Yeast are microscopic, one-celled fungi that humans commonly use to make fermented products like beer, bread and cheese. Yeast obtain food from sugars, which they break down through a process called fermentation to produce the energy that they need to sustain life, as well as the by-products of carbon dioxide and alcohol. It is these by-products that give yeast-fermented foods their properties.
Yeast need sugars to grow and multiply. They are capable of breaking down fructose, glucose and other monosaccharides, but they cannot use starch as a food source because they lack the enzymes to break it down, according to HowStuffWorks. However, enzymes found in some starchy food products, such as germinating barley, change starch into sugar that the yeast can use. This is how sugar is generated for use by yeast in the process of brewing beer.
There are many different species of yeast, but the one most commonly used in cooking and baking is called Saccharomyces cerevisiae, or brewer's yeast. This yeast is sold in a dry, inactive form from which most of the moisture has been removed. The yeast comes to life and begins reproducing when it is exposed to liquid and a source of sugar on which it can feed.