Perhaps you recall from your high-school or college chemistry class experiments that carbon dioxide bubbles are generated whenever water is poured over a dry acid and alkali mixture. Well, that is exactly what happens when you use baking powder, because this cooking ingredient is essentially a blend of acid (calcium acid phosphate, sodium aluminum sulfate, or cream of tartar, to name three) and alkali (sodium bicarbonate, popularly known as baking soda). Add water to this mixture and a chemical reaction results, producing carbon dioxide. The gas generated creates minuscule air pockets, or enters into existing ones, withing the dough or batter. When placed in a hot oven or on a hot griddle, the dough or batter rises, primarily for two reasons. First, the heat helps release additional carbon dioxide from the baking powder. Second, the heat expands the trapped carbon dioxide gas and air and creates steam. The resulting pressure swells the countless air pockets, which in turn expand the food being baked.

Does baking powder lose its potency over a period of time?

Yes. You can easily determine whether you need a fresh supply by conducting this simple chemical experiment. Pour one-quarter cup of hot tap water over one-half teaspoon of baking powder. The fresher the baking powder, the more actively this mixture will bubble. If the chemical reaction is weak or does not occur, your baking powder will not properly raise whatever you are planning to bake.

When substituting buttermilk for sweet milk in a baking recipe, why should I substitute baking soda for all, or some, of the baking powder in the recipe?

In order for a chemical leavening agent to release the optimal quantity of carbon dioxide gas into the dough or batter, the acid and alkali must be in proper proportion, as it is in the case of baking powder. When you substitute buttermilk for sweet milk, you incorporate extra acid into the batter or dough, and you therefore upset the proper proportion of acid to alkali that was worked out in the original recipe. That extra acid will reduce the amount of carbon dioxide generated and, accordingly, inhibit the leavening process. To counteract the hyperacidity, the cook needs only to substitute baking soda (an alkali) for some or all of the baking powder in order to maintain the necessary quantity of alkali.

For each cup of buttermilk that you use in place of sweet milk, reduce by two teaspoons the amount of baking powder called for in the original recipe and replace it with one-half teaspoon of baking soda.

09-21-93