Baker’s yeast is used as a leavening agent to increase the volume of baked goods. It comes as active dry and compressed fresh yeast, and both are used as leavening agents to help a foodstuff like bread to rise. Active dry yeast is comprised of tiny, dehydrated granules of yeast. Although the granules are alive, the yeast cells are dormant due to their lack of moisture; because the cells are dormant, dry yeast has a long shelf life. Active dry yeast becomes active when mixed with a warm liquid. Compressed fresh yeast is moist and extremely perishable. It must be stored under refrigeration and used within one to two weeks.

Brewer’s yeast is a much different foodstuff. As the name implies, it is a special non-leavening agent used in beer making—it converts the sugars in malted barley into alcohol. Dozens of specialized strains of yeast produce by-products that give each type of beer its unique taste, from all types of lagers to ales. And because it is a rich source of vitamin B, it is also used as a food supplement.