By Harold McGee
Baking powders are complete leavening systems: they contain both alkaline baking soda and an acid in the form of solid crystals. (The active ingredients are mixed with ground dry starch, which prevents premature reactions in humid air by absorbing moisture, and gives the powder more bulk so that it’s easier to measure.) When added to liquid ingredients, the baking soda dissolves almost immediately. If the acid is very soluble, it too will dissolve quickly during mixing and react with the soda to inflate an initial set of gas bubbles. Cream of tartar, for example, releases two-thirds of its leavening potential during two minutes of mixing. If the acid is not very soluble, then it will remain in crystal form for a characteristic length of time, or until cooking raises the temperature high enough to dissolve it—and then it reacts with the soda to produce a delayed burst of gas. There are several different acids used in baking powders, each with a different pattern of gas production (see box).