A leavening agent made up of baking soda, cream of tartar, and either cornstarch or arrowroot. Double-acting powder releases carbon dioxide on contact with liquid, creating the air pockets responsible for the light texture in baked goods. Always try to purchase non-aluminum baking powder so that sodium aluminum sulfate is not released into your foods, possibly compromising your health.
Baking powder is more perishable than you might think, not lasting much beyond the expiration date on the can. Store in a cool, dry place and purchase in small amounts for the best shelf life.