Serious bakers know that making bread is more of a science than an art. Recipes should be followed closely, and swapping an ingredient for whatever you might find in your kitchen can have disastrous consequences. That’s why shopping for yeast—a vital component in leavened bread—can be frustrating. Instant and active dry yeast often appear together in the baking aisle, and though you can use them interchangeably in most cases, they’re not exactly the same.
Yeast are single-celled organisms that produce the gases that make dough rise. Whether you use the instant or active dry product in your next baking project, you’re working with living microbes.
Active dry yeast is typically sold in packets and has a granular consistency. As the name suggests, it’s traditionally activated in warm water before it’s combined with other ingredients. In addition to “waking up” the dormant microorganisms, letting yeast sit in warm water for 10 minutes is a way to confirm it's still alive when you see blooms of bubbles. (If the surface of the water never changes, the packet may be a dud or too old to use.)
Instant yeast technically is dormant as well, but you don’t need to proof it before baking with it. The granules are finer and they’re made to dissolve quickly once they’re added to bread dough. It’s impossible to tell if the yeast is alive with this method, however, so if you don’t know how long that sack of instant yeast has been sitting in your pantry, you may still want to proof it in warm water to be sure.
Many bread recipes call for active dry yeast, but there’s no harm in substituting it for instant yeast 1:1. In fact, King Arthur recommends using instant yeast every time no matter what the ingredients list says. On top of being cheaper, it rises faster than the alternative and keeps producing gases for a longer period of time.
If all you have at home is active dry yeast, you can use it the same way you would use instant. King Arthur tested dough made using un-proofed active dry yeast and found it was no different from dough made with yeast activated in water. There are plenty of opportunities to screw up homemade bread, but using the wrong yeast in the wrong way usually isn’t one of them
[h/t King Arthur]