Yeast is the driving force behind fermentation, the magical process that allows a dense mass of dough to become a well-risen loaf of bread. Active dry yeast, which is sold in sealed, foil-lined packets, provides baked goods with a pleasant yeasty flavor. In the packaging process, about 25% of the yeast cells die off, releasing a small amount of glutathione, which causes relaxation of gluten (this makes it a good yeast for pizza dough, but it's not ideal for all dough products).
1 (1/4 oz.) envelope = 2-1/4 tsp.
To substitute instant yeast for active dry, use 25% less instant; 1 tsp. active dry yeast = 3/4 tsp. instant yeast.
Active dry yeast must be rehydrated in warm water (proofed) before adding to other ingredients.
Refrigerate and use by expiration date.