When you’re reading through recipe instructions, particularly for baked goods, you probably come across the phrase “do not overmix” fairly often. The notion of “overmixing” can be confusing if you’re just getting into baking – after all, cookie dough is either mixed, or it isn’t, right?
The final stage of making a cookie dough or cake batter often involves stirring flour, or a flour mixture, into wet ingredients. When the flour is exposed to liquids and stirred around, the gluten (protein) in the flour starts to develop into a network that will hold whatever you’re baking together, giving cookies, cakes, etc. their structure. Gluten can also make baked goods tough if there is too much of it in the dough/batter, and excessive mixing of the dough can develop the gluten to this point.
So when a recipe instructs you not to overmix, what it means is that you should just do the minimum amount of mixing necessary to make a uniform dough. A good rule of thumb is to stop mixing when no streaks of flour remain in your mixing bowl, or if you’re going to be adding chocolate chips or fruit into your mix, you can stop when a few small streaks of flour remain, since you’re going to give the mixture a few extra turns when you stir in your add-ins.