Active dry yeast and rapid rise (instant) yeast may be similar in appearance and origins (both are dried forms of live yeast), but substituting one for the other will yield vastly different results. When we baked our American Sandwich Loaf (May/June 1996), Multigrain Bread (March/April 2006), and Best American Dinner Rolls (September/October 2006) using equal amounts of each, the active dry batches consistently took longer to rise after mixing and after shaping--by almost 50 percent--and baked up denser than the rapid rise batches. Why? These two forms of yeast have different degrees of potency owing to differences in processing: Active dry yeast is dried at higher temperatures, which kills more of the exterior yeast cells (this yeast requires an initial activation in warm water), whereas rapid rise yeast is dried at more gentle temperatures (so it can be added directly to the dry ingredients). What do you do if you have active dry in the cupboard and a recipe calls for rapid rise? Luckily, there's an easy fix: To compensate for the greater quantity of inactive yeast cells in the active dry yeast, simply use 25 percent more of it (for example, if the recipe calls for 1 teaspoon of instant yeast, use 1 1/4 teaspoons of active dry). The inverse holds true as well--use about 25 percent less rapid rise yeast in a recipe that calls for active dry.
Also, don't forget to dissolve active dry yeast in a portion of the water from the recipe, heated to 105 degrees. Then let it stand for five minutes before adding it to the remaining wet ingredients. Skip this step if using instant yeast in recipes that call for active dry.
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