched chain of glucose units. Although the basic chemical structure of starch contains only carbon, hydrogen and oxygen, some starches have other elements associated within the starch structure. One example is potato starch, which contains phosphorus.
In cooking, starch acts to thicken sauces, stews, gravies and other water-containing dishes as they are heated. Thickening occurs because starch is able to bind water. As starch is heated and mixed with water, the starch granules burst, take up water and thickening results. The amount of swelling and thickening that occurs, as well as how the thickened mixture reacts when cooled, depends on the source, size and shape of the starch granules.
Starch from plants is purified and sold for cooking and baking. Common cooking starches are cornstarch and tapioca. Potato starch and arrowroot starch are also available. Wheat flour contains considerable amounts of starch and can be used in place of refined starches. Each type of food starch thickens at a different temperature and you will notice that different amounts of each are required to get the desired thickening.
Cornstarch and flour are perhaps the most common starches used for thickening. The main difference between wheat flour and cornstarch is that while cornstarch is all starch, flour contains ten to twelve percent protein in addition to the wheat starch. Cornstarch tends to be a faster-acting thickener with less flavor and less impact on the appearance of the finished dish than wheat flour.
Even though starch is a more complex molecule than simple sugars like glucose and fructose, eating starch doesn't always result in a smaller increase in blood sugar. Starch is broken down quickly and absorbed in much the same form as glucose and sucrose. Resistant starch, however, is a type of starch that is not digested like typical starch. It is found in beans, some grains and some manufactured foods like fiber drinks. Resistant starch is considered to be more like fiber than starch because of its poor absorption and its potential health benefits.