WHAT IS IT? Baker’s yeast is a leavening agent used to make bread and similar baked products rise. Unlike other raising agents, such as bicarbonate of soda, it’s not a chemical compound but a living organism, called Saccharomyces cerevisiae. One gram of fresh baker’s yeast contains around 15 billion single-cell yeast organisms. Most yeast used in baking is commercially produced, though a similar wild yeast occurs naturally in flour (this is the starting point for sourdough bread).