Baking powder does not last forever. Because it’s sensitive to moisture and humidity, it generally has a shelf life of between six months to one year. Baking powder should be kept in a cool, dry place, such as inside a cabinet, and should be discarded when it is no longer active. (Its cousin, baking soda*, has an indefinite shelf life, although some manufacturers recommend changing it every three years.)
To test if baking powder is still active, spoon 1/2 teaspoon in a bowl and pour 1/4 cup (60 ml) of boiling water over it. Right away it should bubble up violently. If it does, it’s still good. If it doesn’t, discard it and open a new tin.
Why you should use aluminum-free baking powder (and how to make your own)
*The difference between baking powder and baking soda (bicarbonate of soda) is that baking soda requires an acid ingredient in a recipe to activate it, such as vinegar, buttermilk, coffee, or yogurt.
Baking powder has baking soda as one of its components, as well as an acidic ingredient to activate it, so it can be used in recipes that have no other acidic ingredients. Baking soda and baking powder are not interchangeable in recipes.