If my cookie recipe calls for whole eggs, can I get away with just using the equivalent weight of just yolks or whites? By understanding what each part of the egg contributes to our recipe you will be able to decide which way to go.
We can use whites, yolks, whole eggs or a combination. Eggs are proteins. When cooked, the proteins bind together to form the structure ( shape ) of the cookie.
Texture: Egg whites dry things out. Yolks are made up of fat containing lecithin. Lecithin is a natural emulsifier. An emulsifier serves to keep ingredients together that don’t naturally attach together (like fat and water). Yolks add a silky creaminess to a batter. For cookies that you want a crisp outside (whites) and a gooey inside (yolks), a whole egg would be the choice in the recipe.
Spread: A dough with high acidity, will cause the egg proteins to set up faster, hence spread less. For example, if you make a cookie using cocoa powder (acidic) and replace your cocoa powder with a dutch process cocoa (alkaline), you will find the batter to be much looser with the latter and take forever to set up due to the lack of acidity.