Leavening agents are gases that cause the dough to rise. The gases are produced by chemical action or introduced by the mixing process, which forces air into the dough. The common types of leavening agents are steam, air, and carbon dioxide gas. These agents are produced by yeast or bating soda or baking powder.

AIR.- Air is introduced into the dough by blending (creaming) fat and sugar together, by sifting flour, or by folding in beaten egg whites that already contain air. Steam is used to leaven eclairs and cream puffs.

YEAST.- Yeast is a microscopic, one-celled plant that, when conditions are favorable, will multiply by budding or by the division of a cell into two cells. In this process of reproduction, the yeast plant uses available food (sugars) to produce carbon dioxide gas and alcohol. This is known as fermentation.

ACTIVE DRY YEAST.- Active dry yeast should be suspended in about seven times its weight of water at 105F to 110F for 5 minutes before it is used. The proper temperature of the water is important, as water that is too cold or too hot will harm the yeast. Make sure the temperature of the water does not exceed 110F. Active dry yeast does not require refrigeration, but should be stored in a dry and reasonably cool place. When properly stored, dry yeast will keep for many months.

Yeast foods, known as dough conditioners, have other more important functions than to supply food for yeast. Their major purposes are to condition the water and to assist in the proper fermentation of the dough.

Yeast foods contain three types of functional ingredients:

1. Ammonium salts to supply yeast with a supply of nitrogen for growth

2. Calcium salts to produce the correct amount of hardness in the dough water and to firm the gluten

3. An oxidizing agent to give a firmer, less sticky dough

In addition, yeast foods contain starch and salt to add bulk and make weighing easier. The use of yeast foods is often determined by the strength of the flour and the fermentation period desired. Not all flours require yeast food. When the flour requires such material, its addition produces bread of larger volume, better grain and texture, and improved loaf appearance. Too much will produce inferior bread with low volume and coarse grain.

BAKING SODA.- Baking soda acts as a leavening agent only when there is an acid present. Some of these acids are sour milk or buttermilk, molasses, brown sugar, honey, corn syrup, maple syrup, lemon juice, and vinegar. These are used for different types of quick bread. Only a limited quantity of the acid ingredients can be used for leavening purposes due to the pronounced flavor and heavy texture that baking soda and molasses or syrup give to the products. It is also difficult to determine beforehand the amount of gas that these mixtures will produce. Thus, it is difficult to obtain standard results.

BAKING POWDER.- Baking powder is a leavening agent that contains baking soda, a large amount of starch, and a material that forms an acid when it is mixed with water, thus producing a gas. There are several types of baking powder. The Navy uses a combination-type baking powder that contains the acids sodium aluminum sulphate (S.A.S.) and orthophosphate plus sodium bicarbonate and a cornstarch filler. This type of baking powder is moderately double acting; one constituent acts in the batter, while the other does not act until it is heated in the oven. Baking powder is generally preferred over baking soda because it is more reliable.