Leavening is an agent that produces fermentation. The leavening agent produces gas, air, or steam that expands when heated, making the resulting product light and altering grain textures.
Leavening agents include Yeast, Baking Powder and Baking Soda (used with a little food acid). Yeast is a small plant that, if mixed with sugar, will produce carbon dioxide whenever temperature and moisture are right. Baking powder produces a chemical reaction that releases some of its gas when mixed with a liquid and the rest of the gas whenever it is heated. The following are descriptions of products and their category:
Baking Soda is an important ingredient of baking powder. To be used as leaven it must be mixed with a food acid like buttermilk, sour milk, molasses, vinegar, lemon juice, or cream of tartar.
Cream of Tartar by itself does not leaven anything any more than does sour milk or buttermilk. It is often used as a flavoring in foods and beverages.
Brewer's Yeast is a by-product of the fermentation of beer and is a rich source of vitamins, especially the B-complex. It has no leavening properties.
Yeast Extract is an ingredient used in canned or in dehydrated soups. It is only an extract and cannot leaven anything.
Egg Whites. While eggs are not considered leavening agents, the egg whites, when beaten, can leaven by expansion of the air and by steam when heated. They are the only leavening in many angel food cakes.
When purchasing bakery products, especially pies, inquire whether leavening was used in the crust. Always read the label for the list of ingredients used in that particular product. Examining items in the stores in your area before the time arrives will be valuable to you in planning meals during the Days of Unleavened Bread.
The best approach to this season is: plan, prepare, be creative, and read, read, read those labels!