It's possible to make a bread or cake without any kind of leaven. Certainly, unleavened bread figures prominently in the Bible. An interesting study to do is to examine passages which talk about unleavened bread and also passages that mention yeast or leavening.
Why use leavening, then? When used in a dough or batter, leaven softens and lightens the finished product. By chemical processes, the dough rises and becomes lighter and more airy, with little holes left by gas bubbles. There are two main kinds of leaven: biological agents (living organisms) and chemical agents.
The most common biological agent used in the home is yeast. Some other home substances that can provide yeast to dough are beer, buttermilk, sourdough starter, and yogurt. The most common chemical agents used by home bakers are salt and baking powder or baking soda.
When you buy yeast, you'll find it comes in two forms: cake yeast (also called compressed or fresh yeast) and dry yeast. Most of us at home use dry yeast. If you do use a cake yeast, make sure it is soft and moist and that it crumbles easily. Make sure it is fresh smelling and that there are no dark or dried places on the cake of yeast. Remember that cake yeast must be used very quickly.
Dry yeast, on the other hand, has a longer shelf life, and that is why most of us who are home bakers use dry yeast. Dry yeast is the same yeast as cake yeast, but it has been dried and compressed until it has much less moisture than cake yeast. The dry yeast is actually dormant.
Dry yeast comes in two forms: regular active dry and rapid-rise. They work about the same, except that rapid rise obviously rises faster. This is an advantage if you want speed, but some feel that it doesn't give the dough the time to develop that wonderful yeast bread taste that slower acting yeast does.
You can buy dry yeast in foil packages or in a jar. I use either one, depending on how much baking I will be doing at any one time.
Yeast, as a leavening, must have three things in order to grow: moisture, warmth, and food. In order to active dried yeast, which is dormant, you need to proof it. You do this by mixing the yeast in a warm liquid. (Note: When using bread makers, it's seldom necessary to do this, as you add the ingredients and the bread maker allows the time for the yeast to activate.)
Dry yeast has a sweet tooth, and many recipes call for white granulated sugars, which the yeast will break down into a simpler form. Some recipes work by adding a bit of flour to the liquid, instead, and the yeast will also break the flour down instead.
After proofing, you mix the ingredients of the bread together. Then, you knead it. Kneading aerates the dough and also develops the gluten in the wheat. In aerating the dough, that means that the kneading introduces little air pockets into the dough.
Then, you allow the dough to rise or, in other words, you allow the yeast to ferment. In rising, the yeast feeds on the sugar and produces alcohol and carbon dioxide. The carbon dioxide stretches the existing air pockets, and the dough visibly rises and expands. The flavor of the bread is developed, and the bread becomes lighter in texture.
Chemical leavenings usually are used in combination and provide an acid and a weak base to form carbon dioxide or other gases when exposed to moisture and heat. They are used in cakes, cookies, and quick breads. They are used when the longer process of biological fermentation is not practical or desirable. As we know from high school chemistry, when an acid and a base combine, they form a salt. Thus, these leaveners add a little salt to a baked product.
It takes some knowledge of chemistry to determine how much of each chemical leavener to use when baking. For one thing, an improper forumulation could leave a bad taste in the food. Therefore, many recipes call for the use of premixed baking powders, which have been formulated by experts.
Aside from biological and chemical leavenings, creaming sugar and fat together in a mixer introduces air bubbles which provide a little leavening effect. Usually, this is not enough leavening, so creaming is usually supplemented with a little chemical leavening. Similarly whipping cream, eggs, or certain other liquids also introduces air, which causes these liquids to foam. Folding the foam into a batter leavens the product by making it lighter and fluffier. A sponge cake, for example, owes its delicate texture to the effect of whipping egg whites. Creaming and whipping are called mechanical leavening techniques.
Finally, steam can produce a leavening effect. When steam is infused into a batter, it causes it to expand. To achieve this, you have to bake the item at a high enough temperature to make water steam into the item you are baking. Steamed puddings and popovers are "leavened" in this way.
Many home keepers do not think about the fact that so many of the things they do in the home are related to the sciences -- particularly to chemistry. There is an art to doing these things, as well. If you follow basic directions, you can keep a home without any knowledge of the science behind your activities, and that is just fine. However, if you would like to challenge yourself, it's good to dig a little deeper and learn why things in the home work the way they do. This can make your tasks seem more interesting to you. Also, if you have a knowledge of why things work the way they do, you are better equipped to improvise or to come up with creative new methods and recipes on your own. Not only that, but if you face cultural pressure to think of home keeping as something beneath an educated woman, you can answer that with the inner knowledge that your mind is being challenged by your work.
If you are home schooling your children, you can use household activities -- especially cooking -- to teach them about subjects like chemistry.
Soon, we'll bake some things together.