Starch: Effect of Cooking

Cooking affects starch because in dry starch the molecular chains are folded tightly together. When water penetrates the starch the chains unfold and disperse, causing expansion of the granules. The molecules, forced into rapid movement by the hot liquid, collide and make the mixture viscous. This is noticeable when a white sauce thickens. There has to be enough starch in the mixture to bring the molecules close enough together for thickening to take place. Stirring is necessary to prevent a sauce from sticking, which is what happens when it dries and burns on the bottom of the pan. But excessive stirring, especially after a sauce has thickened, tears the molecular chains away from each other and reduces viscosity. Certain starches are particularly badly affected by stirring, for example tapioca.