Very few people get in a snit over cookies. But even though most people like them, everybody has a different idea about what makes a perfect cookie. Should it be chewy, thick, crunchy, or crisp? It turns out there is some fascinating food chemistry behind cookie texture. A lot depends on how much a cookie spreads, and when it sets. When you know how different fats, proteins, and sugars act in cookie dough, you can mix and match for your perfect cookie texture.
Imagine you’re making cookies. You put nice balls of dough on a cookie sheet, pop them in the oven, and a few minutes later these round, flat things come out. What happened to the balls of dough? They have spread as various ingredients liquify and interact. One of the biggest factors in the chewy or crispy fate of a cookie is the rate at which the cookies spread, as well as when they start spreading. The bottom line is, the more spreading that happens, the thinner and crispier the cookie, and vice versa.
So the cookies are in the oven, spreading out into cookie shapes. Now, when the cookie sets has a lot to do with the final texture of the cookie. Setting is baking terminology for when the proteins from the eggs and flour gluten physically rearrange before rejoining in a new overall structure. Once the proteins have settled in a new framework, the cookie has set. The cookie dough is no longer cookie dough, but an underbaked cookie. The faster a cookie sets, the less it spreads, making a thicker, denser cookie, and vice versa.
How to control rates of spreading and setting? Keep an eye on the type and amount of fat, protein, and sugar you use. Simple, right? Except there are a myriad of options and possible combinations.
To promote spreading, use a solid fat in your cookies, such as butter or margarine. Solid fats tend to melt at lower temperatures, causing cookies to start spreading earlier and spend more total time spreading out into a thin, crispy cookie. It takes a higher temperature to get the molecules in liquid fats, such as vegetable oil or melted butter, up and dancing. Cookies containing liquid fats start to spread at higher temperatures, after more time in the oven. Starting the spreading process late means they do less of it, and end up thicker and denser.
The type of sweetener you use also affects the final cookie texture. Sugar is hydroscopic, meaning it readily absorbs water, both liquid and from surrounding air. When more water is absorbed, less of it evaporates during baking, leaving a moister, chewier cookie. Some sweeteners are more hydroscopic than others, however. White granulated sugar turns out to be not very good at absorbing water. Honey, maple syrup, molasses, or brown sugar are examples of more hydroscopic sources of sugar. Brown sugar, much more hydroscopic, will keep absorbing water even after baking, keeping cookies moist and ready to go in your mouth. Because cooking with white sugar leaves a fair amount of water moisture still in the cookie dough, this also helps it spread more than dough made with brown sugar.
Fats, sugars, and… proteins! The two main protein sources in cookies are flour and eggs. Flour provides the protein gluten, which binds ingredients together and provides chewiness. Different types of flour have different gluten content, with cake flour on the low end of the scale and bread flour at the high end. Eggs, or more specifically the egg yolk, also provide binding proteins. The higher the protein content (from gluten and eggs or egg yolks) in cookie dough, the less it will spread, as the proteins reassemble and latch onto each other into a new solid framework. No matter how much protein you have, high amounts of fat and sugar will get in the way of the proteins finding each other, and the cookie setting. Reducing amounts of fat and sugar in the recipe helps cookies set more quickly- the proteins can find each other faster!
The last key ingredient in cookies is… air. How much air is incorporated into the dough affects the final density and the crispiness of the cookie. Liquid fats can’t trap as much air into the dough as solid fats can, especially when the recipe calls for softened butter to be creamed with the sugar, incorporating even more air. When cooked, this air steams up and creates air pockets in the cookies as they set. Without the air, the cookies end up much more dense. Butter and margarine also contain a fair amount of water, which, once it evaporates off as steam, leaves air pockets and a crispy cookie. Egg whites, which are approximately ninety percent water, and only ten percent protein, also add air pockets as their water evaporates, along with some extra protein. Beating them before adding adds even more air, resulting in cookies of the crispy and not-so-dense variety.
Salivating for a rich, moist, chewy, thick, dense cookie? Pull out the brown sugar and melt the butter or use vegetable oil. Find the bread flour, maybe add an extra egg or egg yolk. Don’t use very much of the sugar or fat. Or would you rather a crispy, crunchy, thin, not-so-dense cookie? Go for the solid fats and whip them up with lots of air and sugar. Get some cake flour, and add an egg white beyond what the recipe calls for.
Of course, rarely do all of the stars line up and you have several cups of both brown sugar and bread flour in your pantry, in addition to remembering to melt the butter. But now you know why cookie recipes say to cream the butter and the sugar, why they often call for some white sugar and some brown sugar, and why the number of eggs varies with every recipe. You’ve always wondered, right? Now you can be your own potions master, and whenever you’re hankering for the perfect cookie, you know how to make it. And you’ll know why it works.