Have you ever read a recipe that instructed you not to over mix your dough or batter and thought, "Well duh!" only to later wonder why it said that? What would happen? You see this in so many recipes you might think the universe will implode if stir your muffin batter too long. That probably won't happen, but let me tell you what will.
Our story begins with gluten. I imagine gluten will come up time and time again here in Food Science 101, because it is such an important part of baking. I will spare you all the organic chemistry details and summarize for you. You're welcome. Gluten is a protein that develops by mixing flour with water. It forms an elastic network that allows baked goods to expand, which is essential in proper raising.
Obviously, some gluten is needed in baked goods. But mixing doughs or batters too much through either stirring or kneading can cause excess gluten formation. An elastic network is good, but you don't want to chew your way through too much of it!
How exactly does over mixing or over kneading cause problems in your favorite baked treats? Let's break it down:
Muffins: Since gluten is formed by mixing water with flour, too much mixing can cause too much gluten formation. Again, the elasticity of the gluten causes rising. In a muffin batter, the exterior proteins bake first, and too much gluten causes the interior of the muffin to continue expanding which creates tunnels on the inside the muffin and a tough texture. When mixing muffin batter, it's okay to have a few lumps!
Biscuits: Biscuits rely on carbon dioxide formation to rise. Over working biscuit dough causes not only too much gluten formation, but it also allow the carbon dioxide gas to escape. This leads to toughness and flat biscuits.
Bread doughs: Here's the other side of the gluten coin; if kneaded too long, the gluten strands in bread doughs can actually break apart. That means not enough gluten. Because gluten forms the elastic network that holds the dough together, this causes the dough to lose its elasticity and results in a lumpy, sticky dough that won't properly expand.
No, over mixing will not end life as we know it, but it will result in a less-than-perfect baked good. And I want nothing but the best for you! Happy baking!