Unlike baking soda, which keeps forever, baking powder begins to react and lose its potency the instant you open the can and expose the contents to humidity in the air.
2
For maximum leavening power, i recommend buying baking powder in the smallest can available, dating the can when opened, and discarding the contents after three months.
3
The alternative, uncommon but hardly daunting, is to make your own baking powder.
4
Simply combine one teaspoon of baking soda (an alkaline) and two teaspoons of cream of tartar (an acid) for each tablespoon of baking powder called for.
