Yeast is made up of a single-celled organism, Saccharomyces cerevisiae, which multiplies rapidly when fed sugar in a moist environment. One pound of yeast contains 3,200 billion yeast cells! Yeast also thrives on starch, which it converts to glucose, a simple sugar. This process ferments the sugar, which converts to alcohol and carbon dioxide. As we've mentioned earlier, the carbon dioxide expands the baked good to produce the light, fluffy texture. Of course, yeast is also a vital part of the making of alchoholic beverages, such as beer and wine. But that's another story... The minimal alcohol product burns off during the baking process, and the yeast dies also with the heat.
The ideal temperature for yeast growth is 100 to 115°F., but for leavening purposes, the ideal temperature is 80-95 degrees F. If the yeast grows too quickly, it will produce large bubble pockets in the end product. Yeast begins to die at 120°F. So, it's important to let your yeast dough rise in a spot where the temperature is regulated. One-half an ounce of yeast will raise 4 cups of flour in about 1-1/2 to 2 hours under ideal conditions.
You should also proof your yeast to be sure it's viable before using in a recipe. To check it, mix a bit into 1/4 cup of lukewarm water with 1/4 teaspoon sugar. It should begin to bubble and ferment within about 5-10 minutes. If not, the yeast is dead and should be discarded.
Salt inhibits the growth of yeast. Never mix yeast into salted water. Since most tap water goes through a filtering process which utilizes salt as a refining/cleaning agent, many cooks use only distilled water for baking. However, if you are baking during the hot summer season and find your dough rising too much, the addition of a little extra salt can control that runaway yeast growth.
Most yeast is sold in single-use packets or bulk bags known as dry active yeast. Compressed yeast is not as widely available, but can be used to the ratio of one standard cake of compressed yeast to one scant tablespoon of dry yeast. If the dry yeast is stored in airtight packaging, in a cool dry place, it's not necessary to refrigerate it. Yeast should always be at room temperature to begin a recipe. Standard single-use packets contain about 2-1/2 teaspoons (1/4 ounce) of yeast granules.
Now available on the market is fast-rising active dry yeast, which is smaller-grained than conventional active dry yeast and speeds rising times by as much as fifty percent, often eliminating the need for a second rising period. It may be used interchangeably in measure with active dry yeast. The best method for using this yeast is to mix it directly with the dry ingredients before adding liquid, instead of adding it to warmed liquid and then adding to dry ingredients.
Know that compressed fresh yeast consists of 70 percent moisture and must be stored in the refrigerator. Compressed fresh yeast is highly perishable, as opposed to dry active yeast, and loses its vitality within two weeks, even when properly stored refrigerated in an airtight container. Compressed yeast can be stored in the freezer, but should be defrosted at room temperature and then used immediately. Dried yeast has become the norm for its staying power in the pantry. Yet, even dry yeast must be stored in an airtight container, with no threat of moisture, and it will lose its life over time. Use 2 teaspoons of dried yeast to a 2/3 ounce compressed yeast cake as a substitution.
Yeast measures will also have to be adjusted at higher altitudes. Again, this will take experimentation on your part for your altitude. And, why knead? It helps distribute the yeast cells uniformly throughout the dough, so it doesn't rise unevenly. Kneading also develops a firm gluten structure, providing the framework for the carbon dioxide bubbles.
Yeast for Sourdough is of a slightly different breed, Saccaromyces esigus, and multiplies at a much lower rate than standard baker's yeast. The resulting product is much more compact and dense. It's more acidic because it cannot digest maltose sugar, thus a certain type of indigenous San Francisco bacteria takes over. The byproduct of the bacteria is highly acidic, giving that characteristic sourdough flavor.
Leaveners in general
Salt inhibits the growth of yeast, yet gives a firmer crust, finer crumb and adds flavor. Sugars are not essential to leavened baked goods, but they make the product more tender due to postponement of protein coagulation, allowing the dough/batter to grow to a greater volume before being frozen into stasis by the baking process, as well as adding to flavor. If too much sugar is used, it can slow down the growth of the yeast, with a low-rise result. The relationship of sugar to salt to leavening is crucial to a pleasing final product.
Note: Brewer's yeast has no leavening properties but is added to products for nutritive benefits, as it is rich in the B vitamins. It is also, of course, used in the brewing of beer.