Why are some cookies then and crisp and others, thick and cake-like? It’s a structural question and cookie structure is primarily determined by:
- The sugar
- The flour
- The eggs
There are other contributing factors, of course. The amount and type of fat will affect structure. (A cookie made with shortening instead of butter will tend to be crisper.) A non-stick, well-greased sheet will cause more spread. But if you understand the function of sugar, flour, and eggs, you’ll largely understand what is going on in your oven.
Sugar melts. Sugar in your recipe acts like a liquid in the oven. The more sugar, the more spread if all else is equal. Adding more flour to your cookie will absorb some of that melting sugar and reduce spread.
Sugar also caramelizes in the oven. As it caramelizes, it gives an attractive brown tone to the cookies. (Extra baking soda will also contribute to browning.)
Flour affects structure in three ways. When the starches bake, they set up and create structure. Flour absorbs moisture and reduces spread. Gluten strands in the flour create a chewy texture. Generally, more flour—especially in conjunction with eggs—makes for a more cake-like cookie.
Does it matter what kind of flour you use? Yes. Most cookies do just fine with all-purpose flour and we develop most of our cookie recipes with all-purpose flour. Pastry flour is low in the proteins that form gluten and without the gluten, cookies are very tender and crumbly. If this is the type of cookies that you would like, consider using pastry flour.
On the opposite of the scale is bread flour. With a higher protein content and hence more gluten strands, cookies made with bread flour are much chewier. When we want a chewy cookie, we often use bread flour. (The addition of oats makes for a chewier cookie also. We often add a few quick oats to the flour for a chewier cookie with more structure.)
Eggs add water, fat, and proteins to your cookie dough. About 3/4’s of the egg by weight is water so adding eggs creates a softer dough. The fat creates more of a pleasing “mouth feel.”
But the protein creates structure. A typical cookie recipe calls for the creaming together of the sugars and fat—butter or shortening. The sugar crystals cut through the fat creating tiny air pockets. The eggs are added and the mixture beat until light and fluffy creating even more air pockets. The flour is mixed in until just combined, not collapsing all the air pockets. When baked, these air pockets expand creating lightness in the cookie. If there is not too much moisture or sugar, the proteins in the eggs coagulate and starches in the flour become firm. If there is enough flour and eggs, the cookies are light and cake-like. If the ratio of sugar to flour and eggs is high, the cookie collapses or partially collapses creating a denser, chewier cookie. If the ratio is even higher, the resulting cookies are thin and crisp-like. In our experience, nothing changes the structure of cookies as much as the addition or deletion of eggs.