Stiffener is a term used to describe a substance that is added to a material to increase its stiffness or rigidity. STARCH is a common stiffener, and is often used in the production of paper, textiles, and adhesives. It is also used as a thickening agent in food products such as sauces and gravies. STARCH is composed of long chains of glucose molecules, and is produced by plants as an energy storage molecule.