What is the fuction of Starch

Starch is the main thickener in gravies, sauces and puddings. Starch absorbs water, and  becomes a gel when cooked. As the starch swells up with water, the amylose leaches out, and the amylopectin forms the gel. Some starches have higher amylopectin content, and make better gels than those containing lots of amylose. Remember, higher the amylose  content lower is the swelling power and smaller is the gel strength for the same starch concentration. As a thickener (as opposed to a gel) it is the amylose that has the main function. The long  water-soluble chains increase the viscosity, which does not change much with change in temperature. Amylose chains tend to curl up into helices (spirals) with the hydrophobic parts inside. This allows them to trap lipids (oils and fats) molecules inside the helix, as well as aroma molecules.