Leaveners are used in baked goods to improve texture and visual appearance. They create air pockets within a dough or batter to give the final product a light, fluffy texture. In general, leaveners can be divided into three categories: physical, biological, or chemical.
Carbon dioxide gas is most often responsible for the leavening action in baked goods and can be produced by biological agents like yeast, or chemical agents such as baking soda and baking powder.
There are two types of physical leaveners: air and steam. Air is often incorporated into batters when butter and sugar are creamed together. Briskly whisking butter (or another solid fat) with sugar traps small pockets of air within the fat. Air can also be used as a leavener when whipping egg whites or cream. In both instances, the air becomes trapped within a protein matrix in the cream or egg whites, causing expansion. On a smaller scale, sifting flour also traps a small amount of air and can offer a minimal level of leavening action.
The second physical leavener is steam. When water converts to steam, the volume increases by approximately 1,600 times its original size. When moist batters are introduced to high temperatures, the liquid in the batter rapidly transforms into steam. The steam becomes trapped within the batter, which solidifies as it is baked. Steam is used as a leavener in foods such as popovers, cream puffs, and pie crusts.
Yeast is a biological leavener; it's a living organism that ferments sugars for energy and carbon dioxide gas is a byproduct of this process of fermentation. In order to begin the fermentation process, yeast requires carbohydrates and moisture. Warmth speeds up this reaction, although it is still relatively slow. Because yeast produces carbon dioxide at a slow rate, it is often used in breads that have a strong gluten matrix that can hold the gas in for long periods of time. Liquid batters, like those used for pancakes, are too weak to keep gas trapped for that length of time and they need a faster-acting leavener such as baking soda.
Two chemical leaveners are baking soda and baking powder. Baking soda is a natural alkaline powder that produces carbon dioxide gas when combined with an acid. Since the reaction occurs rapidly, baking soda is an ideal leavener for soft or weak batters like pancakes, muffins, and other quick breads. Buttermilk, vinegar, yogurt, or even cocoa powder can be used as the acid in this reaction.
Baking powder is similar to baking soda but it already contains the acid necessary to react. The acid in baking powder is in the form of a salt, which means that it will not react until combined with water. Baking powder is an ideal leavener for recipes that do not contain a lot of other acidic ingredients, such as cookies. Most baking powders sold commercially today are double-acting, which means that it will produce gas twice—once when water is added and again when the mixture is exposed to heat. Double acting baking powder provides a consistent and reliable leavening action.