Most professional chefs use fresh yeast to make dough. The difference between baking powder and yeast is that baking powder reacts chemically to produce carbon dioxide, which makes the baked goods rise, while the carbon dioxide produced from fresh yeast is the result of the yeast feeding on the dough. It works best in dough that isn’t excessively manipulated. Fresh yeast contains about 70 percent moisture, so it does not need to be dissolved before use, just softened in a little water. When buying fresh yeast, make sure it is nice and foamy with no dark or dry patches, and has a pleasantly fresh, yeasty smell. Fresh yeast can last up to two weeks in the fridge.