Q: What is the difference between baking soda and baking powder and how are they used. Can one be substituted for the other?
Baking soda and baking powder are both chemical leaveners used to make baked goods such as cakes and muffins. Baking soda has some other culinary uses, not discussed here. In recipes calling for baking powder, baking soda can be used, along with some cornstarch and cream of tartar. Baking powder cannot, however, be used to replace baking soda.
The chemical in baking soda is bicarbonate of soda (NaHCO3). When combined with an acidic ingredient, such as vinegar or the lactic acid in buttermilk, baking soda releases carbon dioxide which forms into bubbles in the food. When heated, these bubbles then expand and help to rise or lighten the final product.
Baking powder is a mixture of baking soda and an acid, in powdered form, that combine in liquid to create the same reaction. There are three general types of baking powder -- fast-acting, slow-acting and double-acting; the most commonly available being double-acting.
Double-acting baking powder uses two different acids, one of which reacts at room temperature and the other only during the baking, at higher temperatures. The first reaction helps to form the initial bubbles that are trapped in the batter. As the food cooks, the material around these bubbles starts to set. Carbon dioxide from the second reaction is better trapped within the bubbles and gives a better lift.
Fast-acting baking powder uses only an acid that reacts immediately, while slow-acting contains only the acid that reacts under heat. All three forms will also contain some cornstarch to help keep the mixture dry before use.
Recipes that call for both baking powder and baking soda usually also contain an acid ingredient that will react with the baking soda. The ingredient might be vinegar or buttermilk, mentioned earlier, or molasses, lemon juice, sour cream, honey or chocolate, to name a few. In this case, the amount of baking soda is however much will react with the acidic ingredient. Additional leavening is provided by the baking powder.
Baking soda, combined with an equal measure of cornstarch and twice as much cream of tartar, can be used to replace baking powder. Use about one quarter the amount of baking soda as the recipe calls for baking powder, and then scale the cornstarch and cream of tartar accordingly. For example, if the recipe calls for 1 teaspoon of baking powder, it can be replaced by ¼ teaspoon of baking soda, ¼ teaspoon of cornstarch and ½ teaspoon of cream of tartar.
Generally speaking, baking powder can not be substituted for baking soda since this will leave excess acidic compounds in the food which may affect flavor, texture and color.
For more on this topic, see my posting Baking Soda vs. Baking Powder Redux.
For readers who might be interested, the chemicals commonly used in baking powder, along with the bicarbonate of soda, are cream of tartar (KHC4H4O6), tartaric acid (H2C4H4O6) or monocalcium phosphate monohydrate (Ca(H2PO4)2 • H2O) for the fast acting acid. For the delayed reaction, the chemical may be sodium aluminum sulfate (Na2SO4 • Al2(SO4)3) or anhydrous monocalcium phosphate (Ca(H2PO4)2).