What Is Yeast?
Yeast is a living plant that is used for the purpose of causing fermentation. The yeast we use in baking bread is an artificial yeast—really a dough made of flour and a little common yeast and made into small cakes and dried. If kept free from moisture it retains the power of causing fermentation for some time. The flour and other matter in a cake of yeast are only used to keep the yeast in a form where it can be preserved. It is necessary to add water to start fermentation and that is why we add hot water when we stir in the yeast for a baking.