Baking recipes use a number of techniques and ingredients to give you a light, airy end result. Angel food cakes get their delicacy from beaten egg whites, breads are lifted high by yeasts, while cakes and quick breads usually rely on baking soda in some form. Baking powder combines soda with other ingredients to make it more versatile, and self-rising flour has baking powder mixed in at the factory, but neither can substitute directly for baking soda.
Baking Soda 101
Many a childhood science fair project works by combining an acidic ingredient with a base, or alkaline ingredient, and uses the resulting burst of gas to perform a task. That's exactly how baking soda works in your baked goods. It reacts with acidic ingredients such as lemon juice or vinegar, buttermilk, or cream of tartar to produce carbon dioxide gas. Your dough or batter traps that gas, forming small pockets of carbon dioxide that raise your baked goods and leave them with a light and delicate crumb. Baking powder can be substituted for soda, either plain or in self-rising flour, but it's not a direct substitution.
For baking soda to work in your recipe, it needs to be balanced with the right quantity of acidic ingredients. If there's too much or too little acidity, your baking won't rise or brown properly and it might have an "off" flavor. Baking powder minimizes trial and error by combining the baking soda ahead of time with dry acidic ingredients, and a neutral filler such as corn starch to keep the two from reacting prematurely. Soda accounts for about one-quarter of the baking powder's volume, so you'd substitute 1 teaspoon of powder for every 1/4 teaspoon of soda.
Using Self-Rising Flour
Self-rising flour goes one step further in the search for consistent results. It combines all-purpose flour with baking powder and a small amount of salt, all thoroughly mixed together at the factory. It's convenient for bakers, especially novices, because it reduces the number of steps in a recipe. It also improves results, because the ingredients are already thoroughly mixed. Each cup of self-rising flour contains 1 1/2 teaspoons of baking powder. If you're trying to bake a favorite recipe and find you're out of soda, you can simply substitute self-rising flour for the all-purpose flour in your recipe. You'll also need to make a few small adjustments.
If your recipe is designed to use baking soda it contains acidic ingredients such as buttermilk or natural cocoa. Self-rising flour doesn't need the acidity to work, so you can replace those ingredients with neutral equivalents such as plain whole milk or Dutch-processed cocoa. If a faint buttermilk tang is part of the charm of your recipe, you can keep up to a quarter of the original amount for its taste. Cookies, quick breads and biscuits won't brown as well without soda, but you can compensate by increasing the heat of your oven during the last few minutes or by brushing the tops of biscuits and quick breads with milk, cream or a beaten egg.