Leaveners are classified as aerating, chemical, and yeast.
Whole eggs, egg whites, butter, shortening, lard. Work on the principle that a volatile substance will produce vapors that will be captured within food as it dries and sets during baking to give structure to baked goods.
Whole eggs are beaten until light, egg whites until fluffy. During baking, the moisture in the eggs or whites expands as water vapor. At the same time, the heat of the oven firms the crumb structure.
Butter and solid fats flavor as well as leaven. They are generally low in moisture, so they are worked into the dough to form thin, alternating layers of fat and dough. The moisture in the dough evaporates, producing water vapor which is trapped by the moisture-proof fat, resulting in a flakey, aerated pastry.
Baking powder, baking soda, ammonium carbonate. Chemical reactions occur between two or more ingredients, specifically between alkali and acid, to produce carbon dioxide. The reaction starts when liquid is added to the mixture.
Some example of acid ingredients include sour milk, buttermilk, sour cream, cream of tartar (potassium bitartrate) and vinegar. Baking powder is a combination of both alkaline (sodium bicarbonate) and acid (cream of tartar) ingredients. It was invented to insure consistent results in recipes. Most U.S. baking powders are double acting, i.e., they release gas during mixing and again during baking. The reaction is slower than that of old fashioned single action baking powders.
The oldest known leavener, this one-celled microorganism divides and multiplies at a phenomenal rate when subjected to moisture and heat. Yeast remains dormant at temperatures below 50°F, and are killed at temperatures above 120°F. Adding a small amount of sugar will enhance development, but too much sugar (greater than 6% flour total) will stop yeast growth.
Salt inhibits yeast growth. Whole milk is not good for proofing yeast, because milk fats coat yeast cells and inhibit cell growth.