Baking powder and baking soda don’t work the same and are not interchangeable. Let’s have a little lesson in chemistry. (I love this stuff!)
First, I’ll give you the definitions of both baking soda and baking powder: These are both leavening agents that, when used correctly will create the desired height and texture (lightness) in your baked product. They are not interchangeable, though, since they work differently, given the acidity level in the mixture before baking. The acidity level is determined by the ingredients that you use in your recipe. Some acidic ingredients are citrus juice, molasses and buttermilk.
Baking Soda: Sodium bicarbonate: This is an alkali which, when broken down by heat, produces sodium carbonate, water and carbon dioxide. When baking, it needs to react with an acid ingredient in order to produce carbon dioxide. Carbon dioxide begins to form as soon as a mixture with baking soda is moistened. When this happens, lots of little gaseous bubbles form within a given mixture and, since bubbles are buoyant, their natural inclination is to rise upward. When this rise takes place, the bubbles carry the surrounding mixture with it. (This reaction can happen simply when the mixture is in a bowl, sitting on your counter, without heat.) When this mixture is placed into a hot oven, it continues to rise until the protein (eggs, etc.) in the mixture sets and becomes firm (then it’s baked and ready to leave the oven). It’s best to bake a mixture that’s exclusively leavened with baking soda as soon as possible after it’s been assembled, or you’ll essentially lose (or reduce) it’s leavening power.
Baking Powder: This is the combination of an acid and an alkali, which, when activated by heat, will produce carbon dioxide. (Actually, baking powder has baking soda in it; that’s the alkali part. The acid ingredient is cream of tartar.) As with baking soda, the formation of carbon dioxide is what leavens a mixture in the oven. However, there are some real benefits to using the baking powder that’s most available today. It’s called “double-acting” baking powder and it has the ability to leaven hours (and sometimes even weeks) after a mixture has been assembled. The reason: double-acting baking powder has two different kinds of acid in it. One is cold water soluble, meaning it will react as baking soda would, right away, as soon as it’s moistened (this is the cream of tartar). But it also has a hot water soluble acid in it (sodium-aluminum sulfate), which won’t be activated until it’s exposed to heat. So, you can assemble certain types of cookie batters and leave them in the refrigerator or freezer before baking. Or you can make a muffin batter sometimes weeks ahead and leave it in a well-sealed container in your refrigerator (for weeks) before baking.
Some people object to the use of any product that contains even a trace of aluminum in it, because of the association with Alzheimer’s disease. I think, however, that the benefits of using this product out-weigh any danger involved. (That’s my personal opinion….) If you wish, single acting baking powder is available in health food stores.
To create your own single acting baking powder: Mix 1/2 teaspoon cream of tartar (acid) with 1/4 teaspoon baking soda (alkali). If planning to store this in bulk, add 1 teaspoon cornstarch (as an inert filler), per the other ingredients, and just scale up proportionately.
Here are the differences between how to use these two types of leaveners and when you would choose to use one versus another.
When you would use baking soda: You would use both baking powder and baking soda when you have an acidic ingredient, like buttermilk, in a mixture that you would like to achieve height and/or lightness in after baking (quick breads, muffins, cakes, buttermilk biscuits, etc.). Even certain candy mixtures, like butter-crunch toffee, need baking soda, even though it’s not baked. In this case, when baking soda is stirred into to this very hot mixture, millions of tiny bubbles, filled with carbon dioxide, form within the candy mixture. Because it hardens so quickly, the intact bubbles remain suspended in the toffee, once hardened. This is why butter-crunch candy shatters between your teeth (and doesn’t break a tooth!) when you bite into it.
In the case of cookies, where you’re not looking for height, baking soda helps to create a flatter cookie with a somewhat cracked exterior (a desired look in gingersnaps, for example). These cookies would be leavened with baking soda. Cookies made with molasses, which is an acid (and dark brown sugar also is somewhat acidic, since it contains molasses), these cookies would also be leavened with baking soda.
There is a reason that baking soda affects the appearance of a baked cookie. Since you only put a small amount of cookie batter on a flat surface (a cookie sheet), while baking, the bubbles created by the formation of carbon dioxide quickly rise to the top of this very small mixture. When the bubbles reach the surface, they burst open, creating a lower, flatter cookie, than cookies leavened with baking powder. Baking soda also helps to brown foods that don’t spend a long time in the oven. Many times those soft pretzels that everyone loves are dipped in a baking soda solution to help them quickly brown in the oven. This way, they don’t have to over-bake, which would make them harder than desired.
When you would use baking powder: You would use baking powder in a recipe that does not contain an acid ingredient, since this leavening agent already contains the acid necessary to create carbon dioxide. (Remember how to make baking powder: acid + alkali + starch.)
When you would use both, baking soda and baking powder in your recipe: You would use both baking powder and baking soda in a recipe where you want to strengthen the leavening power (stabilize the rise) in a mixture leavened with just baking soda. Another reason would be if you wanted, or needed, to keep a mixture leavened with baking soda in its assembled state, without baking for an extended period of time (because of its single action). There is rarely a reason to incorporate baking soda in a recipe that requests the use of baking powder, unless you want to increase browning or create a cookie that’s flatter in appearance. Be aware that baking soda and baking powder are both used in precise measurements—for precise reasons—so it’s best to follow your recipe’s instructions for best results.