Flour and Fat Form Body

• Flour gives a cookie its weight. Thin, crisp cookies contain very little flour, whereas thick, dense cookies contain a lot of flour. The ratio of fat-to-flour determines how much cookies spread as they begin to bake. Rising begins when the water in the butter or shortening turns to steam, when the internal temperature of the dough reaches 212 degrees Fahrenheit. Steam rises out of the dough, pushing the remaining ingredients up in the process.

Leavening Agents Give Rise

• The rising that the steam causes isn’t sufficient to give texture to cookies or other baked foods. It’s so slight that it would hardly be noticeable in a baked and cooled cookie. Rising is primarily achieved by a leavening agent, a combination of an acid and an alkali that creates carbon dioxide gas bubbles when you expose them to moisture and heat. The most common leaveners in cookie recipes are baking soda and baking powder. The carbon dioxide gas bubbles create tiny spaces in the cookie, which is why a baked cookie is less dense than unbaked cookie dough.

Eggs Set the Shape

• The body of the cookie would collapse in on the spaces that the carbon dioxide bubbles created -- or “fall” -- if there wasn’t another ingredient in the dough that could maintain a solid structure around the spaces. This is the vital role in rising that eggs play: Egg proteins act like the frame of a building by bonding to the flour particles and coagulating. As the gas bubbles burst and dissipate, the proteins hold the cookie in the shape it had when the bubbles were still intact. When a baker says that a cookie has “set,” he is saying that the proteins have coagulated and fixed the cookie in the shape that the leavening agent created.

Other Eggs-elent Roles

• Eggs play a couple of other roles in cookie baking. Eggs contribute moisture to cookie dough, while the yolks add a little bit of fat and flavor as well. The protein structure that holds the cookie’s shape is also responsible for making the cookie chewy.