About baking powder
Baking powder is a dry chemical raising agent used to increase the volume and lighten the texture of baked goods such as muffins, cakes, scones and North American biscuits. Baking powder works by releasing carbon dioxide gas into a batter or dough through an acid-base reaction, causing bubbles in the wet mixture to expand and thus leavening the mixture. It is used instead of yeast for end-products where fermentation flavors would be undesirable or where the batter lacks the elastic structure to hold gas bubbles for more than a few minutes. Because carbon dioxide is released at a faster rate through the acid-base reaction than through fermentation, breads made by chemical leavening are called quick breads.
Most commercially-available baking powders are made up of an alkaline component (typically baking soda, one or more acid salts, and an inert starch (cornstarch in most cases, though potato starch may also be used).
Generally (in countries where the cup is used as a standard measure in cookery) one teaspoon (5 ml) of baking powder is used to raise a mixture of one cup (200-250 ml) of flour, one cup of liquid, and one egg. However, if the mixture is acidic, baking powder's additional acids will remain unconsumed in the chemical reaction and often lend an unpleasant chemical taste to food. High acidity can be caused by ingredients like buttermilk, lemon, yoghurt, citrus, or honey. When excessive acidity is present, some of the baking powder is replaced with baking soda. For example, one cup of flour, one egg, and one cup of buttermilk requires only ½ teaspoon of baking powder—the remaining leavening is caused by buttermilk acids reacting with ¼ teaspoon of baking soda.
The opposite is sometimes true, too. In baking powders that contain sodium acid pyrophosphate, excess alkaline substances can sometimes deprotonate the acid in two steps instead of the one that normally occurs, resulting in an offensive bitter taste to baked goods. Calcium compounds and aluminum compounds do not have that problem, though, since calcium compounds deprotonates twice are insoluble, and aluminum compounds do not deprotonate in that fashion.
Moisture and heat can cause baking powder to lose its effectiveness over time, and commercial varieties have a somewhat arbitrary expiration date printed on the container. Regardless of the expiration date, the effectiveness can be tested by placing a teaspoon of the powder into a small container of hot water. If it fizzes energetically, it's still active and usable.