Most baking ingredients are quite self-explanatory: we know what eggs, flour, butter, and sugar are, and some of us even have an idea of the purpose they serve in making a cake delicious. However, baking powder and baking soda are a source of confusion for many beginner bakers: they both look identical, have similar names, and if you decide to try either of them on their own, you will be left with a horrible, bitter taste in your mouth.
Because of these similarities, many people think that the two products are interchangeable. They are not, and swapping them around or confusing them can lead to some catastrophic baking fails. In order to avoid disaster, we are going to go over what baking soda and baking powder are, what they do, and the substitutions you can actually use.
Baking soda, also called bicarbonate of soda or sodium bicarbonate, is a base mineral. This means that when it combines with something acidic, it produces carbon dioxide. It's the same scientific process behind the "volcano" school project: combine a base (baking soda) with an acid (vinegar), and you get an explosive chemical reaction. When you use baking soda in your baking, you are replicating that chemical reaction on a smaller, controlled scale. The carbon dioxide bubbles puff up the dough and help it rise in the oven, creating light, fluffy baked goods.
Baking soda can only be used in recipes that also include an acidic ingredient such as buttermilk, lemon juice, yogurt, molasses, or honey. You also need to make sure that you only use as much baking soda as is specified in the recipe: this is the amount that will react with the acid. Adding more does not mean more lift in your baking, it just means there is baking soda left over from the reaction, which can give your baked goods an unpleasant, chemical taste. For this same reason, you should be careful when you measure your baking soda: shake up the container first, then fill your measuring teaspoon and scrape the excess off with a knife for a perfect level measurement.
It is important to place your dough in the oven as soon as it is ready, as the chemical reaction will dissipate if you leave it sitting for too long. This will effectively render the baking soda useless, and your baking will fall flat.
Baking soda's shelf life is more or less infinite as long as you keep it in a dry and airtight container. However, if you have had your current box of baking soda for a long time, it is a good idea to check if it is still good before using it. Remember, if your baking soda does not activate, it can leave a bad taste.
You can test your baking soda by adding half a teaspoon of vinegar to a cup of hot water and then adding half a teaspoon of baking soda. If the mixture visibly fizzes up, you can go ahead and use it in your recipe.
Baking powder is a combination of baking soda, another acid (usually cream of tartar), and a stabilizer that keeps the mixture from reacting when dry (usually cornstarch). Baking powder is thus a concentrated, "pre-mixed" version of baking soda, which activates when mixed with a liquid. This creates the same chemical reaction and the same carbon dioxide "bubbles," which helps baked goods rise.
There are two types of baking powder: single-acting and double-acting. Single-acting baking powder is essentially the one described above: it activates with liquid to create carbon dioxide. Like baking soda, single-acting baking powder needs to be activated quickly or the carbon dioxide will dissipate.
Double-acting baking powder, on the other hand, involves two reactions: some bubbles are created when the liquid is mixed in, but it is the heat of the oven that causes most of the rising. Because of this, double-acting baking powder is more reliable and failsafe, as you can still get a good result even if you forgot to preheat your oven. Single-acting and double-acting baking powders are fully interchangeable, but as an amateur home baker you will most likely encounter double-acting baking powder.
The shelf life of baking powder is approximately a year. This means that if your box of baking powder has been sitting in your cupboard for years, chances are that it is no longer doing anything to your baking. If you're in doubt, you can test your baking powder in the same way as baking soda, except that you can omit the vinegar (remember, baking powder already includes an acid): just stir half a teaspoon of powder into a cup of hot water and see if it fizzes.
So, why would you use one or the other? Why do some recipes call for baking soda while others call for baking powder, or a combination of both? The simple explanation is that different levels of acidity will create different textures in your baked goods, so most recipes will call for a specific combination of both.
The perfect balance of baking soda and acid can be difficult to achieve: you need to make sure you are using the right amount of the acid for the flavor, while also using just enough baking soda to react with it so you are not left with a chemical taste. Using a combination of baking soda and baking powder makes this easier, as you can "top up" with baking powder without worrying about its reaction with other ingredients.
This can also affect flavor. For example, baking soda neutralizes the acidity of any acid it is mixed with, so it is often used to balance out some sourness from recipes such as buttermilk scones. On the other hand, baking powder works well for cases where there is no acid, like a plain cake.
Because of its longer shelf life, chances are you may find yourself with only baking soda to work with. Generally speaking, it is ok to substitute baking soda in a recipe that asks for baking powder, as long as there is an acidic ingredient with which it can react: for half a teaspoon of baking soda, you can use a teaspoon of lemon juice or vinegar. You can also make your own baking powder by mixing baking soda and cream of tartar: however, the latter is not an ingredient many people happen to have on hand.
If you only have baking powder and need baking soda, the substitution is not that easy. You can't feasibly "revert" baking powder into soda, and if a recipe calls for baking soda, chances are it requires a strong rising agent. Baking soda has four times the rising power than baking powder, but multiplying your amount of baking powder by four will most likely just make your batter or dough taste bad.
In conclusion, baking powder and baking soda are similar and are both used for the same purpose, but they are not interchangeable because of the differences in their chemical composition. A well-stocked baking cupboard will include both, and following the correct measurements and proportions in a recipe is be essential for getting a delicious, fluffy baked good.