What's the difference between baking soda and baking powder?

 baking soda and baking powder

Even if you are not an avid baker, there's a good chance you have baking soda in your kitchen because it is used in a variety of other applications. Baking powder might not be in your cupboard unless you do a lot of baking. If you were inspired to bake and didn't have baking powder on hand, you might be tempted to substitute baking soda. However, the two products are different and you need to be careful in substituting one for the other, lest you end up with sunken cakes or super-spreading cookies.

You can get a quick rundown of the differences between baking soda and baking powder here. The basic science behind soda's leavening prowess is that the alkaline soda reacts with acids in the food, creating a gas that provides lift. But there's a lot more to it than that, according to Stella Parks (aka BraveTart). The alkalinity of baking soda also affects how long it takes the eggs in baked goods to set, and it weakens gluten, helping to keep cookies and cakes tender. It also speeds the Maillard reaction in baked and fried foods.

Baking soda requires the dough or batter to be slightly acidic in order for it to work. Baking power does not, because it is a combination of the alkaline baking soda and two acid salts, usually monocalcium phosphate and either sodium acid pyrophosphate or sodium aluminum sulfate. Since the acids are built in to the powder, the dough or batter does not need any acidic component of its own to provide the reaction.

Liquids set off the first reaction (between the monocalcium phosphate and soda), and heat is the catalyst for the second reaction between the remaining acid and the soda. This two-step process is often referred to as "double-acting". Most commercial baking powders are double acting; there are single-acting baking powders but they are rarely seen (at least in the US).

If you have ever wondered why some recipes call for both baking soda and baking powder, The Kitchn provides one explanation. What it boils down to is the amount of acid in your batter. Sometimes there just isn't quite enough to overcome the alkalinity of the baking soda, so powder is required. Using too much baking powder can lead to off flavors such as a metallic aftertaste, so just enough is added to provide the extra oomph to the batter without adding unwanted flavors. (Using too much baking soda can lead to a soapy taste, but you have to add a lot to get to that point.)

You can substitute baking powder for baking soda, with a caveat. You will have to use two to three times the amount of powder than baking soda, thereby increasing the chances of off flavors. While you cannot use baking soda alone to substitute for baking powder, you can add another ingredient to make your own single-acting baking powder. Cream of tartar, mixed 2:1 with baking soda, will work in most recipes calling for baking powder. 

Even though many websites and cookbooks instruct you to toss out old baking soda and baking powder, either product will last for a long time if it is stored properly. Baking soda, for instance, will keep for at least a couple of years when stored at or below 77 degrees Fahrenheit/75% relative humidity. If there is any doubt about the viability of your soda, mix a generous pinch of it with a tablespoon of vinegar: if it bubbles briskly, it is good to go. To test baking powder, add 1/2 a teaspoon to a few tablespoons of warm water; the mixture should fizz moderately. 

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