Yeast, a living organism, is the agent responsible for the light, airy quality of bread. There are many different types of yeast, and using the right kind will ensure that your dough rises well.
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How Yeast Works
The sugar and starch in a dough feed the yeast and help it grow, causing it to produce carbon dioxide, which is what makes the dough rise.
Instant Dry Yeast
Unlike regular dry yeast, instant dry yeast does not requires "proofing," meaning it doesn't have to be dissolved in warm water. Also, breads baked with this yeast only require one rise, instead of the usual two.
Bread Machine Yeast
Like instant dry yeast, bread machine yeast is highly active. Because it is more granulated than regular yeast, it hydrates quickly and can be added along with the dry ingredients. When using bread machine yeast, only one rise is required.
Bread machine yeast may also include ascorbic acid, which is added as a dough conditioner. According to the Cook's Thesaurus website, bread machine yeast and instant dry yeast are very similar and may be substituted for each other in a recipe.
Both instant dry yeast and bread machine yeast are available in jars; instant dry yeast may also come in packets. Store them in a cool dry place if unopened, and in the refrigerator after they have been opened. Be sure to observe the expiration date, says the website AllRecipes.com.