Baking powder is a leavening agent, most frequently used in recipes like cakes, quick breads, and pancakes. It's often advertised as "double acting," which means that it contains two types of leavening ingredients. Unlike baking soda, baking powder frequently adds an additional ingredient, usually an acid, to bicarbonate of soda. This results in a higher rise in baked goods, because the rise begins at room temperature, instead of when foods are baked.
The most common combination of acids and alkalines that produce baking powder are cream of tartar and sodium bicarbonate. Sodium bicarbonate activates at high temperatures, but combining it with an acid like cream of tartar activates it earlier. Not all recipes require this, and in fact, some baked goods can be ruined by the double rise mechanism. When a recipe calls for baking soda, using powder can create too much rise. Similarly, substituting soda for powder may result in flatter baked goods.
If a baker runs out, he or she can combine one part baking soda and two parts cream of tartar. For instance, in a recipe that calls for 3 teaspoons of baking powder, the baker would use 2 teaspoons cream of tartar and 1 teaspoon of baking soda. The powder may also contain a small amount of cornstarch, which helps to keep the ingredients from absorbing moisture, but this isn't unnecessary when mixing up a small substitution batch.
There is some concern the aluminum content in baking powder, especially for its potential to increase the risk for the later development of Alzheimer's disease. People who are concerned about this possible risk can find a number of brands that are aluminum free. This version is commonly available in natural food and health food stores.
Since baking powder begins to activate at room temperature, many recipes for baked goods that tell the cook to have all ingredients at room temperature. Many ignore this advice, and still produce fine baked goods. Bakers who are trying to make a high, light cake need to pay attention to these instructions, however, to make sure that the rising process begins before the cake reaches the oven.
Recipes containing acids like lemon juice and buttermilk will activate baking powder more quickly and render it more effective. Often, recipes recommend using about 1 teaspoon (4.6 g) of powder to 1 cup (125 g) of flour, but this amount should be lower when high acid content foods are added. The amount of leavening required in a recipe that contains about 1 cup (236.5 ml) of lemon juice or buttermilk usually decreases by a half.