The yeasty smell of warm bread dough filled me with anticipation as I patted the sticky dough into the edges and corners of a loaf pan and gently smoothed the surface with my wet hands. Though eager to bake the bread immediately, I forced myself to have patience and allow the dough rise first. When I returned some time later to put the risen bread dough into the oven, I was dismayed to find that the dough had not risen at all. What had happened to my bread?

Yeast, the leavening agent that causes yeast bread dough to rise, is a fungus. It feeds on sugars in the flour and produces carbon dioxide gas. The gas bubbles are trapped by the sticky dough, and they cause the dough to rise. Baking the dough kills the yeast, but the trapped gas bubbles yield a light, porous bread.

That was what was supposed to happen. That it did not happen with my bread dough on that occasion clearly demonstrated to me that things could go seriously amiss in the process. After many more years of baking experiences and reading what other bakers and food scientists have written on the subject, I now have a better understanding of the way yeast works.

• Yeast grows better when it is warm. Cool temperatures will cause yeast to become dormant.

Use warm liquid to make the dough. Heat the liquid to 105° to 115° when using regular yeast. When using rapid rise yeast, the optimal temperature for the liquid is higher, about 125° to 130°.

Though the liquid used in the dough may be the right temperature, if all the other ingredients are cold, the dough may not be warm enough for yeast to grow. With the exception of the liquid, all ingredients should be at room temperature.

In a cool or drafty room, dough may cool enough not to rise well. To create a warm environment with an electric oven, turn on the oven light and put the bread in the oven to rise. The small amount of heat from the light bulb makes the oven just warm enough to promote rising. With a gas oven, the heat from the pilot light may keep the oven warm enough for rising bread. To warm a microwave oven for rising dough, bring a cup of water to a boil in the oven. When the oven turns off, put the dough in the microwave oven, leaving the cup of hot water in the oven. The heat from the water will keep the microwave oven warm.

• At higher temperatures (above 140°), yeast dies. If liquid added to the flour mixture is too hot, it may cook the yeast before the yeast has a chance to grow.

I used to check the water temperature with a thermometer designed for making yogurt. (A candy thermometer would work also.) By feeling the water when it showed the right temperature on the thermometer, I eventually recognized the correct temperature without having to use a thermometer. It just took some practice.

• If yeast is past its expiration date, it may fail to grow. To check its viability, dissolve a teaspoon of yeast and a teaspoon of sugar in a half cup of warm water. Let the mixture stand for five minutes. If it foams and smells like baking bread, the yeast is still active.
• If not enough yeast is used, the dough will not rise sufficiently. However, if too much yeast is used, the bread may rise too rapidly and collapse.
• Too much salt inhibits yeast growth. Accidentally adding salt twice or measuring it incorrectly may keep the dough from rising.
• Garlic inhibits yeast growth. Keep this in mind when trying to make herb bread.
• Too much liquid in the dough makes the dough too thin to support the gas bubbles, and the dough will collapse after rising.

Though learning to bake bread requires some trial and error, having a better understanding of yeast has helped me to have more success in my baking and fewer failures. I revel in the successes; I consider the less successful endeavors as learning experiences; and I make the failures into breadcrumbs to use in other cooking projects.