Compressed fresh yeast is a perishable product that must be refrigerated and, if properly stored, has a shelf life of up to four weeks. It has a moisture content of 70 percent, produces the most carbon dioxide per cell out of all the types of yeast mentioned here, and requires no hydration time. It should not be frozen. It is very easy to scale and can be added directly to a dough mix. Although the general recommendation for substituting fresh yeast for active dry yeast in a formula is two parts fresh to one part dry, it is always best to check the manufacturer’s package directions. To test whether fresh yeast is viable, follow the same instructions as for active dry yeast above.