The Mystery of "YEAST"
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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.