yeast is a living microorganism, a single-celled fungus. While there are more than 1,000 known species of yeast, bakers use domesticated strains of the species Saccharomyces cerevisiae—the name roughly translates to “the sugar fungus found on beer.” Baker’s yeast, also called brewer’s yeast, is a microscopic rod-shaped organism. In the wild, Saccharomyces cerevisiae is principally found on fruit skins, including grapes. A mature yeast cell is about 4 micrometers in length; a grain of salt is about a thousand times larger, and even skin cells are nearly 10 times the size of a typical baker’s yeast cell.
