Donut shops are often the social center of a neighborhood, a place to linger over coffee and chat with friends. Making your donuts at home means losing the social time, but eating them while they're fresh and hot is pretty good compensation. Commercial donuts are usually yeast-raised, but old-fashioned cake donuts use baking powder. If you happen to run out of baking powder, you can save a trip to the market by substituting baking soda.
Baking Powder vs. Soda
Baking powder and baking soda both do the same thing in your baked goods. As the batter heats up, they both create bubbles of carbon dioxide gas that are trapped in the batter, making it light and fluffy. To make the carbon dioxide, baking soda requires moisture and an acidic ingredient to react with. Baking powder is nothing more than baking soda with dry acidic ingredients added. When it's moistened by the batter and heated, it creates a similar reaction. To substitute soda for baking powder you simply need to combine it with a suitable form of acidity.
If you have a set of small measuring spoons, it's easy enough to make up a direct substitute for your baking powder. For every teaspoon of baking powder called for in your recipe, you'll need one-quarter teaspoon of baking soda and a half-teaspoon of cream of tartar. You'll also need to add a quarter-teaspoon of cornstarch, which keeps the soda and cream of tartar from reacting too quickly. Mix the corn starch with the cream of tartar first, then add the soda.
Unfortunately, not everyone keeps cream of tartar on hand, which makes it problematic as a substitution. Another alternative is to use baking soda and rely on acidic dairy ingredients to provide the necessary reaction. Use a quarter-teaspoon of soda for every teaspoon of baking powder called for in your donut recipe. To provide the acidity, substitute a half-cup of buttermilk, sour milk, plain yogurt or kefir for the same amount of regular milk. The acidity in that much cultured or sour milk, combined with the quarter-teaspoon of soda, will replace one teaspoon of baking powder.
Lemon Juice or Vinegar
If you don't have buttermilk or plain yogurt on hand either, you can fall back on lemon juice or vinegar to provide the acidity you need. For every teaspoon of baking powder, you'll need to use one-quarter teaspoon of baking soda and approximately 1 1/2 teaspoons of lemon juice or vinegar. You can add them along with the other wet ingredients or stir the lemon juice or vinegar into the milk you use for the recipe. The effect is much the same as adding soured milk or buttermilk.