Commercial yeast is sold to home and restaurant cooks in three common forms, each a different genetic strain of S. cerevisiae with different traits.
Cake or compressed yeast is a moist block of fresh yeast cells, direct from the fermentation vat. Its cells are alive, and produce more leavening gas than the other forms. Cake yeast is perishable, must be kept refrigerated, and has a brief shelf life of one to two weeks.
Active dry yeast, which was introduced in the 1920s, has been removed from the fermentation tank and dried into granules with a protective coating of yeast debris. The yeast cells are dormant and can be stored at room temperature for months. The cook reactivates them by soaking them in warm water, 105–110°F/ 41–43°C, before mixing the dough. At cooler soaking temperatures, the yeast cells recover poorly and release substances that interfere with gluten formation (glutathione).
Instant dry yeast, an innovation of the 1970s, is dried more quickly than active dry yeast, and in the form of small porous rods that take up water more rapidly than granules. Instant yeast doesn’t need to be prehydrated before mixing with other dough ingredients, and produces carbon dioxide more vigorously than active dry yeast.