Yeast Information Center

YEAST
Yeast is a living organism: a one-celled fungus. Various strains of yeast are present virtually everywhere. Yeast feeds on carbohydrates, converting them to carbon dioxide and alcohol in an organic process known as fermentation:

Yeast + Carbohydrates = Alcohol + Carbon Dioxide

When yeast releases carbon dioxide gas during bread making, the gas becomes trapped in the dough's gluten network. The trapped gas leavens the bread, providing the desired rise and texture. The small amount of alcohol produced by fermentation evaporates during baking. As with most living things, yeast is very sensitive to temperature. It prefers temperatures between 90˚F and 110˚F (32˚C-43˚C). At temperatures below 34˚F (2˚C), it becomes dormant; above 138˚F (59˚C), it dies. Salt is used in bread making because it conditions gluten, making it stronger and more elastic. Salt also affects yeast fermentation. Because salt inhibits the growth of yeast, it helps control the dough's rise. Too little salt and not only will the bread taste bland, it will rise too rapidly. Too much salt, however, and the yeast will be destroyed. By learning to control the amount of food for the yeast and the temperatures of fermentation, you can learn to control the texture of your yeast-leavened products. Types of Yeast Baker's yeast is available in two forms: compressed and active dry. (You may also encounter a product called brewer's yeast; it is a nutritional supplement with no leavening ability.)

Compressed Yeast
Compressed yeast is a mixture of yeast and starch with moisture content of approximately 70%. Also referred to as fresh yeast, compressed yeast must be kept refrigerated. It should be creamy white and crumbly with a fresh, yeasty smell. Do not use compressed yeast that has developed a sour odor, brown color or slimy film. Compressed yeast is available in 0.6-ounce (17-gram) cubes and 1-pound (450-gram) blocks. Under proper storage conditions, compressed yeast has a shelf life of 2-3 weeks.

Active Dry Yeast
Active dry yeast differs from compressed yeast in that virtually all the moisture has been removed by hot air. The absence of moisture renders the organism dormant and allows the yeast to be stored without refrigeration for several months. When preparing dough's, dry yeast is generally rehydrated in a lukewarm (approximately 100˚F (43˚C)) liquid before being added to the other ingredients. Dry yeast is available in 1/4-ounce (7-gram) packages and 1- or 2-pound (450-gram or 1-kilogram) vacuum-sealed bags. It should be stored in a cool, dry place and refrigerated after opening.

Instant Yeast Instant
or quick-rise dry yeast is also available. It must be blended with the dry ingredients in a bread formula, then activated with hot (approximately 125˚F-130˚F [52˚C-54˚Cl) water. It dramatically speeds the rising process. Instant yeast can be substituted measure for measure for regular dry yeast. Instant yeast is still a living organism and will be destroyed at temperatures above 138˚F (59˚C). The flavors of dry and compressed yeasts are virtually indistinguishable, but dry yeast is approximately twice as strong. Because too much yeast can ruin bread, always remember to halve the specified weight of compressed yeast when substituting dry yeast in a formula. Likewise, if a formula specifies dry yeast, double the amount when substituting compressed yeast. All the formulas in this text requiring yeast use regular active dry yeast.

Sourdough Starter
Prior to commercial yeast production, bakers relied on starters to leaven their breads. Early starters were simple hut magical mixtures of flour and liquid (water, potato broth, milk) left to capture wild yeasts from the air and then ferment. Only a portion of the starter was used at a time. The rest was kept for later use, replenished periodically with additional flour and liquid so that the magic could continue. Today, starters are generally fortified with yeast to provide consistency and reliability; they are prized for the unique, sour flavors they impart. Prepared dry cultures are often used commercially to give bread a sourdough' flavor without requiring the time and space necessary to develop and maintain an active starter.