Did you know that most beer is made by a fungus? Saccharomyces cerevisiae is the most widely used species of yeast (which is a fungus) when making beer. This yeast consumes the glucose and maltose released from barley during the mash. In the absence of oxygen, the yeast uses fermentation to keep its metabolism running. The byproduct of the process is alcohol and hundreds to thousands of other chemicals that give beer its distinct flavor and aroma.