What's the difference between baking soda and baking powder?
Dear Straight Dope:
Now that the whole Y2K thing seems to have passed, let's get to some issues that are worthy of your genius: What's the difference between baking soda and baking powder? I always thought they had something to do with making baked goods "rise." Why do some recipes call for both and others just for one or the other? Can they be substituted? Also, why do so many baking recipes call for a small (1/4 to 1/8 teaspoon) amount of salt? Such a tiny pinch can't do much for flavor especially for sweets. Does the salt effect the baking process somehow?
Baking soda and baking powder are both used to make dough rise--that is, become filled with tiny bubbles of carbon dioxide. But there's a small but crucial difference between the two.
Baking soda is actually sodium bicarbonate, a simple compound that, when mixed with an acid, releases CO2 gas. That's why sodium bicarb is a remedy for "sour stomach": the bicarb reacts with the acids in the stomach, neutralizes them, and generates gas in the process. A couple of belches later and all is well again (at least until those nachos get to the intestines, anyway).
Baking powder, by contrast, contains sodium bicarbonate plus cream of tartar and a drying agent. Cream of tartar is an acid and will react with the sodium bicarb when dissolved in water. The drying agent, normally a starch, keeps the mix dry and prevents an explosive reaction from occurring in the can. Baking powder deteriorates with age, so don't buy a ton of it unless you plan on using it all soon. Eventually moisture gets in, overpowers the drying agent, and causes carbon dioxide to be slowly released.
Many baking powders are advertised as "double-acting." This simply means that the powder contains two acids, one that works at temperatures of 120 degrees and higher (i.e., the oven), and the other at room temperature. Double-acting baking powders are useful in recipes where cooking takes a long time--a cake, for example.
Why do some recipes call for baking soda while others require baking powder? That's governed by the other ingredients in the recipe. Since baking powder contains an acid and a base, it has an overall neutral effect in terms of taste. Hence, recipes that call for baking powder often call for milk as well, which is also neutral. By contrast, baking soda is purely basic and as such, gives a bitter taste unless it can be neutralized by the acidity of another ingredient. Usually, the other ingredient is buttermilk or yogurt, and the two produce a balanced taste. I don't know any recipes that call for both baking soda and baking powder and can only speculate that they need great leavening power but have delicate flavors.
As a rule, baking powder can be used in place of baking soda, but not vice versa. Since baking soda includes no acid, it won't work in recipes where baking powder is called for without some kind of adjustment. You can make your own baking powder by mixing two parts cream of tartar to one part baking soda.
On to salt. In breadmaking, salt affects yeast, which in turn defines characteristics such as taste, texture, and crust color. In other baked goods, salt is used to enhance flavor, much as MSG or vanilla do, without being detectable itself. In Understanding Baking, professional bakers are advised to add 1.75 to 2.25 pounds of salt to every 100 pounds of flour. Since those quantities aren't useful when baking for any group smaller than a suburb, I did some measuring in the kitchen and found that the recommended salt/flour ratio works out to about a half teaspoon per cup. That's the amount called for in many baking recipes, give or take a pinch.
"Soda or Powder? The Lift Question." Cook''s Illustrated. Jan/Feb 1997, p. 23.
"Double-Acting Baking Powder and Waffles Don''t Mix." Cook''s Illustrated. Nov/Dec 1993, p. 28.
Amendola, Joseph, and Donald Lundberg. Understanding Baking, Second Edition. New York: Van Nostrand Reinhold, 1992. Pages 41-49, 99-90.
Windholz, Martha, ed. The Merck Index, Tenth Edition. Rahway, NJ: Merck & Co., Inc., 1983. Page 1230.