baking powder [beyk-ing pou-der] noun
A chemical leavener composed of baking soda and a dry acid, such as cream of tartar. Cornstarch is often added to keep the mixture dry and flowing. The blend is typically two parts dry acid to one part baking soda (the cornstarch is quite minimal). The leavening action is simple: mixed with liquid, the baking powder begins to release carbon dioxide gas bubbles that will help the bread or cake to rise and have a light texture.
There is more than one kind of baking powder, but the most popular is “double-acting.” The double-acting leavening process first begins when the baking powder comes in contact with liquid (while mixing batter or dough) and then a second time when it is exposed to the heat of the oven. It is the only kind we use and we like both Davis and Rumford brands. (In the image above you can see that the Davis brand has a ledge for you to use to scrape your measuring spoon against, making it very easy to measure. Also, the lid is plastic and airtight, which is important for storage).
Single-acting tartrate and phosphate baking powders release their gases as soon as they’re moistened and can lose “oomph” by the time they hit the oven; because of this, they are not very popular and are not as widely available. Baking powder is perishable and should be kept in a cool, dry place. Always check the date on the bottom of a baking powder can before purchasing.
To test if baking powder still has leavening power, combine 1 teaspoon with 1/4 cup hot tap water. If it bubbles enthusiastically, it’s still good to use.