Yeast

Yeast is a single-celled microorganism that is neither a plant nor an animal—it is a member of the kingdom Fungi—and it requires suitable conditions to thrive. These conditions include moisture, oxygen, food, and appropriate temperatures, and when they are provided, the life cycle of the yeast will become activated, resulting in both reproduction and alcoholic fermentation. Alcohol fermentation is the conversion by yeast of sugars into alcohol and carbon dioxide, and it is primarily this attribute of yeast that pertains to the bread baker. (By contrast, lactic fermentation is the bacterial conversion of sugars into primarily lactic acid.)