Baking powder is a raising agent made from a base and a weak acid that release carbon dioxide, making cakes rise. Its main base is usually sodium bicarbonate, while the acidic component may include phosphate or other acid salts.
When baking powder becomes wet, its acid and bicarbonate react and produce carbon dioxide gas. In double-acting baking powder, part of the gas is released during mixing and more is produced when heat activates the mixture in the oven. Gas bubbles expand, and the batter’s structure sets around them.
Commercial baking powder commonly includes cornstarch or another starch. The starch absorbs moisture, keeps the acid and base from reacting prematurely, and makes the powder easier to measure. This built-in acid is why baking powder can work in recipes that do not contain buttermilk, yogurt, or another acidic ingredient.
English chemist Alfred Bird developed an early commercial baking powder in 1843. Baking powder differs from yeast, which produces carbon dioxide through fermentation and adds more time and characteristic fermented flavours. Cream of tartar is an acid used in some formulations, not the complete raising agent described here.