Here's the break-down:
- Active dry yeast: this is the stuff you probably picture when yeast come to mind, it's dry and granular with a consistency similar to cornmeal. To use active dry yeast, you have to dissolve the granules in warm water, then add it to the rest of the ingredients. It behaves "typically" and it will give your dough two rises.
- Instant yeast: also commonly referred to as "bread machine yeast," is milled to much finer particles. You do not have to dissolve this yeast in water prior to using, so you can add it along with the other dry ingredients. This yeast will give your dough two rises.
- Rapid-Rise yeast: this is essentially a type of instant yeast that has had additional enzymes and other additives included to make it rise faster; it is milled finer and does not need to be dissolved in water prior to use. With this yeast type, you can skip the first rise and shape the dough right after kneading.
Can you use the types interchangeably?
- You can use active dry yeast and instant yeast interchangeably on a 1:1 basis (i.e. 1 tsp active dry = 1 tsp instant). If subbing one for the other, keep in mind that you may need to adjust the amount of liquid in your recipe to account for the warm water necessary for dissolving active dry yeast.
- Rapid-Rise yeast cannot be substituted for either active dry yeast or instant yeast because it behaves so differently.