After busily cleaning out the pantry, I discovered two open, but only partially used, containers that I didn't know I had. One was baking powder and one was baking soda. It's always useful to have extra on hand in case of a baking emergency, but I figured that it would be a good idea to test them to see if they still worked, since chemical leaveners can lose their potency over time for various reasons, including poor storage conditions.

The procedure for testing these two products is simple. Get out two small glasses or bowls and fill one with 3-4 tablespoons of white vinegar and fill the other with 3-4 tablespoons of room temperature water. Add about 1/2 teaspoon baking soda to the bowl containing the vinegar and the same amount of baking powder to the water bowl. Each mixture should fizz up, with the soda/vinegar producing a bigger reaction. If they both work, mark the containers with the date and check them again in 6-12 months (if you haven't used them up by then) to make sure they're still active. And if they don't fizz up, you better stop by the store before the next time you want to whip up a batch of cookies.

Baking powder and baking soda are both types of chemical leaveners used in baking. The two have different properties, so they cannot simply be substituted for one another directly.

Baking soda is also known as bicarbonate of soda. It reacts with acidic ingredients, such as buttermilk, yogurt and molasses to create bubbles of carbon dioxide, which causes batter to rise. It starts to work immediately when exposed to the acidic ingredient, so a batter made with baking soda should generally be baked as soon after mixing as possible. Baking soda can also aid in browning during baking.

Baking powder is actually a combination of baking soda, cream of tartar and a bit of cornstarch. Cream of tartar is an acidic ingredient, so it prompts the baking soda to work even when no other acidic ingredients are present in a batter. The cornstarch absorbs moisture and prevents the baking powder from being activated too quickly, or before it makes it into the batter in the first place. It works by releasing CO2 bubbles, just as baking soda does. Many baking powders, and most of those used in the US, are "double acting," meaning that they work once when exposed to moisture and again when exposed to heat. This gives the impression that baking powder doughs and batters will rise higher than those made with baking soda alone, but this is not necessarily the case.

 

I often get asked whether substitutions can be made in a recipe. Baking is really a sort of science experiment, much more so than actual cooking, so while substitutions will work sometimes it is often best to simply follow the recipe, if at all possible. I never thought to devote an entire post to the subject, instead answering inquiries on a one-on-one basis, but Rose Levy Beranbaum finally said what I, and other bakers, have been thinking for a long time:

Can you substitute ingredients? Yes. Should you? Usually, no.

There are many parts of recipe for breads, cookies and cakes that cannot change without fundamentally altering the final product. For example, if a recipe calls for baking powder you cannot substitute yeast. They may both be leaveners, but they are completely dissimilar. You could try to substitute whole wheat flour for all purpose, but the outcomes will not be identical because the properties of each type of flour are different. Flavorings and add-ins, like chocolate chips, blueberries, nuts or vanilla extract, can be substituted for other, similar ingredients, but the mainstays of a recipe should not be changed if you are expecting to reproduce the outcome of the original recipe. Rose suggests changing one ingredient at a time, if you really want to experiment, and watching the results. You might not get edible cookies, but you will gain a better understanding of how your ingredients work should you want to make changes in the future. Websites like FoodSubs.com can help, but if you want chocolate cake right now, try to stick with the recipe for the best results.