When baking bread, each recipe tells you the kind of yeast that you need to use. Typically, for home bakers, there are two kinds of yeast that you need to worry about: active dry yeast and instant yeast. Sometimes you can get fresh yeast at specialty food stores, but the shelf life is short, so most home bakers use commercially grown dried yeast.
What is the difference?
Active Dry Yeast
Active dry yeast is yeast that needs to be “proofed” before using. Proofing just means to wake up the yeast by placing them in a small amount of warm water for several minutes prior to including it in your bread recipe. Why? According to Peter Reinhart’s The Bread Baker’s Apprentice, it is because, “active dry yeast is grown on larger grains of nutrient that have to be dissolved in warm water.”
Instant yeast can be added directly to a bread recipe. There is no need to proof the yeast. Why? Again, according to Peter Reinhart, “Instant yeast comes on such small grains that it instantly hydrates when the dough hydrates so can be added directly to the flour.” It is often called quick rise yeast or bread machine yeast. Because of the way it is processed, you need to use slightly less instant yeast than active dry yeast in a recipe, about 25 percent less.
Either way, it is best to keep both kinds of dried yeast in the refrigerator or freezer after the jar has been opened to maintain freshness. If your recipe calls for a packet of yeast, and you typically purchase your yeast in a jar, the conversion is 2 1/4 teaspoons per packet of yeast the the recipe calls for.
Source: The Bread Baker’s Apprentice