Cupcakes, like a full-sized cake, should be light and airy. A cupcake that is heavy and dense is disappointing, regardless of its flavor or how beautifully it is decorated. Baking soda is a chemical leavener that helps create the soft, even texture you expect from a cupcake. It's an essential ingredient of any cupcake mix, but you have a couple of options if your baking soda box is empty.
What Baking Soda Does
Baking soda, or bicarbonate of soda, is an alkali that reacts with acidic ingredients in the cupcake batter to form carbon dioxide. Gluten proteins in the flour trap the carbon dioxide, allowing the batter to rise. As cupcakes bake, the carbon dioxide escapes and is replaced by air. These air pockets give cupcakes their soft, airy texture.
Packaged Versus Homemade Mixes
Commercial cupcake mixes already have leaveners incorporated into the mix, along with flour, sugar, and other ingredients. You do not need to add baking soda or another leavener when you mix the batter from a prepackaged mix. When you're making a cupcake mix from scratch, however, the leavening agent is crucial. If you want to create a homemade dry cupcake mix to store and bake later but do not have any baking soda, you can mix the other dry ingredients together and store the mix without the leavener. Be sure to note on the recipe or mix packaging that you need to add baking soda when you are ready to bake.
Baking Soda Substitutions
If your recipe calls for baking soda but all you have is baking powder, you don't need to make an emergency trip to the grocery store. Baking powder contains baking soda and an acid to activate the baking soda. You need to use about four times as much baking powder as baking soda to create the same amount of rise in your cupcakes. Keep in mind that recipes developed for baking soda have some other acidic ingredient, such as buttermilk, to activate the baking soda. Using baking powder, which already has an acid, will cause your finished cupcakes to have a more pronounced tart flavor than they otherwise would. To avoid this, substitute a nonacidic ingredient, such as regular milk for buttermilk.
Don't Overdo It
Don't be tempted to add extra baking soda to make your cupcakes even taller or softer. Cake and cupcake recipes are carefully balanced to achieve the right texture without overdoing any element. It takes little baking soda - just 1/4 teaspoon per cup of flour - to make the whole batch of cupcakes rise. Adding extra baking soda will cause the batter to rise faster than the gluten in the flour can keep up, and the cupcakes will fall. The resulting cupcakes will be tough and dense.