Fermentation Fermentation happens when the usual process of respiration through which an organism produces energy is inhibited in some way, changing the chemicals that are produced. Yeast fermentation occurs when there is a lack of oxygen. It produces carbon dioxide and alcohol. Yeast Yeast or Saccharomyces is a fungus that is commonly used in fermentation. Bread Yeast is added to bread to make it rise. Fermentation produces carbon dioxide gas that makes the dough swell. Alcohol is also produced but it evaporates while the bread is baked. Alcohol Yeast is used to produce alcohol in wine and beer. Yeast, together with another fungus, Aspergillus oryzae produces alcohol in sake. Other Fermentation Products Different fungi can produce different chemicals during fermentation. The dye, carotene, is produced by Rhizopus stolonifer. Penicillin is the fermentation product of Penicillium chrysogenum. Aspergillus niger produces citric acid or vitamin C that is added to soft drinks.