LEAVENERS
Leaveners also contribute to baked goods' taste, coloring and texture. For a successful recipe you want to balance the leavening system to achieve a neutral pH, otherwise your recipe can taste bitter, have an off or dull color, not set properly, fall apart easily, crack, and so on. Baking soda begins to create carbon dioxide gas when moistened. Double-acting baking powder (which most baking powders are these days) produces an initial set of gas bubbles when mixed with wet ingredients and then a second set when heated. The first reaction forms many small gas cells in the batter; the second reaction expands the bubbles to create a light texture. In the oven, heat not only assists these reactions, but also transforms the water in the recipe into steam that also contributes lift.
Baking powder is basically a blend of acid (most commonly calcium acid phosphate, sodium aluminum sulfate or cream of tartar) and baking soda (alkali - sodium bicarbonate), creating carbon dioxide bubbles when both moistened and heated. It also contains an inert starch (i.e. corn starch) to act as a filler and prevent the components from reacting prematurely. It is known as a double acting chemical leavener -- it begins release carbon dioxide as soon as it is moistened, and again when heated in the oven. (There are single acting baking powders, but are rarely used by home bakers). Always use double acting in recipes; it is virtually the only type available in grocery stores. Baking powder does not need an acidic ingredient to release its leavening power, as baking soda does, because it contains its own; baking powder contains 30 percent baking soda (alkaline) and an acid, such as cream of tartar.
BAKING SODA Baking soda (bicarbonate of soda), another chemical leavener, is used when there is an acidic ingredient in the recipe. The acid might be hidden such as in honey and molasses. When baking soda comes in contact with an acidic ingredient and is moistened, the alkali/acid combination creates carbon dioxide (CO2), water and a neutral salt. It also renders a neutral, tasteless residue. Baking soda is four times as strong as baking powder. The general rule is to use 1 to 1-1/4 teaspoons baking powder per cup of flour. On the other hand, baking soda should be added at 1/4 teaspoon per cup of flour. Adding buttermilk (an acid) instead of milk: if the recipes uses 2 teaspoons baking powder, and you add 1/2 cup buttermilk, instead use 1 teaspoon baking powder + 1/4 teaspoon baking soda or 1 teaspoon baking powder = 1/2 teaspoon baking powder + 1/8 teaspoon baking soda