Yeast is a tiny living plant that produces carbon dioxide gas, causing dough to rise. There are 2 kinds of yeast available today: active dry yeast, granulated in form, and cakes of compressed fresh yeast. Fresh yeast works quickly in a warm dough. It can be refrigerated. Dr. Oetker Instant Dry Yeast has all the properties of fresh yeast. It can be mixed with flour without premixing in a warm liquid and it can be stored for up to 18 months in a cool, dry place without losing its strength.