Old-fashioned fresh yeast, also called cake or compressed yeast, is best avoided because it's so perishable that it's not unusual to buy a block that's already dead. Instead, we recommend using active dry yeast or instant yeast (the kinds most often found in supermarkets), which are far more reliable. Granulated active dry yeast is made by putting yeast on a dryer to remove 90 percent of its moisture. It's a harsh process that produces many dead yeast cells, which form a hard coating around each granule. That's why the granules must be proofed, or soaked in water at a temperature of 100 to 115 degrees before they're used, in order to dissolve that coating and awaken the active yeast within. No proofing is required for instant yeast, otherwise known as quick-rise, fast-rise, or rapid-rise yeast. The drying process for this kind of yeast, developed in the 1970s, is gentler; fewer cells are killed, so the yeast is faster acting and can be added directly to dry ingredients. The liquid you add to make the dough, however, should be between 115 and 130 degrees. Any lower, and the yeast might not work; any higher, and you might kill it. Also, because instant yeast is more potent, you should use less of it: three-fourths of a teaspoon to a single teaspoon of active. Either type of dried yeast should keep for up to a year at room temperature.