Yeast

Yeast is a unicellular fungus that converts sugar and starch into carbon dioxide bubbles and alcohol.   This has made it a useful ally in the production of bread, beer, and wine.  

There are many varieties of yeast.  Bread is made with baker's yeast, which creates lots of bubbles that become trapped in the dough, making the bread rise so it's light and airy when baked.  A small amount of alcohol is also produced, but this burns off as the bread bakes.   Beer yeast and wine yeast are used to convert sugar into alcohol and, in the case of beer and champagne, bubbles. 

Raw active yeast should never be eaten, since it will continue to grow in your intestine and rob your body of valuable nutrients. Once deactivated through pasteurization, however, yeast can become a good source of nutrients.  It is a a good source of protein, an excellent source of B-vitamins including folic acid and B12.