If you are going to be a bread baker, you should know yeast. Yeast is the magic ingredient of the baking world. It's alive and master bakers have learned to cultivate yeast as a living thing in their bread and pastry doughs. In this section we will explore the different types of yeast and learn how to cultivate yeast in our products to make the best breads.

Our grandparents used-and many commercial bakers still use-fresh yeast rather than the dry yeast that we buy in the store. Fresh yeast performs marvelously well but is fragile, must be kept refrigerated, and used right away-hardly the conditions of today's carefree baking.

Instead of fresh yeast, most of us use dry yeast, either instant active dry yeast or active dry yeast. The difference in the two is how the yeast cells hydrate or absorb water. Instant active dry yeast does not have to be hydrated in water for five to ten minutes prior to mixing as active dry yeast does. Active yeast is mixed in water, the particles are dissolved, and the yeast is allowed to grow until the mixture becomes foamy. Then it is added to the flour. The cells of instant dry yeast are porous to absorb water and can be put directly in the flour without waiting for the yeast to hydrate. However, so that the yeast does not have to compete with the sugar or other ingredients for moisture, it is best to mix the yeast in only a portion of the flour. A method that works well is to mix the yeast with about one-third of the flour to create a very wet batter where the yeast cells will hydrate easily and then, add the remaining flour.

And yes, yeast is alive. It is neither plant nor animal but a fungus. We add it to the flour in its dormant state and expect it to thrive in our dough with moisture and the proper temperature. Under the right conditions, the yeast cells feed on sugar and multiply. A loaf of bread, ready to go into the oven, may contain millions of yeast cells. (The little particles found in a yeast packet are not yeast cells. They are an agglomeration of yeast cells mixed with dextrose or starch into larger balls containing many yeast cells.) As the yeast cells feed, they expel carbon dioxide and alcohol. The carbon dioxide gas rises through the bread dough and is captured by the gluten structure in the dough to form air cells. The alcohol and other excretions impart a "yeasty" flavor to the dough. Master bread bakers manipulate the ratio of carbon dioxide to alcohol-usually with temperature and acidity--to control the rise time and the flavors in the breads.

So how do we nurture these little creatures? Like most other living creatures they require moisture, food, and a hospitable environment. In a moist environment, yeast will grow rapidly. Most of the time, you will want your bread dough as moist as you can handle without being sticky. A bread dough that is too dry will take a long time to rise because the yeast will not multiply as rapidly and because the dry dough is stronger and more difficult to lift.

Yeast feeds on sugar or converts the starch in the flour to sugar for food. Without the capability to convert starch to sugar for food, yeast would not thrive in sugar free breads such as French bread. Salt impedes the growth of yeast so you can slow down the rise with salt. Conversely, you speed up yeast growth with sugar. An extra half teaspoon of salt will significantly slow the rise of the dough.

Moist dough between 78 degrees and 80 degrees is an ideal environment for yeast growth. Since yeast is very sensitive to temperature, temperature is a major factor in how fast yeast multiples. Yeast is dormant and will not grow at 40 degrees and grows only slowly at 55 degrees. Yeast dies instantly at 140 degrees. We recommend not using water warmer than 120 degrees to avoid accidentally killing the yeast.

A thermometer has been called the baker's secret weapon. In all breads, it is very useful to be able to measure the temperature of the water, the dough during mixing, and the bread as it comes from the oven. In using a bread machine, the exact water temperature is critical to a uniform outcome. Bread is baked when the internal temperature is between 190 degrees and 210 degrees. As mentioned, the ideal dough temperature for the proper growth of yeast is 78 to 80 degrees. At higher temperatures, the dough may rise too quickly creating a crumbly texture to the bread. At less, the bread will rise more slowly and will have a higher alcohol content-though some marvelous, complex flavors can be created at lower temperatures.

Understanding yeast and how it works is an essential lesson for the bread baker.