Sugar

Sugar makes several contributions to cookie structure and texture. When creamed with the fat, or beaten with egg, it introduces air bubbles into the mix and lightens the texture. It competes with the flour starch for water, and raises the starch gelation temperature nearly to the boiling point: so it adds hardness and crispness. A large proportion of pure table sugar, sucrose, contributes to hardness in another way. The proportion of sugar in some cookie doughs is so high that only about half the sugar dissolves in the limited amount of moisture. When the dough heats up during baking, more sugar can dissolve, and the added liquid causes the cookie to soften and spread. Then when the cookie cools, some of the sugar recrystallizes, and the initially soft cookie develops a distinctive snap—a process that may take a day or two. Other forms of sugar—honey, molasses, corn syrup—tend to absorb water rather than crystallize (chapter 12), so when heated they form a syrup that permeates the cookie, helps it to spread, and firms as it cools, making it moist and chewy.