Have you ever wondered which yeast you should use when baking? What's the difference between active dry yeast and instant yeast? What the heck is compressed yeast and fresh yeast? Can you use Rapid Rise yeast if a recipe calls for active dry yeast? So I'm going to clear it up for you and demystify the whole yeast question.
1There are basically two kinds of yeast - fresh, compressed yeast in cakes and dry yeast. Fresh yeast is something most people do not use anymore and many stores don't bother to carry it. It has a very short shelf life (two weeks or less) and is highly perishable. Because of this, manufacturers developed dried yeast, or "active dry yeast." It enables yeast to be stored for a long time. In this particular drying process, some yeast cells are killed and so that is why it is best to "proof" active dry yeast in some warm liquid, to make sure that enough yeast cells are still alive to do the trick with your dough. Manufacturers got even more clever and came up with a different drying process, one that left a whole lot more yeast cells alive - this is "instant dry yeast", better known as Rapid Rise (Fleischmann's brand) or Quick Rise (Red Star brand) or Perfect Rise (SAF brand). They do not require proofing or rehydration to work. They are designed to be added right in with your dry ingredients. These instant dry yeasts are all the same thing and are packaged the same, also - in 1/4 ounce little envelopes. Instant, or quick rising yeasts, will do the job of rising your doughs about 50% faster. They are more stable and reliable and a lot of people prefer them. Also, the yeasts that are labeled "For Bread Machines" are instant yeasts.
2Are they interchangeable? Remember, the biggest difference is that active dry yeast needs to be proofed and instant yeast does not. Aside from that, you can substitute one for the other in a recipe - you just use a little less instant yeast if a recipe calls for active dry yeast.
3Why use active dry yeast, then, over instant yeast? Some people believe they can taste the difference, but this is really in the rising times, not which yeast you use. The less yeast that goes into a dough and the longer the dough rises, the better the flavor the dough will have. Bakers know slow rises are better for taste, but, as I said, this is a function of the length of time the dough is allowed to rise, not which type of yeast you use. Remember, yeast is yeast is yeast. It's all the same - if you want a slow rise and you are using instant yeast, just use less.
4Instant Yeasts are Rapid Rise (Fleischmann's), Perfect Rise(Red Star) and Quick Rise (SAF). They're all the same thing.
No rehydration is required of instant yeasts. Fleischmann's says on their web site that RapidRise™ yeast actually loses its fast rising capabilities if dissolved in liquid, and will require two complete rises. I assume that holds true for other instant yeasts.
Active Dry yeast has larger granules and is necessary to dissolve completely for the yeast to work. Therefore, Active Dry works best if dissolved in warm water (100° to 110°F).
One envelope (2-1/4 tsp) of yeast (active dry or instant) can raise 4 cups of flour (or about 1 pound)
Yeast dies at 140 degrees F, so be sure that the liquid you add to your dough is not hot. It should be warm, about 95 - 110 degrees F. Use a thermometer until you remember how warm it should feel and then you can just do it by touch. Mr. Food Science himself, Harold McGee, says that yeast activity is best at 95 degrees F/35 degrees C.
Salt can kill yeast or decrease its effectiveness if it comes in direct contact. For this reason, add the salt in a recipe along with the bulk of the flour, when you add that.
1 envelope of yeast is about 2 -1/4 teaspoons.
You don't have to refrigerate yeast, but if you do, it's better to bring it to room temperature before using.