Cornstarch is made from the endosperm of corn grain. It is primarily used as a thickener for sauces and gravies. Corn starch contains a high percentage of the starch amylose, which allows it to form gels. In smaller quantities, amylose is unable to gel the entire sauce, so it functions as a thickener. However, when enough cornstarch is added in the presence of heating the mixture becomes a gel through a process known as gelatinization.
Because of its lack of protein, cornstarch is a better thickening agent than flour. It requires half as much cornstarch as flour to thicken a mixture to about the same consistency as flour. Cornstarch is added to cold water before being added to the mixture being thickened because it stops the cornstarch from forming clumps.
Cornstarch is also used in to thicken chocolate milk, as an anti-caking agent in powdered sugar and in baking powder to separate the acids from the base. It also has medical and industrial uses.