Baking soda and baking powder are used in baked goods to expand the air bubbles that are created when you cream butter and sugar or whip eggs. They do not make the bubbles themselves. Both of these leaveners depend on chemical reactions between alkali and acidic ingredients to create the carbon dioxide that leavens the batter or dough. And although their names are similar, they are not interchangeable.
Baking soda (bicarbonate of soda), an alkali, reacts with the acidic ingredients in a recipe, such as buttermilk, yogurt, sour cream, molasses, and brown sugar, to form carbon dioxide that expands the air bubbles in a batter. Batters and doughs with baking soda should be baked immediately after mixing, before the carbon dioxide can dissipate.
Baking powder does not rely on a specific combination of ingredients for its rising power. It is made from alkaline baking soda mixed with acids (often aluminum sodium sulfate and monocalcium phosphate) and a bit of cornstarch. Most baking powders are called double-acting because they are first activated when the batter is moistened, and then again from the heat of the oven. Some bakers get a metallic aftertaste from commercial baking powders made with the commonly used aluminum sodium sulfate, but there are brands without it, so be sure to check the label.
You can make your own baking powder. For 1 tablespoon baking powder, mix 2 teaspoons cream of tartar and 1 teaspoon baking soda. This mixture will be activated as soon as it is moistened, so get the batter into the oven without delay, before the carbon dioxide is exhausted.
Baking soda has a shelf life. Slip the box into a resealable plastic bag to keep out the air, taking note of the expiration date on the box, and store in a cool, dry cupboard. To check its freshness, stir 1/2 teaspoon baking soda into 2 tablespoons vinegar. If the mixture bubbles and fizzes, the baking soda is still good.
Refer to the use-by date on baking powder to estimate its shelf life and store in a cool, dry place. For a freshness test, stir 1/2 teaspoon baking powder into 1/4 cup hot tap water. If it fizzes, it’s still good.
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